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LA SILLA OBSERVATORY

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1. epee lt y IDLE DSLR mem o n yh Kener TOS bee tea OAK IWO w Seq nering tao AS Arctica ee OS cS i nama e t a ew peg mene 2 rL E TO w wer Eepos Zort Doae rme Pa a ww 00 00 00 Ove ab we we Esnox Pra Eogocwe me wire ii omea Le Core med Exposure Paraneters DIT fea f 1 16 gt pe Di MOT T i e XV we RTD MO Sarpin 3 pro Aso hal Dairy Readout Lode Double gt Tano i D o a icon Window Parameters MAK inm omer Xa GTARTE 1 stany y x mes NY ore ta line 009190100 JEDI DATA CANCEL sn any Acomiog Der Spare Espasa Matar y 106 3 005 4s H CLEAR 2 CUEAR rer a Ld Ld ee Figure 4 1 OS of SOFI Acquisition templates Move to take an image off the target at a offset position defined by the user in the template To expedite the process one can even store a fixed pattern during the movement of the telescope first make sure the DIT and NDIT have been updated to the values in the acquisition template but this may lead to dark negative lines instead of dots on the subtracted image The RTD can display either each individual frame or it can display the average of NDIT frames INT mode The option is selected from the button labeled DIT or INT respectively The RTD can also display the image zoomed or reduced by an integer factor clicking on the button to the right of label Scale will rise a pop up menu with the available options The menus on the top left han
2. In this particular case the number of cycles is 4 so the total exposure time is 60 x3x3x2x4 72 minutes If the number of cycles is greater than 1 the telescopes moves in the following pattern A B B A A B B A etc Between consecutive A A or B B positions there is no telescope offset but one can include random offsets between consecutive A B or B A positions In other words every A B or B A cycle can be shifter with respect to the previous The size of the region along the slit in which are placed the random offsets is defined by the parameter Jitter Box Width In the example given here the 1st and 2nd A positions will differ Additionally the 1st and 3rd B positions will also differ In principle the nodding frequency should be similar to that used during direct imaging However the desire to reach background limited performance forces longer exposures in the spectroscopic modes As a general rule one should limit the time spent at any one position to less than 15 minutes Although guiding is optional we recommend that you guide To do so set the parameter Combined offset to true The Non Destructive Read Out is recommended for faint objects that require long DIT This is done SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 with the templates SOFI_spec_obs_AutoNodNonDestr and SOFI_spec_obs_GenSpecNonDestr In addition to the parameters of Table 3 9 these two templates include those showed in Table 3 10 The arr
3. In all templates except acquisition templates it is possible to window the detector The window is described with the coordinates of the origin and the window size Table 3 16 Note that the entire array is read the windowing parameters only determine which part of the image will be stored in the raw data file Therefore the windowing does not decrease appreciably the overheads Signature Keyword Default Value Explanation Number of rows DET WIN NX 1024 Number of rows Number of columns DET WIN NX 1024 Number of columns First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Table 3 16 Detector Window Signatures and Keywords Chapter 4 Phase 2 Preparation and Observing with SOFI 4 1 General Issues Observations at the NTT are done under the normal ESO procedures Observers are expected to arrive one to two days before their observations start and with the help of the support astronomer to fully specify their observations during this time Some hints e The visitors will be given a visitor account to use the computers in the visitors office there are possibilities to link a laptop to the local network e The visitors are expected to drive themselves around the observatory so a valid international driving license is needed if they can not drive the observatory staff will drive them e The visitors are encouraged to call the La Silla safety number officer at
4. SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 21 Parameter signature Value Exposure Name SOFI_Map DIT 10 NDIT 6 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 NJITT 6 NEXPO 9 Jitter Box Width arcsec 40 Filter wheel 1 K Filter wheel 2 open Instrument Mode LARGEFIELDIMAGING Combined Offset T F F Return to Origin T F T RA Offset List arcsec 0 450 450 0 450 450 0 450 450 DEC Offset List arcsec 00 0 450 0 0 450 0 0 Table 3 3 Parameters of the SOFIimg obs AutoJitterArray template with commonly used values SOFT_img obs_AutoJitterArray_1 as the area mapping There are two distinct cases i Observations of semi extended objects as large of a quarter to half of the Sofl filed of view these may be moderately distant galaxies small Milky Way clusters or LMC SMC clusters They are too big for simple jittering mode observations but still leave room for more efficient observing strategy where the observer does not need to sample clear sky 50 of the observing time Instead the user can observe the sky simultaneously with the target adopting clever offsets that would move the in the centers of the four quadrants or two halfs of the array In the latter case a suitable rotation offset may be necessary to align the side of the array with the major axis of the object A typical example of a 4 point observation is shown in Figure 3 1 The figure shows the target
5. a round object with diameter 90 arcsec as it will appear on the RTD and on the Sofl images The best choice is to use the template SOFI_ img_obs_AutoJitterArray_1 As it is described in Section 3 2 4 this template allows the user to take a sequence of jittered images around user defined positions first all positions are imaged ones then a random jitter is added to the entire pattern and it is repeated as many times as necessary Note that the offsets are executed before the images are taken and the offsets are defined along RA and DEC in arcseconds On the figure the images in the sequence are numbered to show the order in which the target will move during the observations The zero number is the location of the target acquisition and it will be discussed further The parameters of both templates are listed in Table 3 4 Note that for 4 point observation the NEXPO parameter must be equal to four Therefore the total integration per position is controlled by the time spent per on each image DITxNDIT usually 1 3 minutes and by the number of jittered images taken at each individual position NJITT In this example the total integration time is NJITxNEXPxNDITxDIT 12x4x10x6 2880 sec In general the template allows to define manually the offsets for each individual image However this is inconvenient if the number of images is large To simplify the matter we suggest to the user to define a closed loop where after four images the
6. 0 100 200 300 Flat Field Age days Figure 3 4 Stability of the Special Dome Flat images accuracy of the photometry as a function of the flat field age generated by this fit is normalized to one Then the flat is multiplied by this surface to create a corrected flat field Alternatively one can think of this surface as a second flat field and instead of correcting the dome flat the user can divide the data by the regular dome flat and then by this surface This surface is sometimes called an illumination correction Without this correction the low frequency variability from edge to edge of the array is between 1 and 3 depending on the filter the objective and the type of flat used After the illumination correction the low frequency variability improves to better than 1 Illumination Correction surfaces can be downloaded from the SOFI Web page The observatory staff produces them typically once a month Templates for observing a standard star in a ar ray of 16 positions are available in the impex directory of SOFI on wsofi in the calib directory SOFT Tllum_Correction_J SOFT Tllum_Correction_H etc NOTA BENE The illumination correction frame refers only to the flat field that was used to create it and it must not be used with other flat fields The reason for this is that the illumination correction removes the effect of different dome screen illumination and in general every time the user obtains dome flats the intens
7. Data Reduction bul Basie Concepts 220 can asf A Rie antes We od OH eck RO A O22 MA PING ec ay Saeed eas a St An ee Sle Bee Be ae Sones Me ae Ga a 5 2 1 Inter quadrant Row Cross Talk ee 5 2 2 Masking the Bad Pixels 2 2 2 2 2 0 0002 eee ee ee eee 5 2 3 Subtracting the Dark Bias Frame a tice ay Bet pa AR e Re EG 52 A Sky Subtraction a noe hoy Ae ot an Bi tod Dee he ba hed aet 5 2 5 Flat Fields and Illumination Corrections 2 5 2 6 Image Alignment and Combination oao e ea e 020220004 5 3 Long Slit Spectroscopy sosoo a 5 3 1 Inter quadrant Row Cross Talk ee 5 3 2 Sky Subtraction 425 844 522 a a OR eee ee 539 Flat Fields ies gi e ace Ee hla A BO a ee ORS 5 3 4 Removing Slit Curvature e DOr AAT CS ag pt SR Mes Sk co 2 oe Os S o nl te a ot A DO to i 5 3 6 Removing of the Atmospheric Absorption Features and Flux Calibration 5 3 7 Alignment and Combination 0 02 00 eee eee eee DA Polarimetry eir 4 88 ce A be ee a ee Be ee E Enr Calibration Arcs B Atmospheric Absorption SOFI Templates A Reference Guide Cul General Points aea mea E AE Rated SOT ee AA Se 2 SOFT Imagine Templates se ogee Bets Ge ee ee Ee ee eB C 2 1 SOFI Imaging Acquisition Templates 0 222004 C 2 2 SOFI Imaging Science Templates 2 a C 2 3 SOFI Imaging Calibration Templates 2 0 ee C 3 SOFI Polarimetric Template 2 2
8. FE o o o o o o o o o o o Do o oO oO o mo Z gt x lt gt lt gt lt x lt x lt gt x lt 6x10 pa i ot a ie ng a a al B 4 gt a i n E i ra 5 o o y Sa im Y el st 2 4x10 P SS gt i co o amp D o zZ Z 2x104 o E E 1 L 1 L 1 1 1 1 i 1 1 L 1 1 1 j 1 1 2x10 1 25x10 1 3x10 1 35x10 1 4x10 1 45x10 Wavelength A Figure A 4 A Xenon and Neon arc spectrum taken with the medium resolution grism at the J atmospheric window The main lines are marked SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 i 2 a o o 12 me 7 st Ce Cos op M co o ce 383x104 E ine QQ wow LO oD Ke 52 0 y Q P OO WO YC Ji oD en a a LO ud LO O co co pe n i rs a A o oO oo o o Do o oo Z gt x lt lt lt x lt gt x lt Z x lt Z Z L a eS PSPs es PS a ES SS 2x 104 pi e po T n ds H 3 i ig L J o 5 S S o 2 L 5 oO ES o La d aar 104 j eee G A o ed 1 1 f 1 1 1 1 1 1 5104 1 6x 101 1 7x101 1 8104 Wavelength A Figure A 5 A Xenon and Neon arc spectrum taken with the medium resolution grism at the H atmospheric window The main lines are marked SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 T T 8x104 o o o eo aot a p c LO oD sto 0 oO co oO Oo CO
9. Line identifi cations are given in the Appendix A The halogen lamp is used to flat field spectroscopic data These flats are called Nasmyth flats Note that the Nasmyth flats introduce an extra spatial response by the lamp that has to be removed by additional night sky spectroscopic flat The halogen lamp is not suitable for flat fielding of imaging data Use the dome flat field lamp for this see Sec 3 5 2 for details 2 7 Instrument Performance and the Exposure Time Calculator Table 2 5 we list the approximate zero point of the broad band filters with the large field objective as measured over many nights starting from July 1998 to May 2005 Limits are similar for the small field and spectroscopic objectives For more recent detection limits in the spectroscopic modes and for detection limits with the narrow band filters together with recent measurements of the ZPs refer to the SOFI web page Filter ZP Average Background Detection Limit Mag sq arc second Point Source Extended Source Z 22 6 ENS EN i J 23 2 15 5 16 1 22 7 22 1 Js 23 1 a rn oe A 23 0 13 4 14 7 21 8 21 4 K 22 4 12 8 13 3 20 8 20 4 Table 2 5 Measured SOFI performance for the broad band filters 1 hour exposure The point source detection limits are based on the following assumptions e Signal to noise ratio of 5 computed over 21 pixels e Pixel scale 0 288 arc seconds pixel e 1 hour exposure made up of 60 one minute integrations e Seeing 0 75 arc secon
10. the individual data reduction steps The users must read it carefully before preparing their observations because the good understanding of the data reduction will help you to plan the observations better and more efficiently There is also a WEB page dedicated to SOFI It is accessible from the SciOp home page http www 1ls eso org lasilla sciops There you will find the most up to date information about the instrument recent news effi ciency measurements and other useful data that do not easily fit into this manual or a subject of frequent changes This WEB page is updated regularly Applicable documents VLT MAN ESO 14100 1510 OS Users Manual 2 VLT MAN ESO 14100 1531 DCS Users Manual 3 VLT MAN ESO 14100 1094 ICS Users Manual 4 VLT MAN ESO 00000 000 1 1 P2PP Users Manual 5 1 3 VLT MAN ESO 00000 000 SOFT TSF Parameters Reference Guide obsolete Abbreviations and Acronyms The abbreviations and acronyms used in this manual are described in Table 1 1 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Acronym BOB DCR DCS DEC DIT EMMI ESO FWHM ICS IR ISAAC NDIT NDR NINT NTT OB OS P2PP PSF RA RON SOFI TCS TSF VLT ZP Description Broker of Observing Blocks Double Correlated Read Detector Control System Declination Detector Integration Time ESO s Multi Mode Instrument European Southern Observatory Full Width at Half Maximum Instrument Control System Infra Red IR Spectrogr
11. 11 737 0 009 11 431 0 006 11 337 0 008 11 336 0 005 9109 S055 D 04 18 18 9 69 27 35 11 552 0 002 11 326 0 002 11 255 0 027 11 269 0 002 9111 S361 D 04 49 54 6 35 11 17 11 246 0 006 11 031 0 006 10 992 0 033 10 980 0 006 9113 252 D 05 10 25 6 44 52 46 11 059 0 005 10 776 0 005 10 708 0 034 10 713 0 005 9115 363 D 05 36 44 8 34 46 39 12 069 0 007 11 874 0 005 11 826 0 007 11 831 0 005 9116 S840 F 05 42 32 1 00 09 04 11 426 0 009 11 148 0 009 11 077 0 014 11 058 0 008 9118 S842 E 06 22 43 7 00 36 30 11 723 0 011 11 357 0 009 11 264 0 016 11 261 0 010 9119 S121 E 06 29 29 4 59 39 31 12 114 0 006 11 838 0 005 11 765 0 009 11 781 0 005 9121 S255 S 06 42 36 5 45 09 12 12 719 0 004 11 434 0 004 11 372 0 004 9125 S005 D 07 19 38 6 84 35 06 10 885 0 007 10 598 0 006 10 514 0 013 10 522 0 008 9129 209 D 08 01 15 4 50 19 33 10 914 0 007 10 585 0 006 10 487 0 021 10 496 0 009 9132 312 T 08 25 36 1 39 05 59 11 949 0 006 11 669 0 005 11 608 0 004 11 609 0 004 9133 S495 E 08 27 12 5 25 08 01 11 521 0 007 11 048 0 008 10 965 0 016 10 960 0 010 9134 P545 C 08 29 25 1 05 56 08 11 881 0 007 11 624 0 005 11 575 0 005 11 596 0 006 9135 S705 D 08 36 12 5 10 13 39 12 362 0 010 12 098 0 011 12 040 0 014 9136 S165 E 08 54 21 7 54 48 08 12 489 0 008 12 214 0 008 12 138 0 018 12 142 0 011 9137 S372 S 09 15 50 5 36 32 34 11 153 0 007 10 891 0 007 10 830 0 019 10 836 0 010 9138 S852 C 09 41 35 8 00 33 12 11 354
12. 4444 in case of medical or other emergency and to warn a member of the observatory staff for any outdoor activity remember that the closest hospital is a few hours away e The visitors are expected to contact their support astronomer upon arriving note that your support astronomer may be sleeping late if the previous night was dedicated to service mode observations or to technical work e The visitors are expected to read carefully the La Silla web page to acquaintance themselves with the rules of the operation of the observatory and the instrument they are going to use e The visitors should not edit by hand an OB or write a script to produce them as it may cause crashes 4 2 The VLT environment P2PP BOB OS TCS DCS ICS Observations are described by observing blocks OBs essentially scripts for driving the telescope and the instruments The OBs consist of one or more templates in effect commands in the scripts The templates have a number of parameters much like the functions in computer programming A typical science OBs is made up of four components a target an acquisition template an obser vation description and a set of constraints The observation description is itself made up of one or more observation and calibration templates The constraints describe the acceptable conditions 39 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 under which an observation can be carried out in service mode and therefore th
13. A A Ai 3 a yb E et e ea ser o a eae 20 e CSKF 12 14 CSKF 1ZA gt CSKE 14f D 154 R Po uda SA 911 E 13A e a e le ay 140 Figure E 5 Finding charts for the photometric standards of Persson et al 1998 V ___000_ 105 at s
14. Exposure Name DET DIT DET NDIT NSAMP NSAMPPIX Number of columns Number of rows First column of window First row of window Spectro Mode Which Slit Combined offset F T Jitter Box Width arcsec Return to Origin T F Nod Throw arcsec NINT Number of AB or BA cycles Table C 30 SOFI_spec_obs_AutoNodNonDestr Header Keyword DET EXP NAME DET DIT DET NDIT DET NSAMP DET NSAMPIX DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY INS SMODE INS WHICHSLIT SEQ COMBINED OFFSET SEQ JITTER WIDTH SEQ RETURN SEQ NODTHROW SEQ NINT SEQ NABCYCLES Value SOFI NODEFAULT NODEFAULT 4 4 1024 1024 1 1 NODEFAULT NODEFAULT False 1 LSO MAN ESO 40100 0004 Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Spectroscopic Mode Which Slit T guiding ON F guiding OFF Jitter box size Returns the telescope to the original pointing if True Nod throw arcsec Number of exposures in each A or B position Number of AB cycles Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of Samples Sample Number per Reading Number of columns in the window Number of rows in the window First colu
15. Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Instrument Mode INS IMODE NODEFAULT Instrument Mode Combined offset F T SEQ COMBINED OFFSET F T guiding ON F guiding OFF Return to Origin T F SEQ RETURN T Returns the telescope to the original pointing if True RA offsets list arcsec SEQ OFFSETALPHA LIST NODEFAULT list of offsets along RA Dec offsets list arcsec SEQ OFFSETDELTA LIST NODEFAULT list of offsets along Dec Pupil Rotation SEQ ROTPUPIL T T rotate N not rotate Table C 14 SOFIimg_obs_JitterOffset SOFI_img_obs_JitterOffset This observation template Table C 14 is very similar to SOFI_img obs_AutoJitterOffset The only difference is that the user fully specifies the offsets in SEQ OFFSETALPHA LIST and SEQ OFFSETDELTA LIST instead of executing randomly distributed offsets It is assumed that offsets are given so that the telescope alternates between object odd numbered frames and sky The object frames are identified as SCIENCE and the sky frames are identified as OTHER The telescope offsets are executed before the taking the images Guiding is only possible for the object frames if SEQ COMBINED OFFSET is set to True T must be done by the observed during the preparation of the Observing Block and guiding is started before the template starts done by the operator after warning from the observer Compare the following example Table C 15 with the example for SOFIi
16. SEQ OFFSETDELTA LIST giving more freedom and flexibility Complicated patterns can be used pointing the telescope exactly on areas of interest However this comes to a price of more difficult preparation of the observation SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name NGC6118 DIT 6 NDIT 10 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposures 10 Filter wheel 1 Kg Filter wheel 2 open Instrument Mode LARGE_FIELD_IMAGING Combined offset T F F Jitter Box Width arcsec 20 Return to origin T F T Sky Offset Throw arcsec 600 Rotate Pupil T Table C 10 SOFLimg obs_AutoJitterOffset Example The offsets in the list are relative to the previous position they are in RA and DEC and they are in arcsec Note that the telescope offset are executed before the corresponding image is taken In other words if you want the first image to be at the position you have acquired the target the first offset has to be 0 0 If the number of the images is longer than the number of the offsets the software loops back to the beginning of the list and executes again the first offset If the number of the images is shorter than the number of the offsets the software executes only as many offsets as necessary starting from the beginning of the list The RA and DEC lists can have different length but this can be extremely confusing
17. SOFT User s Manual 2 0 Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset F T Jitter Box Width arcsec Return to Origin T F Header Keyword DET EXP NAME DET DIT DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS FILT1 ID INS FILT2 ID INS IMODE SEQ COMBINED OFFSET SEQ JITTER WIDTH SEQ RETURN Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 1 NODEFAULT NODEFAULT NODEFAULT F 40 T Table C 7 SOFI_img_obs_AutoJitter LSO MAN ESO 40100 0004 Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures in the sequence Filter wheel 1 position Filter wheel 2 position Instrument Mode T guiding ON F guiding OFF Jitter box size Returns the telescope to the original pointing if True Parameter signature Value Exposure Name NGC6118 DIT 10 NDIT 6 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposures 6 Filter wheel 1 Kg Filter wheel 2 open Instrument Mode LARGE_FIELD_IMAGING Combined offset T F F Jitter Box Width arcs
18. SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name NGC6118 DIT 6 NDIT 10 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposures 4 NJITT 3 Jitter box width arcsec 20 Filter wheel 1 Ks Filter wheel 2 open Instrument Mode LARGE _FIELD IMAGING Combined offset T F F Return to origin T F T RA offset list arcsec 500500 Dec offset list arcsec 50 50 0 50 Table C 17 SOFIimg_obs_AutoJitterArray and SOFI_img_obs_AutoJitterArray_1 Example defined by the parameter SEQ NEXPO BOB will start again with the first offset It repeats simply the list of offsets you enter until it has the same number of exposures It is possible to randomly jitter around each position of the map within a box whose size is set by SEQ JITTER WIDTH The number of jittered frames at each offset array position is SEQ NJITT and the telescope moves to the next map array position only after they are completed The total number of frames acquired by this template will be the product of the number of array positions by the number of jitters around each one of them i e SEQ NEXPO x SEQ NJITT A modified version of this template is called SOFI_img_obs_AutoJitterArray_l It has identical pa rameter set and it takes the same number of images as the original template but in a different order exposures are taken at the positions of the array as defined
19. a medium resolution grism Of the two low resolution grisms one covers the region from 0 95 to 1 64 microns and the second covers the region from 1 53 to 2 52 microns Both grisms are made of KRS5 refractive index 2 44 and the entrance surface of both is inclined to the optical axis The blaze angle of the grooves is equal to the apex angle of the prism The wavelength ranges the resolving powers and the resolutions of the grisms are given in Table 2 3 The medium resolution grism gives about twice the resolving power of the two low resolution grisms It is used with the H and K filters as order sorting filters in the 3rd and 4th orders to cover respectively the H and K atmospheric transmission windows The wavelength ranges the resolving powers and the resolutions of the grisms are given in Table 2 4 The wavelength ranges are defined by the H and K filters SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 9 T T T T T T T T T T T T T T T 1 06 1 19 Fe II 1 26 E J a 1 F 1 08 1 21 Pf 1 28 J 2 E Y pr a a E E i n L la n E Transmission Hel 2 06 H 2 12 2 19 2 25 CO 2 34 2 09 Bry 2 17 2 28 Transmission Figure 2 3 SOFI filters The solid red lines indicate the currently available broad band J J H and K filters The short dashed magenta lines are the available narrow band filters The long dashed red lines show the soon to be commissioned J filter to
20. and any remaining time to complete the last DIT that was started during the telescope offset recall that the sequencer is not interrupted unless the DIT is changed in case of guiding there is an extra overhead necessary to reacquire the guiding star therefore the intervals between the offsets have to be as long as possible to minimize these losses and this can be achieved with increasing NDIT i e NDITxDIT 2 3 min for imaging in case of excellent sky conditions but not too long as to compromise the sky subtraction e there is a 0 1 second delay between the reset of the array and the first read at every DIT this implies that to minimize the overhead the user should try to use as long DIT as possible but not too long as to saturate the detector or to work in the non linear regime above 10000 ADU e it takes 1 second to read out the array It is difficult to give a precise estimate of the instrumental overhead but here is a common example A one hour exposure made up of 60 one minute unguided exposures with typical for H K or K DIT 10 and NDIT 6 with telescope offsets for jittering in between will take a total of 80 minutes Guided exposures will take 35 to 50 overheads Chapter 3 Observing in the IR 3 1 The IR Sky Observing in the IR is more complex than observing in the optical The difference arises from a higher and more variable background by stronger atmospheric absorption and telluric emission throughout the 1 to 2
21. arcsec The small jitter is acceptable because during every cycle we take four images at different array location reducing the effect from the bad pixels the major reason for jittering in this case Note that this is usually not the case SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name SOFT 4 point DIT 6 NDIT 10 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 NJITT 12 NEXPO 4 Jitter Box Width arcsec 20 Filter wheel 1 K Filter wheel 2 open Instrument Mode LARGEFIELDIMAGING Combined Offset T F F Return to Origin T F T RA Offset List arcsec 148 0 148 0 DEC Offset List arcsec 0 148 1 148 Table 3 4 Parameters of the SOFIimg obs AutoJitterArray 1 template with commonly used values for 4 point observation of a semi extended object with SOFI_img_obs_AutoJitter and template observations of uncrowded targets where the jitter is needed to produce a sky image for the sky subtraction There is one more albeit simple complication in this observing strategy the offsets are executed before the images are taken so the acquisition must place the target at the location of the fourth image in the cycle This is the position shown in the figure with a dashed circle and marked with zero Finally the thick arrows show the apparent movement of the target on the RTD from one position to another North and East are also marked This
22. at different positions on the array Note that Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence List of DITs SEQ DIT LIST NODEFAULT Detector Integration Time individual exposure sec List if NDIT SEQ NDIT LIST NODEFAULT Number of DITs averaged into an individual image Table C 20 SOFIimg_cal_Darks 82 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence List of DITs SEQ DIT LIST NODEFAULT Detector Integration Time individual exposure sec List if NDIT SEQ NDIT LIST NODEFAULT Number of DITs averaged into an individual image Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Inst
23. certain minimum specified withing the template This is intentionally done to ensure that the 5 frames before and after any frame are not too close spatially and can be safely used for creating a sky image without large residuals for the sky subtraction By default there is no telescope offset before the first exposure Tf SEQ RETURN is set to True T the telescope slews back to its original position at the end of the template if not the telescope is not moved This feature is useful if more exposure of the same field are taken i e with different filters without running the corresponding acquisition template In this case the parameters should be set to True If SEQ COMBINED OFSET is set to T and if guiding was started before the start of the template guiding is Star To Box Considering that in imaging mode the time between the offsets is limited typically to 1 3 min by the sky background variation the guiding is rarely used in imaging mode The value of SEQ JITTER WIDTH corresponds to the full width of the box in which the offsets are generated The choice of box size is a compromise between to constraints 1 Too wide a box may lead to insufficient image overlap The signal to noise ratio will degrade toward the edged of the final combined image 2 Too small a value may lead to poor sky subtraction near extended objects including bright stars with noticeable wings Setting this parameter to zero is equivalent to staring the obse
24. column of window First row of window Spectro Mode Which slit Combined offset F T Jitter Box Width arcsec Return to origin T F Nod Throw arcsec NINT Number of AB or BA cycles LSO MAN ESO 40100 0004 Value NGC6118 40 2 1024 1024 1 1 LONG_SLIT_BLUE long slit_1 T 20 T 100 3 3 Table C 31 SOFI_spec_obs_AutoNodOnSlit Example Table C 32 SOFI_spec_obs_AutoNodNonDestr Example Parameter signature Exposure Name DIT NDIT NSAMP NSAMPPIX Number of columns Number of rows First column of window First row of window Spectro Mode Which slit Combined offset F T Jitter Box Width arcsec Return to origin T F Nod Throw arcsec NINT Number of AB or BA cycles Value NGC6118 50 2 30 4 1024 1024 1 1 LONG_SLIT_BLUE long slit_1 T 20 T 100 3 3 91 92 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures Spectro Mode Which Slit Return to Origin T F X offset list arcsec Y offset list arcsec Observation Type S or O Guiding N B S Parameter signature Exposure Name DET DIT DET NDIT NSAMP NSAMPPIX Number of columns Number of rows First column of window First row of window Number of exposures Spectro Mode Which Slit Return to Origin T F X offset
25. contains both Xenon and Neon lamps The Xenon lamp produces an even spread of lines for both the red and blue grisms It is well suited for wavelength calibration Figure A 1 show the main Xenon lines for the blue grism There are two electronic ghosts caused by the very bright lines near one micron between 1 35 and 1 4 microns Figure A 2 shows the main Xenon lines for the red grism The continuum in the red is thermal emission from the lamp Both the Neon and Xenon lamps should be used to calibrate the medium resolution grism Figures A 3 A 4 A 5 and A 6 show the main Xenon and Neon lines 58 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 oo a MANO DO co co oD po i o 0 TINO 0 co mo co co QQ T Sa a Kej lt 00 H O oO mN M st 0 oD QQ LO a OOO co LO O E st o o os NN WY oD oD te 4000 F 2000 Relative Intensity LA E 9802 4 9925 9 Ghost L Ghost L 1 L 1 L L 1 L 1 2x10 1 4x10 1 6x10 Wavelength A BR or A Figure A 1 A Xenon arc spectrum taken with the blue grism The main lines are marked Relative Intensity SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 4000 T T T sO E E 10 Sn SS R bad a 4 x S se x Ye a ow Lo E oD o D El F E Y lt e E ve co o oe ce W a Y a oO a 3000 H A E 2000 i E 10
26. example can easily be generalized one can observe a slightly small target at five positions including one at the center of the array In this case NEXPO 5 On the contrary an edge on galaxy with a major axis of 3 4 arcmin a minor axis of 1 2 arcmin must be observed at only two array positions so NEXPO 2 In this case the user should define a suitable rotator offset to align the major axis with the array edge ii Observations of objects that need 2 3 fields to cover the entire target with additional images to sample the sky In principle the template SOFI_img_obs_AutoJitterOffset can accommodate such observations if they are carried in a sequence of one field at a time However this means 100 overhead for the sky sampling Instead the user can reduce the overhead by alternating between one sky position and two three target positions A typical case is observing a large 6 10 arcmin galaxy in a sequence sky target field 1 target field 2 sky target field 1 target field 2 sky This is a 3 point pattern One can even alternate between two or more differnet sky positions to improve the sky subctraction sky1 target field 1 target field 2 sky2 target field 2 target field 1 sky1 The last pattern is in effect a 6 point sequence An example for this observation scheme is given in Figure 3 2 and the offsets are listed in Table 3 5 Note that the rotation offset has to be defined in the acquisition template Also the
27. imaged on the array produces a ghost that affects all the lines where the source is and all the corresponding lines in the other half of the detector For instance if the bright source is on row 300 the cross talk affect the row 300 512 812 and vice versa if the source is on row 750 the other affected row will be 750 512 238 The cross talk is seen either on high S N data or if the bright source reaches near saturation level 48 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Though the effect is not completely understood it is well described and can be easily corrected The intensity of the ghost is in fact 1 4 x 107 times the integrated flux of the line An IRAF script for removing the cross talk is available from the SOFI web page http www 1s eso org lasilla sciops ntt sofi reduction sofi scripts The script has to be given input and output lists of images The script ca remove the cross talk only if the source that causes it is not saturated In case of saturation the counts of the source will be decreased by the non linearity and the subtraction of the ghost will be incomplete However even in those cases the users can improve the data The cross talk manifest itself on the final reduced image as i residual of the ghost and ii residual of the line leaks i e horizontal lines with increased counts While there is nothing one can do for the formen effect short of masking the latter one can be removed This can b
28. instrument web pages or the instrument scientists for the latest status of the instrument There is a focal elongator in the grism wheel which can be used in combination with the large field objective to image with a pixel scale that is identical to the pixel scale of the small field objective The image quality is usually superior than that of the small field objective but this mode is 10 to 15 less efficient because of the additional optical elements Imaging can be done through the standard IR broadband filters J Js H Ks a Z filter which peaks at 0 9 microns and many narrow band filters The filters together with their central wavelengths and widths at half maximum are listed in Table 2 2 Listed also are the cut on wavelengths for the two order sorting filters These filters are used with the two low resolution grisms Transmission curves of imaging filters together with the atmospheric transmission are shown in Figure 2 3 see also Appendix B The K short or K filter is different from both the standard K filter and the K filter defined by Wainscoat and Cowie 1992 AJ 103 332 The long wavelength edge of the K filter is similar to that of the K filter but the short wavelength edge is similar to that of the K filter Thus the K filter avoids both the atmospheric absorption feature at 1 9 wm and radiation from the thermal background beyond 2 3 um The difference between K and K is given by K K 0 005 J K S
29. keeping NSAMP small the analog signal can be sampled several times for each reading this is done with the parameter NSAMPIX The read out noise is reduced approximately by the square root of the number of samples NSAMP This method is also less susceptible to 50 Hz pickup noise The drawback of the NDR is that it takes slightly longer to process the data although the disadvantage becomes smaller as the exposure time increases The shortest integration time in this mode is given by 1 64 x NSAMP NDR is only available for the spectroscopic modes For most applications we recommend NSAMP 30 and NSAMPIX 4 There is a noiseless component of the signal introduced by the reset which is somewhat unstable a short time after the reset Thus there is a small delay of 100 ms between the reset and the first read This component decays with time for this reason dark frames taken with long DIT have negative counts The Hawaii array like the NICMOS III array is read out simultaneously in four quadrants This leads to a characteristic jump in the count level at rows 1 and 513 This jump depends on the DIT and on the incident flux on the array The jump is stable with time and it disappears after the sky subtraction 2 5 2 Features on the Detector There is a number of features of the detector The most prominent ones are a few circular depressions in the quantum efficiency with radius if 20 40 pixels Their presence is not a source of concern because they
30. length At the end of the template the telescope is returned to the original position if SEQ RETURN is set to True T If not the telescope is not moved at the end of the template Consider the following example Table C 19 It contains 7 offsets with the option to guide Star To Boz the only other option is No Guiding Box To Star has been dsiabled The object is always observed with a NDIT equal to 3 whereas the sky is always observed with NDIT equal to 2 At the end there are 4 images for the object each of them is the average of 3 NDIT expositions of 20 sec DIT And 3 images of the sky each of them is the average of 2 NDIT expositions of 20 sec DIT SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 81 C 2 3 SOFI Imaging Calibration Templates SOFI_img_cal_Darks This calibration template Table C 20 produces dark frames both for imaging and spectroscopy The instrument is set to the DARK mode where both filter wheels and the grism wheel are in the closed position The number of frames is defined in SEQ NEXPO Dark frames can be taken with different DIT and NDIT values defined as lists in SEQ DIT LIST and SEQ NDIT LIST Note that SEQ DIT LIST and SEQ NDIT LIST do not need to have the same length If the number of exposures is larger than the number of elements in a list then the list is restarted from the beginning as many times as needed until the correct number of frames have been acquired This template should be
31. list arcsec Y offset list arcsec Observation Type S or O Guiding N B S Header Keyword DET EXP NAME DET DIT DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS SMODE INS WHICHSLIT SEQ RETURN SEQ OFFSETX LIST SEQ OFFSETY LIST SEQ OBSTYPE LIST SEQ GUIDING LIST Header Keyword DET EXP NAME DET DIT DET NDIT DET NSAMP DET NSAMPIX DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS SMODE INS WHICHSLIT SEQ RETURN SEQ OFFSETX LIST SEQ OFFSETY LIST SEQ OBSTYPE LIST SEQ GUIDING LIST Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 1 NODEFAULT NODEFAULT T NODEFAULT NODEFAULT NODEFAULT NODEFAULT Value SOFI NODEFAULT NODEFAULT 4 4 1024 1024 1 1 1 NODEFAULT NODEFAULT T NODEFAULT NODEFAULT NODEFAULT NODEFAULT Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures Spectroscopic Mode Which Slit Returns the telescope to the original pointing if True List of offsets in X arcsec List of offsets in Y arcsec Observation type O object S sky Guiding B Box To Star S Star To Box N none Table C 33 SOFI spec obs GenericSpectro Description File name prefix Detector Integ
32. of an OB is not constrained The example corresponds to the maximum duration acceptable Parameter signature Value Exposure Name NGC6118 DIT 10 NDIT 6 Number of exposures 36 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Filter wheel 1 K Filter wheel 2 open Instrument Mode LARGEFIELDIMAGING Combined Offset T F F Jitter Box Width arcsec 20 Sky Offset Throw arcsec 600 Rotate Pupil T F T Table 3 2 The parameters of the template SOFI_img_obs_AutoJitterOffset with typical values In this template one observes alternatively the object and sky There are 36 exposures in total 18 20 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 on the object and 18 on the sky The sky positions are randomly chosen to lie on a circle that is centered on the object The diameter of this circle is set by the parameter Sky Offset Throw Each of the six sky positions is different Each of the six object positions are also different they are randomly distributed within a square box that is centered on the object The box size is set by the parameter Jitter Box Width Auto guiding is set by the parameter Combined offset If selected guiding is done when the telescope is pointing to the object field As the NTT tracks very well and as one usually spends no more than a few minutes on a single position is not recommended At the end of the template the telescope returns to the orig
33. of exposures Detector Integration Time individual exposure sec Number of DITs averaged into an individual image List of Spectroscopic Modes B R Z J NB 1 061 H K Slit list 0 6 1 2 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Header Keyword Value Parameter signature Exposure Name Number of columns Number of rows First column of window First row of window NSAMP NSAMPPIX Number of exposures List of DITs List of NDIT Spectral Mode List Slit List DET EXP NAME DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY DET NSAMP DET NSAMPIX SEQ NEXPO SEQ DIT LIST SEQ NDIT LIST SEQ SPECTROMODELIST SOFI 1024 1024 1 1 4 4 1 NODEFAU NODEFAU NODEFAU NODEFAU Description File name prefix Number of columns in the window Number of rows in the window First column of window First row of window Number of Samples Sample Number per Reading Number of exposures Detector Integration Time individual exposure sec Number of DITs averaged into an individual image List of Spectroscopic Modes BRZJNB_1 061 H K Slit list 0 6 1 2 SEQ SLIT LIST Table C 39 SOFI_spec_cal_NonDestrDomeF ats Appendix D Frame Types There are several basis frame types which are identified by three keywords in the FITS header The full list is given in Table D 1 Keyword Value Type DPR CATG SCIENCE Exposure on target object DPR CATG OTHER Exposure off target sky DPR
34. offset facility of this task The final image will have the largest possible dimensions and the signal to noise ration will most likely degrade toward the edges because of the smaller and smaller number of images that cover the outer regions Remember to set the lower threshold of imcombine appropriately to exclude both the marked bad pixels and the pixels that were given bogus values at the edges of the shifted images 54 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 5 3 Long Slit Spectroscopy 5 3 1 Inter quadrant Row Cross Talk This step is a must for reducing spectroscopic observations because unlike the imaging the spectra are always aligned along the array columns so a ghost of the upper half of the spectrum always appears in the lower one and vice versa unlike the imaging when the observers may be fortunate not to have bright star ghosts on top of their targets or they can even rotate the instrument to avoid it The effect is particularly dangerous when one looks for emission lines because their ghosts are emission line like and can be confusing More details on the cross talk can be found in Section 5 3 1 5 3 2 Sky Subtraction In most cases the user simply has to subtract one image from another This should remove most of the night sky emission The subtraction of the average of every preceding and succeeding image works particularly well The residuals can be removed during the extraction of 1 dimensional spectrum The t
35. one corresponding raw in the upper lower half of the array 5 2 2 Masking the Bad Pixels The SOFI detector array suffers only minor cosmetic defects it has 0 1 or 1000 bad pixels A mask of the bad pixels is available in the SOFI web page http www 1s eso org lasilla sciops ntt sofi reduction bad pix html We consider the following types of bad pixels e Dead pixels they are computed from the histograms of flats taken during SOFI Calibration Plan the distribution of the pixels values is fitted with a Gaussian mean sigma and we select the dead pixels as the ones whose value lays 40 bellow the mean value e Hot pixels they are computed from the histograms of darks taken during SOFI Calibration Plan the distribution of the pixels values is fitted with a Gaussian mean sigma and we select the hot pixels as the ones whose values lays 4g above the mean value e Noisy Pixels they are computed from the histograms of the sigma image obtained when averaging the flats and the darks of the previous 2 steps e Frame Pixels the mask also marks as bad those pixels that are masked by the edges of the focal plane field mask 50 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 NOTA BENE The masking must occur before the images are shifted and aligned It is up to the user how to exclude the masked pixels Perhaps the simplest way is to use two masks The first one will have zero values at the bad pixels and one at good ones Eve
36. photometric standards of Persson et al 1998 TL 103 Finding charts for the photometric standards of Persson et al Finding charts for the photometric standards of Persson et al SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Chapter 1 Introduction SOFT or Son OF ISAAC is the infrared spectrograph and imaging camera on the NTT In many ways it resembles its parent ISAAC and the EFOSCII on the 3 6m Both are focal reducing instruments capable of imaging spectroscopy and polarimetry SOFT offers the following observing modes e imaging with plate scales of 0 144 0 144 0 273 and 0 288 arc second per pixel in the following modes Small Field Large Field Focal Elongator Spectroscopic Field and Large Field respectively broad and narrow band filters in the wavelength range from 0 9 to 2 5 microns are available low resolution R 600 varies across the wavelength range 0 95 2 52 micron spectroscopy with fixed width slits of 0 6 1 and 2 arc seconds and slit length of 4 92 arcmin medium resolution R 1500 varies across the wavelength range H and K band spectroscopy with fixed width slits of 0 6 1 and 2 arc seconds and slit length of 4 92 arcmin e 0 9 2 5 micron imaging polarimetry with the large field objective 0 288 arcsec per pixel and the set of filters available in imaging mode This manual is divided into several chapters For proposal writers it is sufficient to read chapter 2 giving a gen
37. section is cited verbatim as Appendix A in A User s Guide to Reducing Slit Spectra with IRAF by Phil Massey Frank Valdes and Jeannette Barnes These documents are available on the WWW but for user s convenience we have placed copies of them at http www 1s eso org lasilla sciops ntt sofi reduction other manuals For the medium resolution grism the slit curvature is larger As yet we have not tried to do a 2 dimensional wavelength solution to arc spectra taken with this grism SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 55 5 3 5 Arcs For the red and blue low resolution grisms it is sufficient to use the Xenon lamp for wavelength calibration For the medium resolution grism both the Xenon and Neon lamps should be used The main Xenon and Neon lines are identified in Appendix A For the medium resolution and red grisms a cubic fit to calibrate the dispersion is adequate For the blue grism a quartic fit is better 5 3 6 Removing of the Atmospheric Absorption Features and Flux Calibration The IR spectra are dominated by atmospheric absorption features To remove these features it is customary to observe a star with a featureless spectrum at a similar airmass to the that of the target This star is called telluric standard The reduction involves the following steps e the spectrum of the target has to be divided by the spectrum of the telluric standard This will remove the atmospheric absorption because presumably it
38. seen in Figure 3 4 Therefore it s not necessary to take flats every day during especially if high precision photometry is not required to achieve the scientific goals of your program However it is advisable to take at least two sets of flats to check their consistency 3 5 3 Illumination Corrections It is often the case that the flat field generated from the dome the twilight sky or the night sky itself does not represent completely the low frequency sensitivity variations of the array In other words the dome flat field contains a residual large scale variation that is usually due to the fact that the illumination of the screen can never represent accurately the illumination of the sky In addition the illumination of the screen can change with time due to the variations in the lamps used to illuminate it and due to variation of the screen and dome temperature for gt 2 3 micron This problem can be solved by observing a bright star preferably a standard in a rectangular grid pattern of 9 16 positions across the array After sky subtraction and flat fielding with the uncorrected flat the intensities not the magnitudes are fitted with a low order polynomial Next the surface 34 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 3 5 T T T T T T T T T T T T T T T T 3h gt i F e ES Y x ee e 7 S254 7 o L 4 e f i o L 3 5f 2 2 u G P al a J 1 5 H 4 1 L 1 1 1 L 1 L L L 1 1 L
39. takes polarimetric dome flats for a list of rotation angles SOFT User s Manual 2 0 Parameter signature DET DIT DET NDIT Filter wheel 1 Filter wheel 2 Alpha Offset arcsec Delta Offset arcsec Add Velocity Alpha Add Delta Velocity Rotation Offset on Sky Combined offset F T Preset Telescope F T Save Image F T Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures Filter wheel 1 Filter wheel 2 Combined offset F T Return to Origin T F X offsets list arcsec Y offsets list arcsec Rotator Offset Header Keyword DET DIT DET NDIT INS FILT1 ID INS FILT2 ID TEL TARG OFFSETALPHA TEL TARG OFFSETDELTA TEL TARG ADDVELALPHA TEL TARG ADDVELDELTA TEL ROT OFFANGLE SEQ COMBINED OFFSET SEQ PRESET SEQ SAVE LSO MAN ESO 40100 0004 Value NODEFAULT NODEFAULT NODEFAULT NODEFAULT 0 0 0 0 0 0 0 0 0 0 False True False Table C 24 SOFIimg_acq_Polarimetry Header Keyword DET EXP NAME DET DIT DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS FILT1 ID INS FILT2 ID SEQ COMBINED OFFSET SEQ RETURN SEQ OFFSETX LIST SEQ OFFSETY LIST SEQ ROT OFFANGLE Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 1 NODEFAULT NODEFAULT F T NODEFAULT NODEFAULT 45 Table C 25 SOFI_img_obs_Polarimetry Description Detector Integrat
40. the non linearity limit the user may consider defocusing the telescope for standard star observations or switching to one of the imaging modes with finer pixel scale for observations that can not The DCS has to cope with backgrounds that range from a fraction of a ADU sec pix as seen in the spectroscopic modes to six hundred ADU sec pix as seen during summer with the K filter and the large field objective The high count rate from the background particularly at Ks limits a single integration to about 10 seconds and it can drop down even to 6 seconds depending on the humidity clouds cover etc Thus in order to accumulate sufficient photons without saturating the detector the observer should acquires many individual integrations and then averages them on the fly into a final frame The control system allows to do exactly that In this context we define DIT the Detector Integration Time as the amount of time during which the signal is integrated onto the detector diodes and NDIT as the number of detector integrations that are obtained and averaged together These averaged frames make up the raw data and in normal co add mode they are the smallest block of data presented to the user So that the total exposure time of a single image single raw data file available to the user is NDITx DIT Note that the RTD Real Time Display can show either a single DIT or the averaged NDITxDIT Additionally some of the science templates allow the u
41. to make sure that at least 12 15 images at different locations are available In general the more frames you have the better the results but up to a point because of such effects as pupil rotation and increasing overheads There are two basic strategies for sky subtraction i creating a single sky image and then subtracting it from each individual frame Here is a step by step description of the procedure to remove the sky from a stack of n jittered frames where n is SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 51 large a For each image take the 10 images that were taken closest in time These images will be used to estimate the sky In case of jittered observations when the target field is on all images these 10 images will indeed include the target In case of observations alternating between the object and the sky the list must include only the 10 closest sky images b Scale these 10 images to a common median To first approximation this is the sky image However it is possible to improve it One possibility is simply to reject the most deviating values i e rejection of the highest and the lowest 1 or 2 values 30 rejection etc We strongly encourage the user to experiment here Another option is a more sophisticated 2 iteration rejection After making a zero order sky as described above return to the list of the images you used to create this sky From each of them subtract the sky The result will be images with ne
42. tool can be obtained from Maiolino Rieke amp Rieke 1996 AJ 111 537 web sites http www arcetri astro it maiolino solar solar html and http nicmos2 as arizona edu marcia solar For an example how to treat early spectral type telluric standards how to stellar models and for an empirical library of spectra of early type stars refer to Hanson et al 1996 ApJS 107 281 Good empirical IR spectrophotometric standards are not available The only remaining way to do flux calibration is to observe the science target in one or more broad band filters and to scale the spectrum so that it agrees with the broadband fluxes This method can achieve an absolute calibration of 5 10 For example of IR flux calibration in the more general case of extended objects see Ivanov et al 2000 ApJ 545 190 See also the discussion on the flux calibration in Section 3 4 3 5 3 7 Alignment and Combination There are two general approaches to combining spectra i to combine 2 dimensional images after an appropriate geometric correction that will align the wavelength axis and straighten the spectra 56 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 ii to combine 1 dimensional spectra usually after wavelength calibration that were extracted from the 2 dimensional images Every one of these methods has certain advantages and disadvantages and the user will have to decide what is the best way to reduce the data Obviously the first method is e
43. transformation of the image in spatial direction However these are complicated and time consuming steps that involve heavy modification of the data prior to any scientific measurements and it is preferable to avoid them if possible 30 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 The division by the telluric standard introduces into the target spectrum a number artificial emis sion features because of the intrinsic absorption features of the telluric standard itself By choosing carefully the spectral type of the telluric standards one can minimize the number of such features but there are no stars with completely featureless IR spectra and therefore some such emissions will always remain To remove them after the division by the telluric standard one should multiply by the absolute spectral energy distribution of the telluric standard However this information is usually not available so one has to model the spectral energy distribution of the standard For hot stars a blackbody fit to the star may be appropriate but for stars of spectral type later than B a more accurate description of the energy distribution may be required Another possibility is to use a solar analog for telluric standard taking advantage of the fact that the true intrinsic IR solar spectrum of the Sun is available It was corrected by observing the Sun at differ ent zenith angles and extrapolating to secz 0 extremely time consuming technique tha
44. used to take dark frames at the end of the night for all the detector integration times that have been used during the night though it is not strictly necessary since the dark signal will be removed along with the subtraction of the sky background SOFI_img_cal_DomeFlats and SOFI_img_cal_SpecialDomeF lats Imaging dome flats are taken with these calibration template Table C 21 For each element in SEQ DIT LIST and SEQ NDIT LIST four images are taken one with the dome lamp off two with the dome lamp on and a fourth with the dome lamp off The intensity of the dome lamp is controlled manually SOFLimg cal_SpecialDomeFlats takes an additional set of images with the aperture wheel partially masking the array These images are used to estimate the bias pattern The reasons for using this template rather than the first are explained in the SOFI Users manual The intensity of the dome lamp is controlled semi automatically the template maintains a database and once it is populated for a given configuration the lamp intensities are taken from there The parameter sets for both templates are identical SOFI_img_cal_StandardSatr This calibration template Table C 22 is intended for standard star observations in one filter If a standard has to be observed in a few different filters this template has to be called as many times as necessary with the appropriate filter defined in each template call The telescope can be offset to position the object
45. vapor varies so will the amount of absorption The edges of the atmospheric windows are highly variable which is important for the stability of the photometry in J and K filters These difficulties have led to the development of specific observing techniques for the IR These techniques are encapsulated in the templates that are used to control SOFI and the telescope In this section we link common observational scenarios with specific templates and we give some concrete examples In later sections we discuss some finer points The templates are described in detail in The SOFI TSF Parameters Reference Guide 3 2 Imaging It is not unusual for the objects of interest to be hundreds or even thousands of times fainter than the sky Under these conditions it has become standard practice to observe the source together with the inevitable sky and subtract from it an estimate of the sky Since the sky emission is generally 16 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 17 variable the only way to obtain good sky cancellation is to do this frequently The frequency depends on the wavelength of observation and respectively on the nature of the sky background emission and on meteorological conditions Ideally one would like to estimate the sky more quickly than the time scale of the sky variations While this could be done quickly with the traditional single and especially double channel photometers the overhead in observing with array detecto
46. 0 006 11 041 0 006 10 981 0 015 10 982 0 008 9140 262 E 09 45 42 8 45 49 40 11 409 0 011 11 085 0 008 11 022 0 012 9141 S708 D 09 48 56 4 10 30 32 11 081 0 008 10 775 0 008 10 715 0 035 10 718 0 010 9143 P550 C 10 33 51 8 04 49 05 12 344 0 007 12 121 0 005 12 067 0 006 12 081 0 005 9144 S264 D 10 47 24 1 44 34 05 11 642 0 009 11 335 0 008 11 263 0 018 11 280 0 010 9146 217 D 12 01 45 2 50 03 10 12 323 0 007 11 002 0 005 10 931 0 003 10 936 0 004 9147 S064 F 12 03 30 2 69 04 56 12 111 0 007 11 803 0 007 11 722 0 013 11 724 0 007 9150 S791 C 13 17 29 6 05 32 37 11 661 0 008 11 310 0 007 11 250 0 014 11 267 0 008 9153 P499 E 14 07 33 9 12 23 51 11 947 0 008 11 605 0 008 11 560 0 013 11 540 0 008 9154 S008 D 14 23 45 5 84 09 58 11 232 0 007 10 990 0 007 10 904 0 009 10 915 0 008 9155 S867 V 14 40 58 0 00 27 47 12 045 0 008 11 701 0 005 11 622 0 005 11 633 0 005 9157 S273 E 14 56 51 9 44 49 14 11 341 0 007 10 924 0 005 10 851 0 004 10 849 0 004 9160 S870 T 15 39 03 5 00 14 54 10 914 0 008 10 701 0 008 10 649 0 010 10 659 0 009 9164 P565 C 16 26 42 7 05 52 20 12 180 0 007 11 895 0 006 11 842 0 007 11 844 0 006 9170 S875 C 17 27 22 2 00 19 25 11 132 0 005 10 835 0 005 10 739 0 006 10 744 0 005 9172 S279 F 17 48 22 6 45 25 45 12 477 0 009 12 118 0 006 12 026 0 006 12 031 0 006 9173 S024 D 18 18 46 2 80 06 58 11 039 0 007 10 778 0 007 10 693 0 009 10 711 0 008 9175 S071 D 18 28 08 9 69 26 03 12 252 0 006 11 91
47. 00 rr Jos e Oo Lad wi PA 1 1 1 1 1 1 1 1 1 1 pe 1 4x10 1 6x10 1 8x10 oe LOH 2 2x10 2 4x10 2 6x10 Wavelength A Figure A 2 A Xenon arc spectrum taken with the red grism The main lines are marked SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 2x10 I T T T T F T T T I T T T T uN F E ES POE Gn SOR Co co N en oR XN d st te roo Fr OD ef 0 lt LO LO lt c ITV DD BD Oo a Q LO c F O o oor Q co ron e 7 cel o gt O ao OO ce o cel co co co O O EJE E Ss 22 2 2 T o D o o o o o oo Do v El E ach P lt a e lt a gt lt P lt P lt 1 5x10 LE og moi q j at F 4 un zi 5 i 1 ia o o o g 1 2 y i yoke Ue ae 10 E 5 4 e e i E ta gs f i L 4 pey 3 V Az 4 5x104 OF 1 8000 8500 9000 9500 Wavelength A Figure A 3 A Xenon and Neon arc spectrum taken with the medium resolution grism at the Z atmospheric window The main lines are marked SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 T T T as Ss 4 a 8x10 5 B co or m1 A o 2 n a q o fej o o q own Ww EDO os z7 oO o Q co oD x D co Q2 ogee a mM ka 9 co o co ou oD O p oO E e QQ Q Y SN AN SN mn 2 x N 3 jaa M a Lo LO LO LO LO co LO Lo o OC in wo oO oO co co oO RS
48. 00 Rotator offset 0 Table C 26 SOFIimg_obs_Polarimetry Example Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix DIT individual exposure DET DIT NODEFAULT Detector Integration Time individual exposure sec NDIT number of DIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence List of Rotator angles SEQ ROT OFFANGLE NODEFAULT Rotator offset list degrees Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Return to Origin T F SEQ RETURN T Returns the telescope to the original pointing if True Table C 27 SOFIimg_cal_PolarimDomeFlats SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 C 3 4 SOFI Spectroscopic Templates C 3 5 SOFT Spectroscopic Acquisition Templates SOFI_img_acq_MoveToSlit This acquisition template Table C 28 is very similar to the SOFT img acq_MoveToPixel template The selected slit is drawn on the real time display RTD superimposed on the image of the field In most cases operators will use the option to move the selected object to the cente
49. 20 20 020000002 a C 3 1 SOFI Polarimetric Acquisition Template oa aa a C 3 2 SOFI Polarimetric Science Template o o e C 3 3 SOFI Polarimetric Calibration Template 2 C 3 4 SOFI Spectroscopic Templates 2 o e ee ee C 3 5 SOFI Spectroscopic Acquisition Templates 2 4 Al 48 48 48 48 49 50 50 52 53 54 54 54 54 54 55 55 55 56 58 65 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 C 3 6 SOFI Spectroscopic Science Templates C 3 7 SOFI Spectroscopic Calibration Templates D Frame Types E Photometric Standards vil 88 95 98 99 List of Figures 2 1 2 2 2 3 2 4 3 1 3 2 3 3 3 4 4 1 4 2 A l A 2 A3 AA A5 Optical layout of SOF aasa ra sk eA a a a aa ee Orientation of SOFI for rotator angle Odeg In imaging the North is to the left and the East to the bottom of the image The slit for the spectroscopy is oriented North South Nota Bene To align the North South axis of the field of view or the slit in case of spectroscopy long a certain Position Angle one has to apply in the acquisition template a rotaton angle equal to this Position Angle 2 SOFT filters The solid red lines indicate the currently available broad band J Js H and K filters The short dashed magenta lines are the available narrow band filters The long dashed red lines show the soon to be commission
50. 24 1 Ks open LARGE_FIELD_IMAGING 10 2 5 5 5 5 10 2 5 5 Hoos Table C 15 SOFIimg_obs_JitterOffset Example Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures NJITT Jitter Box Width arcsec Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset F T Return to Origin T F RA offsets list arcsec Dec offsets list arcsec Header Keyword Value Description DET EXP NAME SOFI File name prefix DET DIT NODEFAULT Detector Integration Time individual exposure sec DET NDIT NODEFAULT Number of DITs averaged into an individual image DET WIN NX 1024 Number of columns in the window DET WIN NY 1024 Number of rows in the window DET WIN STARTX 1 First column of window DET WIN STARTY 1 First row of window SEQ NEXPO 1 Number of exposures in the sequence SEQ NJITT 1 Number of Jittered exposures around each Array position SEQ JITTER WIDTH 40 Jitter box size INS FILT1 ID NODEFAULT Filter wheel 1 position INS FILT2 ID NODEFAULT Filter wheel 2 position INS IMODE NODEFAULT Instrument Mode SEQ COMBINED OFFSET F T guiding ON F guiding OFF SEQ RETURN T Returns the telescope to the original pointing if True SEQ OFFSETALPHA LIST NODEFAULT list of offsets along RA SEQ OFFSETDELTA LIST NODEFAULT list of offsets along Dec Table C 16 SOFIimg_obs_AutoJitterArray and SOFIimg_obs_AutoJitterArray 1
51. 5 micron wavelength region Short ward of 2 3 microns the background in dominated by non thermal emission principally by aurora OH and Oz emission lines The vibrationally excited OH lines are highly variable on a time scale of a few minutes Pronounced diurnal variations also occur The lines are strongest just after sunset and weakest a few hours after midnight A complete description and atlas of the sky emission lines can be found in the paper Rousselot et al 2000 A amp A 354 1134 Long ward of 2 3 microns the background is dominated by thermal emission from both the telescope and the sky and is principally a function of the temperature The background in K can vary by a factor of two between the winter and summer months but is more stable than the J or H band background on minute long time scale It also depends on the cleanliness of the primary mirror Imaging in broadband K and the wide field objective can result in backgrounds of 600 700 ADU sec depending strongly on the temperature and humidity The IR window between 1 and 2 5 microns contains many absorption features that are primarily due to water vapor and carbon dioxide in the atmosphere These features are time varying and they depend non linearly with airmass The atmosphere between the J and H bands and between the H and K bands is almost completely opaque The atmospheric transmittance between 1 and 2 5 microns as seen by SOFI is plotted in Appendix B As the amount of water
52. 545 190 The choice of which standard to use depends on which part of the spectrum is of interest All stars have absorption lines so the idea is to choose a star which does not have strong features near the wavelength of interest Hot stars provide relatively featureless spectra however they have strong hydrogen absorption lines so they should not be used as flux calibrators if the region around the hydrogen lines is of interest Later type stars such as G stars have weaker hydrogen lines but are contaminated by multiple weak absorption lines These stars have the additional advantage of being very numerous Stars of later type should not be used as these stars contain numerous weak lines throughout their spectra However such stars have very weak hydrogen absorption and may be usefully employed to determine the strength of hydrogen absorption in the hot stars In general IR standards are significantly brighter than optical standards nevertheless stars should be fainter than seventh magnitude An moderately high resolution IR spectral library for stars withe well known parameters including spectral types is available from Ivanov et al 2004 ApJS 151 387 A list of O F and G type stars selected from the HIPPARCOS catalog with magnitudes appropri ate for SOFI spectroscopy is available at the telescope and on the SOFI web page together with web based tool for automated search for stars with given spectral type and brightness near the
53. 6 0 007 11 834 0 011 11 839 0 007 9178 S808 C 19 01 55 4 04 29 12 10 966 0 007 10 658 0 008 10 566 0 014 10 575 0 008 9181 234 E 20 31 20 4 49 38 58 12 464 0 011 12 127 0 008 12 095 0 007 12 070 0 007 9182 S813 D 20 41 05 1 05 03 43 11 479 0 005 11 142 0 005 11 082 0 010 11 085 0 005 9183 P576 F 20 52 47 3 06 40 05 12 247 0 004 11 940 0 004 11 873 0 007 11 880 0 005 9185 S889 E 22 02 05 7 01 06 02 12 021 0 005 11 662 0 004 11 586 0 012 11 585 0 005 9186 S893 D 23 18 10 0 00 32 56 11 403 0 009 11 120 0 006 11 045 0 006 11 055 0 006 9187 S677 D 23 23 34 4 15 21 07 11 857 0 003 11 596 0 003 11 538 0 009 11 542 0 003 Table E 1 NICMOS photometric standard stars Persson et al 1998 A J 116 2475 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 101 Figure E 1 Finding charts for the photometric standards of Persson et al 1998 I 102 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Figure E 2 Finding charts for the photometric standards of Persson et al 1998 IL SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 103 Figure E 3 Finding charts for the photometric standards of Persson et al 1998 III 104 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Figure E 4 Finding charts for the photometric standards of Persson et al 1998 IV SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 CskD 8 SAMY cskD 16 18 CSKD 20 21 CSKD 34 371 j 3 hile o a at A ssl i 5 a 4 gt Y
54. AN ESO 40100 0004 Grism Order Sorting Wavelength Resolving Dispersion Number Filter Range microns Power A pixel Blue GBF 0 95 1 64 930 6 96 Red GRF 1 53 2 52 980 10 22 Table 2 3 The wavelength range and the resolution of the low resolution grisms The resolution is that measured for the 0 6 arc second slit The resolution scales inversely with the slit width Grism Order Order Sorting Wavelength Resolving Dispersion Name Filter Range microns Power A pixel 3 Ks 2 00 2 30 2200 4 62 4 H 1 50 1 80 1500 3 43 5 J 1 20 1 28 1400 2 71 6 J 1 17 1 24 1400 2 22 7 Z 0 89 0 93 1400 1 87 8 Z 0 86 0 95 1400 1 58 Table 2 4 The wavelength range and the resolution of the medium resolution grism The resolution is that measured for the 0 6 arc second slit The resolution scales inversely with the slit width The Wollaston prism is not achromatic so the exact separation between the two beams is a function of wavelength At J the separation is 48 3 arc seconds while at K the separation is 47 4 arc seconds The beam separation is also a function of position To measure the Stokes parameters and hence the degree and position angle of polarization a second set of images with the Wollaston prism rotated 45 degrees with respect to the first pair are required This is achieved by rotating the entire instrument The Stokes parameters are then determined as follows I i 90 i 0 i 45 i 135 Q i 0 i 90 U i 45 i 135 w
55. C 2 Optical Layos ck hate BN SAY Be tee oe AD EA oN ot Ue SN 262 MACIN oas a a Aye Bae ek RO eee A doe ve aeons senda CON Gg Rake de Hie as eRe aoe 2 3 Long Slit Spectroscopy sser 6 6 tok a toe Gh ee a Ree 24 1 Polarimetry it AR ee es do aes AA Suede ts Son ee i 2 5 The DCS Detector Control System e 2 51 Readout Modes ceuta a o ee A A ae 2 5 2 Features on the Detector e 2 5 3 Windowed Reading 2 0 0 ee ee 2 6 Calibration Unit sees iar Wax amp bo Poe ee A AA ee ete 2 7 Instrument Performance and the Exposure Time Calculator 2 8 Instrumental Overheads aoaaa ee Observing in the IR IL The IR Sky iu parda ea 5 ead eee be ea a Gs 2 A RN ade amp 3 2 1 Selecting the best DIT and NDIT 20 0002 eee eee 3 2 2 Small Objects or Uncrowded Fields 2 2 2 3 2 3 Large Objects or Crowded Fields 0 0 0 00 eee 3 2 4 Maps of Large Fields 2 2 ee 3 2 5 Imaging of Moderately Large Object 0 o e 0220004 3 2 0 Standard Stars 2s2 aah hon ee daa dsd 3 3 Polarimetry 2 4 fa cee i ee ele PE eM eee he eS oe Rake 34 Spectroscopy cacos 2 a E a ee A A ee aE Se ee 3 4 1 Small Objects and Uncrowded Fields 0 0 2 000000 002 ee 3 4 2 Extended Objects and Crowded Fields 0 2 00 2 00004 3 4 3 Telluric Standards and Flux Calibration 0 000 4 3 0 Calibrati
56. CATG CALIB Calibration frame DPR TECH IMAGE Exposure in imaging mode DPR TECH SPECTRUM Exposure in spectroscopic mode DPR TECH POLARIMETRY Exposure in polarimetric mode DPR TYPE LAMP Arc spectrum comparison lamp DPR TYPE DARK Dark frame DPR TYPE STD Standard Star DPR TYPE FLAT Flat Field Frame Table D 1 FITS header keywords defining the content of the images 98 Appendix E Photometric Standards The majority of users prefer to calibrate their data with the NICMOS photometric standard stars Persson et al 1998 A J 116 2475 Pre prepared OBs for observations of most of the standard stars from this list are available at the telescope For convenience these standards that are observable from the Southern hemisphere are listed in Table E 1 Finding charts with 2x2 arcmin field of view are shwon in Figures E 1 E 2 E 3 E 4 and E 5 North is up and East is to the left 99 100 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 No HST RA J2000 DEC J o J H o BM K o K Ks o Kgs 9101 P525 E 00 24 28 3 07 49 02 11 622 0 005 11 298 0 005 11 223 0 008 11 223 0 005 9103 294 D 00 33 15 2 39 24 10 10 932 0 006 10 657 0 004 10 596 0 005 10 594 0 004 9104 S754 C 01 03 15 8 04 20 44 11 045 0 005 10 750 0 005 10 693 0 010 10 695 0 005 9105 P530 D 02 33 32 1 06 25 38 11 309 0 010 10 975 0 006 10 897 0 006 10 910 0 005 9106 S301 D 03 26 53 9 39 50 38 12 153 0 007 11 842 0 005 11 772 0 010 11 788 0 006 9108 P533 D 03 41 02 4 06 56 13
57. DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS FILT1 ID INS FILT2 ID INS IMODE SEQ COMBINED OFFSET SEQ RETURN SEQ OFFSETX LIST SEQ OFFSETY LIST Table C 22 SOFT img cal_StandardSatr Parameter signature Exposure Name DIT NDIT Number of columns Number of rows First column of window First row of window Number of Exposures Filter Wheel 1 Filter wheel 2 Instrument mode Combined offset F T Return to origin T F X offset arcsec Y offset arcsec LSO MAN ESO 40100 0004 Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 1 NODEFAULT NODEFAULT NODEFAULT F T NODEFAULT NODEFAULT Value S9104 1024 1024 1 1 5 J open Large field T T 0 45 90 0 90 0 45 0 90 0 Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures in the sequence Filter wheel 1 position Filter wheel 2 position Instrument Mode T guiding ON F guiding OFF Returns the telescope to the original pointing if True list of offsets along rows list of offsets along columns Table C 23 SOFIima_obs_StandardStar Example 84 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 C 3 SOFI Polarimetric Template C 3 1 SOFT Polarimetric Acquisition Template SOFT_img_a
58. ET DIT NODEFAULT Detector Integration Time individual exposure sec NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Instrument Mode INS IMODE NODEFAULT Instrument Mode Add Velocity Alpha TEL TARG ADDVELALPHA 0 0 Additional tracking velocity in RA arcsec sec Add Delta Velocity TEL TARG ADDVELDELTA 0 0 Additional tracking velocity in DEC arcsec sec Rotation Offset on Sky TEL ROT OFFANGLE 0 0 Rotation offset on the sky degrees Table C 5 SOFI_img_acq_Preset SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Header Keyword Value Description DIT DET DIT NODEFAULT Detector Integration Time individual exposure sec NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Instrument Mode INS IMODE NODEFAULT Instrument Mode Alpha Offset arcsec TEL TARG OFFSETALPHA 0 0 Alpha Offset for sky subtraction arcsec Delta Offset arcsec TEL TARG OFFSETDELTA 0 0 Delta Offset for sky subtraction arcsec Add Velocity Alpha TEL TARG ADDVELALPHA 0 0 Additional tracking velocity in RA arcsec sec Add Delta Velocity TEL TARG ADDVELDELTA 0 0 Additional tracking velocity in DEC arcsec sec Rotation Offset on Sky TEL ROT OFFA
59. EUROPEAN SOUTHERN OBSERVATORY Organisation Europ enne pour des Recherches Astronomiques dans H misph re Austral Europ ische Organisation fiir astronomische Forschung in der s dlichen Hemisph re LA SILLA OBSERVATORY SOFI User s Manual Doc No LSO MAN ESO 40100 0004 Issue 2 0 26 04 2006 L J Prepared Lidman a aby a 16 08 2000 ee ee O Name Date Signature Revised be VEDEL dao 4b ete O AA Name Date Signature Revised A a r ae eens O sc ina weve Name Date Signature Revised vic fe Oe PMO y cee ueen ODA ds ane rise Name Date Signature Reviewed cea ye eee wes oe ei ETEN Name Date Signature Released O a e pa nee ek Name Date Signature il SOFI User s Manual 2 0 LSO MAN ESO 40100 0004 SOFT User s Manual 2 0 Issue Rev Date 02 05 98 14 08 98 12 11 98 25 02 99 16 08 00 05 11 02 26 04 05 LSO MAN ESO 40100 0004 iii Change Record Section Parag affected Reason Tnitiation Documents Remarks Creation New templates New Grism New IRACE and New Templates Some addition and corrections Merged with the template manual major aditions to the data reduction section lv SOFT User s Manual 2 0 This page was intentionally left blank LSO MAN ESO 40100 0004 Contents Introduction 1 1 A First and Final Word sots soomuse fon re A a pt 1 2 Applicable documents 0 aia Basra a ee Ae 1 3 Abbreviations and Acronyms 2 0 ee ee SOFI Son of ISAA
60. FT User s Manual 2 0 LSO MAN ESO 40100 0004 19 Parameter signature Value Exposure Name Hubble_Deep_Field DIT 10 NDIT 6 Number of exposures 60 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Filter wheel 1 K Filter wheel 2 open Instrument Mode LARGEFIELDIMAGING Combined Offset T F F Jitter Box Width arcsec 40 Return to Origin T F T Table 3 1 Parameters of the SOFIimg obs AutoJitter template with commonly used values that on any given position of the array most of the images will have sky and only a minority will have objects Clearly minimum three sky images are necessary for this technique but the experience shows that a reasonable minimum number of the sky images and respectively the object sky pairs is 5 7 to ensure a good removal of the objects from the sky frames Note that this may lead to an extra overhead because in some cases the NDIT has to be reduced artificially contrary to the optimization strategy discussed in Sec 2 8 to a number bellow the optimal just to split the total integration into 5 7 images adding an extra overhead for the telescope offsets This technique is encapsulated in the template called SOFI img_obs_AutoJitterOffset The pa rameters of this template are listed in Table 3 2 While preparing such observations please keep in mind that the total duration of an imaging OB in Service Mode can not exceed 1 hour in Visitor Mode the duration
61. IST 60 Filter wheel 1 J Filter wheel 2 open Instrument Mode LARGE_FIELD IMAGING Table 3 12 Parameters in the SOFIimg_cal_DomeFlats template with commonly used values The template actually takes four exposures in total the first with the lamp off then two exposures with the lamp on and then a final exposure with the lamp off The intensity of the dome flat field lamp is controlled by a panel on the tcs machine The instrument operator or the support astronomer will adjust the voltage until a level of a few thousand usually 3000 4000 ADU counts level is reached when the lamp is on In this template one can enter a list of DITs NDITs If the number of exposures is greater that SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 the number of elements in either of these lists the list is repeated until the the correct number of exposures have been completed Note that each exposure generates a sequence of 4 frames as explained before If the parameter Number of Exposure is gt 1 a corresponding sequence of frames will be generated The template SOFI_spec_cal_DomeFlatNonDestr contains in addition the parameters of Table 3 10 It was pointed earlier that shading one of the components that make up a dark frame is a function of the incident flux This means that the method of creating dome flats described above does not remove perfectly the shading pattern The effect for most observations is small and manifests itself as a disconti
62. In addition the user is provided with number of tools for quick look data reduction based on the pipeline recipes that allow rough data quality assessment The main tools are e Imaging tool it carries out cross talk removal sky subtraction flat fielding alignment and combination of images specified in a list provided through Gasgano observations with large offsets at clear sky fields are not reduced e Spectroscopic tool it carries out sky subtraction flat fielding alignment and combination of the 2 dimensional spectra and it subtracts and wavelength calibrates a 1 dimensional spectrum of the brightest object Naturally this tools works best for bright objects the fainter the object the higher the chance it will fail Zero Point tool cross talk removal sky subtraction flat fielding simple aperture photometry Note that the tool is sensitive to the position of the star on the array and if it is not centered within 20 arcsec it may fail to recognize the standard The SOFI web page contains the most recent instructions how to use the pipeline and the pipeline derived tools The products from them are by no means science grade 4 6 2 The Archive All data taken at the NTT are archived into the ESO archive For more retails see http archive eso org 4 6 3 The Calibration Plan The calibration plan aims to ensure that all data stored in the archive can be fully calibrated regardless of the specific needs of the program In
63. It is good practice to use either lists with a single value or lists of equal length If SEQ COMBINED OFSET is set to True must be done by the observer during the preparation of the Observing Block and if guiding was started before the start of the template will be done by the operator after a warning from the observer the guiding is Star To Box Given the short time spent on a single position typically 2 3 min determined by the sky level variations guiding is usually rarely used If SEQ RETURN is set to True T the telescope slews back to its original position at the end of the template If not the telescope is not moved Consider the following example Table C 12 it gives to you 5 images at 5 different positions Each image is the average of 3 NDIT exposures of 20sec DIT The first image is taken at the position at which the target was acquired The following example Table C 13 will produce 10 images repeating the sequence of 5 offsets Note that the sums of the offsets along RA and along Dec are non zero so the second offset pattern will be shifted relative to the first one by 2 5 arc to the East and 2 5 arcsec to the North This example illustrates an interesting usage of this template to define a closed loop offset sequence that will bring the telescope at the original position or close to the original position i e 2 5 arcsec away to imitate jittering minimizing the effects of the array cosmetic defect
64. NGLE 0 0 Rotation offset on the sky degrees Combined offset F T SEQ COMBINED OFFSET False T guiding ON F guiding OFF Preset Telescope F T SEQ PRESET True T full preset F fine tunning of the pointing Save Image F T SEQ SAVE False T preserve the acq image F no Table C 6 SOFIimg_acq_MoveToPixel To move the target from one position of the array to another the operator simply draws an arrow on the screen with the left hand side button of the mouse At this point a window which lists the pixel co ordinates at the start and the end of the arrow will appear The operator can accept the offsets cancel or edit the co ordinates directly If the offsets are accepted the telescope offsets by the desired amount Finally the operator is given the possibility to redraw the arrow for refining the position of the target if necessary Once the operator is satisfied the template finishes Guiding is possible defaulted to the Telescope Control System s option Star To Box only if SEQ COMBINED OFFSET is set to T and if guiding is on before the start of the template The experience has shown that the guiding is necessary if the observer plans to stay at one pointing more than 15 min This is rarely the case in imaging mode but can often happen in spectroscopic observations The SEQ PRESET parameter allows the user to use the fine tunning pointing functionality of the template without presetting the telescope On other wor
65. R LENS SOFI instrument concept SLIT MASK WHEEL Figure 2 1 Optical layout of SOFI Objective Pixel Scale Field of View Large Field Objective 0 288 4 92 x 4 92 Spectroscopic Objective 0 273 4 66 x 4 66 Small Field Objective 0 144 2 46 x 2 46 Large Field Objective Focal Elongator 0 144 2 46 x 2 46 Table 2 1 The fields of view and the pixel scales available with SOFI SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 il Rotator angle 0 NCS Object Instrument N gt LL 7 E Figure 2 2 Orientation of SOFI for rotator angle O deg In imaging the North is to the left and the East to the bottom of the image The slit for the spectroscopy is oriented North South Nota Bene To align the North South axis of the field of view or the slit in case of spectroscopy long a certain Position Angle one has to apply in the acquisition template a rotaton angle equal to this Position Angle The spectroscopic objective can be used to take images with a scale that is a bit finer than that of the large field objective however this objective is chromatic as mentioned above This means that the pixel scale is a function of wavelength It also has an illumination pattern with a central light concentration All the aforementioned objectives suffer various amounts of image degradation over their respective fields of view We routinely try to improve the image quality during interventions Please consult the
66. acquisition has to put the telescope at the location marked on the figure with zero However the first image in the sequence is actually a sky on the location marked with number one The parameter NEXPO has to be set to six The sequence in which the images are taken is 24 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 shown in the figure on the right The target is marked with thick dark line and it is covered by two overlapping field The jitter boxes around each position are not shown One 6 point cycle will obtain two images of 1 min at each target field 12 cycles will accumulate 24 min of integration on each of the two galaxy fields and 12 min on each of the two sky fields The total observation will require about 1 5hr including the overheads 600 arcsec a He gt re Les arcsec o oO Nn o pan 4 Oo a q 0 2 6 8 12 Dec 120 arcsec 600 arcsec R A arcsec Figure 3 2 Example of 6 point observation of semi extended objects alternating between two target field and one of two different skies Nota bene The reduction of data obtained with the patterns described in this subsection is usually more complicated and requires special attention and more manual intervention than the classical observations In addition the sky sampling is rarefied However in most of the cases these are small prices to pay for 20 30 more data The users must read carefully Section 5 2 4 where the sky su
67. affects the same way both spectra e the science spectrum has to be multiplied by the true intrinsic spectrum of the standard This will remove the artificial emissions features introduced to the target spectrum by the intrinsic absorptions in the spectrum of the standard Removing the atmospheric features is a critical operation In general the spectra of the atmospheric standard and the spectra of the science target will not have exactly the same wavelength scale They may differ by as much as half a pixel This could be caused by internal flexure within the instrument or by the science object and the standard being on different parts of the slit Thus before dividing the standard into the science target one should use the atmospheric absorption features in both to realign the spectra The IRAF task telluric may be useful for this The true intrinsic spectrum of the standard is usually not known with any great precision As discussed in Section 3 4 3 usually the telluric standards are selected to be wither solar analog because a good Solar spectrum is available or early type stars because their spectra are eas ier to model The IR Solar spectrum is available from the National Solar Observatory An atlas of the solar spectrum in the infrared from 1850 to 9000 cm 1 Livingston W amp Wallace L N S O Technical Report 91 001 July 1991 A more detailed description how to use solar analogs as telluric standards and an IRAF based
68. ampling is determined by the product DITxNDIT plus the overhead The user should try to keep this time close to 1 2 min in average sky conditions or 3min in exceptionally good conditions To verify the choice of sky sampling the observer should frequently subtract sequential images from one another and monitor how large is the average residual Ideally it should be smaller than or comparable to the expected Poisson noise but this is rarely the case Usually a few tens or hundreds of ADU are considered acceptable by most users Finally the total integration time is accumulated by obtaining a certain number of images usually 18 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 specified by the total number of exposures and or the number of jittered images at each position depending on which template is used In case when relatively large integrations are necessary it is simply a matter of increasing the number of exposures However in the cases when the total required time can be accumulated in less than 5 7 exposures it might become difficult to create a good sky for the sky subtraction especially if the field is crowded because the sky image may contain residuals from the stellar images that will produce holes in the sky subtracted data This situation will require to increase artificially the number of exposures to 5 or 7 It might be possible to compensate this increase by decreasing correspondingly NDIT to keep the total integration time
69. an be removed The residual shade pattern can be removed if the SOFI_img_cal_SpecialDomeF lat template was used see sec 3 5 2 This template takes the usual sequence of images with the dome lamp on and off and in addition it takes the same set of images with the mask wheel vignetting the array Figure 3 3 The vignetted part of the array is relatively free of scattered light so it can be used to estimate and remove the shade pattern However this estimate of the shade pattern is valid for the frame that is vignetted and it is not valid for un vignetted frame The difference can be estimated from a region that is common to both the vignetted and un vignetted frames The method is outlined below i Collapse the following image sections along detector rows to form one dimensional images columns 50 to 150 in the vignetted image columns 500 to 600 in vignetted image and columns 500 to 600 in the un vignetted image For reference call these images A B and C respectively ii The difference between the shade pattern in the vignetted and un vignetted images is C B iii The shade pattern in the un vignetted frame is then A C B and this one dimensional image should be subtracted from the un vignetted frame SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 iv Steps i to iii should be done for the images with the lamp on and the images with the lamp off Once the shading pattern is removed for both one subtracts the image with lamp of
70. aph And Array Camera Number of DITs Non Destructive Read Number of NDITs New Technology Telescope Observing Blocks Observing Software Phase 2 Proposal Preparation Point Spread Function Right Ascension Read Out Noise Son Of ISAAC Telescope Control System Template Signature File Very Large Telescope Zero Point Table 1 1 Abbreviations and Acronyms used in this manual Chapter 2 SOFI Son of ISAAC 2 1 Optical Layout SOFT is mounted on the Nasmyth A focus of the NTT The light from the tertiary mirror of the telescope enters the front window which has no optical power Immediately after the front window is a cryogenically cooled mask wheel which coincides with the telescope focus The mask wheel contains several masks one for each imaging objective three for long slit spectroscopy a pinhole mask and a special mask used with polarimetry The optical layout is shown in Figure 2 1 The mask wheel is followed by a collimating lens used to focus the instrument two filter wheels a grism wheel an objective wheel and then the detector itself The re imaged pupil of the telescope the primary mirror is located on a slightly undersized stop just before the grism wheel The first filter wheel contains the standard broad band near IR filters several narrow band filters two order sorting filters for low resolution spectroscopy an open position and a fully closed position The second filter wheel contains more narrow band fi
71. are completed It is the best template to use when pointing is not critical The NTT typically points with an error of a few arc second Most observers find this template sufficient for the purposes of their programs SOFI_img_acq_MoveToPixel This acquisition template Table C 6 has two parts First it presets the telescope to the coordinates of the Target associated with the Observation Block Next it requests the operator to interact with the RTD so that the user specified object is moved to a user specified location on the array In order for objects to be clearly seen the telescope can do a small offset after the preset The size and direction of the offset are template parameters TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA The operator is then prompted to store a fixed pattern that is subsequently subtracted from incoming images via the real time display RTD features After storing the fixed pattern the telescope returns to the original position At this point in time the operator is offered the possibility to change DIT and NDIT This is useful if the target can not be identify securely because of the low signal to noise If the operator changes the values the telescope offsets again and the operator is required to store another fixed pattern before the telescope returns to the nominal position This loop can continue until the operator has identified the target Parameter signature Header Keyword Value Description DIT D
72. arly zero background and only the stars above the background Study the distribution of pixel values with image histograms and determine the border value between the background and the stars Usually the limit is 3 times the background standard deviation Next mask out the values above this limit The easiest way to do it is just to replace the values above the chosen limit by a high or low value i e le5 to 1e5 so when you combine the masked images to produce a second iteration sky these pixels will remain out of the range of the good values used in the combination The good values are usually defined as parameters of the combining task Be careful before modifying the files with replacement It is better to copy them into dummy files first c Scale the sky image to have the same median as the image from which it will be subtracted and then carry out the subtraction This method reduces the negative traces of stellar images on the sky subtracted data The number of 10 images used to create the sky is somewhat arbitrary It can be 5 7 or even all available images ii subtracting from each image the average of the previous and the next image In the more general case the average or the median of the previous 2 3 and the following 2 3 images is subtracted Strictly speaking this is a specific case of the method described above However our experience shows that in most cases using only the two images neare
73. asier for reducing ex tended objects because it will become trivial to extract spectra at different locations of these objects However the geometric correction involves heavy modification of the data and some degradation of the resolution may be possible The second method is easier to apply on point source spectra and it has an added advantage of giving a straightforward estimate of the observational error this is just the standard deviation obtained during the combination 5 4 Polarimetry As in the case of imaging you should start to use a bad pixel mask that you can retrieve from http www ls eso org lasilla sciops ntt sofi reduction bad pix html Then you have to apply the flat field to correct for the pixel to pixel sensitivity variations across the chip The flat field exposures should be performed using the same filter as the object exposures and the Wollaston prism in the optical path Since the whole instrument rotates in order to provide a certain orientation of the Wollaston prism separate exposures at different orientations are not required We recommend to take dome flats because when a sky map is used some ghosts of the object on consecutive images may result in a wrong flat field especially at the position s of the object Exposures with the lamp OFF and with the lamp ON must be combined to obtain the final flat field FF lamp_ON_a lamp_OFF_a lamp_ON_b lamp_OFF_b The final flat field is polarized i e th
74. ay is read within each DIT a number of times equal to NSAMP and for each read out the signal is sampled NSAMPIX times The minimum DIT that can be used for a given NSAMP can be calculated in seconds is NSAMPx1 64 sec Parameter signature Value NSAMP 30 NSAMPPIX 4 Table 3 10 Parameters in the templates with Non Destructive Read Out and commonly used values 3 4 2 Extended Objects and Crowded Fields For more complex nodding patterns or for observations of either extended objects or crowded fields please use the template SOFI_spec_obs_GenericSpectro This template is discussed in detail in Appendix C It allows to place the slit on a user defined sequence of positions alternating between the object sky and the sky Similar to the imaging of extended objects an effort should be made to take advantage of the large slit length of 4 92 arcmin to obtain observations with the simple nodding procedure described above because the integrating of the clear sky increases the total time spent on the object by factor of two for the same integration time on the target NOTA BENE Frames taken with the NDR mode must be flat fielded using dome flats taken with the same mode For this reason a specific templates exists SOFI_spec_cal DomeFlatNonDestr 3 4 3 Telluric Standards and Flux Calibration The issue of flux calibrating IR spectra is still very much an issue of some debate This section is written not as the definitive method to flux calibrat
75. bservations obtained following the schemes described in Sec tion 3 2 5 As pointed there the sky sampling in those observing strategies is somewhat rarefied and 52 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 the user should check the residuals from the sky subtraction In case of observations in which the target has been kept on the array during every image the user should mask the target out before constructing a sky image This can be done in many ways perhaps the most straightforward solution is to follow the same procedure as for masking out the bad pixels In the case of 4 point observations described in Section 3 2 5 you have to create four pairs of masks one with zeros in the target quadrant and one everywhere else and another with a large negative value i e 1e6 at the target quadrant and zero everwhere else Then every image has to be multiplied by that of the first masks that matches the location of the cluster and summed with that of the second masks that again matches the target quadrant Remember to produce those manipulations on a copy of the original data Finally a suitable low threshold in imcombine will make sure that the target has been masked out from the sky An alternative to this masking is to use a floating sky constructed from the six in case of the 4 point scheme images immediately preceding and succeeding every image Those images have the target located on different locations than on the sky sub
76. btraction is described SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 25 Parameter signature Value Exposure Name SOFT 6 point DIT 6 NDIT 10 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 NJITT 12 NEXPO 6 Jitter Box Width arcsec 10 Filter wheel 1 K Filter wheel 2 open Instrument Mode LARGE_FIELD_IMAGING Combined Offset T F F Return to Origin T F T RA Offset List arcsec 0 0 120 0 0 120 DEC Offset List arcsec 600 600 210 600 600 210 Table 3 5 Parameters of the SOFIimg_obs_AutoJitterArray_l template with commonly used values for 4 point observation of a semi extended object 3 2 6 Standard Stars The IR window between 1 and 2 5 microns contains several large absorption features that are pri marily due to water vapor and carbon dioxide in the atmosphere The edges of the atmospheric windows are highly variable Unfortunately the edges of some IR filters particularly J and K are defined by these absorption features rather than the transmission curves of the filters themselves Thus when the column density of water vapor is variable accurate photometry can be difficult to achieve On good nights generally when the humidity is low and it is cold it has been possible to achieve better than 1 photometry however on most nights this should be considered as the best limit and the typical accuracy is 3 5 Good planing of the observation and careful data red
77. by the offsets lists then the random offset is applied and the entire array pattern is executed again This template is preferable to the original one because it gives better sky subtraction Also if the weather conditions are poor i e the sky background variations are strong there are thin clouds etc the entire mapped area is imaged under relatively more similar conditions in comparison with the SOFT img_obs_AutoJitterArray tem plate On the other hand if the conditions grow even worse and make it impossible to complete the mapping the observer may be left with a shallow image over the entire area as opposed to a deep image of part of the mapped area The observer must decide which strategy is better suited to the goals of the program before deciding which template to use Consider the following example Table C 17 you will have 4 main positions defined in the RA and DEC offset lists Around each of the offset position you will have 3 images with jitter inside a box of 20 arcsec defined by the parameter Jitter Box Width In total this means 3 x 4 images 12 images Each of them is the average of 10 exposures NDIT of 6 sec DIT so the total exposure time is 10 x 6x 3 x 4 12 min without the overheads The template SOFI_img_obs_AutoJitterArray will take a sequence of 3 jittered images around the first offset position then move to the next offset position and take 3 more jittered images and so on until the entire mapped region is co
78. constant Still there will be some increase in the overheads Summarizing under average conditions for faint targets one can safely use DIT 30 15 and 10sec for J H and K filters respectively The narrow band filters can tolerate DIT of 60 120sec The array level will be dominated entirely by the sky Brighter targets require to reduce these times down to 12 8 and 6sec for the broad band filters for 12 15 mag stars and all the way down to the minimum DIT of 1 183sec for 10mag stars The users may even have to consider splitting their observations into shallow and deep sequences optimized for different magnitude ranges These issues are discussed again in Sec 3 6 3 2 2 Small Objects or Uncrowded Fields If the object of interest is uncrowded stellar field it is not necessary to take separate sky observations In this case one can dither within the field and use the object frames to create sky frames As a rule the offsets should be greater than 10 arc seconds and if very deep exposures are required the offset vector should not be replicated For example an offsetting scheme that is based on a rectangular grid of points will in a deep exposure show faint negative images arranged symmetrically around each real image As a last resort these can be minimized with appropriate masking during the data reduction Similar to the uncrowded fields small objects can be observed in the same manner In this case the user should mak
79. cq Polarimetry This acquisition template Table C 24 is very similar to the SOFT img acq MoveToPixel template The polarimetric mask is displayed on the RTD and is superimposed on the image of the field Then the operator is prompted to define an offset by drawing an arrow on the RTD holding the left mouse button The offset fine positions the object into the transparent region of the mask Since the POLARIMETRY mode uses the large field objective the instrument mode is not a param eter of the template C 3 2 SOFI Polarimetric Science Template SOFI_img_obs_Polarimetry This observation template Table C 25 is used for polarimetry It is quite simple as it works as an imaging template The number of exposures must correspond to the number of the offsets specified in SEQ OFFSETX LIST and SEQ OFFSETY LIST Note that unlike most of the imaging templates here the offsets are defined along detector rows and columns in arcsec so that the users can move the object easily along the strips of the polarimetric mask Most importantly the user have to decide which angle are necessary one should observe the same object at several angles to determine the Stokes parameters cf section 2 4 All frames can be given an orientation relative to the previous position angle by setting SEQ ROT OFFANGLE parameter in degrees When the template starts the instrument is rotated on the sky by SEQ ROT OFFANGLE and remains at this position until the end
80. d corner of the RTD can be used to activate sub windows with various tool which the observer may find useful e The pick object sub window can determine the centroid and FWHM of selected objects in pixels It is useful for monitoring of the image quality e The statistics sub window can be used to display the statistics of a region that can be defined by the cursor It is useful for checking if the counts in the core of an image is bellow SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 SOFI Briet Tiin Minjibyy 2 4 Miniti Figure 4 2 Real Time Display with the Store Fixed Pattern option enabled Note the offset between the negative and the positive star pattern This offset is necessary to avoid self cancellation of the objects in the field The green dot on the top of the image flashes every time when the RTD updates the image The yellow arrow is a remnant from a previous acquisition it indicates the direction and the size of the offset that was carried out the non linearity limit of 10 000 ADU e The cuts sub window can be used to plot a trace that can be defined by the cursor which is especially useful to check the count levels in spectra and the general background level Zooming withing this sub window is possible with the left mouse button the right mouse button un zooms Last but not least the RTD display is also used with the acquisition templates to place objects of interest into slits in spectroscopic modes int
81. deed it is easy to imagine a situation that the data may be used later after their proprietary period expires from the archive for the purpose of another object The calibration plan ensures that any limitations in that respect will not come from the lack of appropriate calibrations Therefore daily calibrations are taken as a mater of routine both by the observatory support staff and in Visitor mode runs by the observers wishing to calibrate their own data The standard procedure is to run every morning after the end of the nigh time observations a special tool calobBuilt that inspects all files taken during the night and determines what calibrations are necessary Furthermore this tool prepares a calibration OB that can be loaded directly into BOB and executed However Visitor mode observers are encouraged not to rely on our calibration plan to provide them with these data but to take their own calibrations in the afternoons before their observations start SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 This has a number of advantages they would do their own quality control of the calibrations and they will have at their disposal appropriate calibrations if they wish to reduce the data during the night The calibration plan includes e Imaging special dome flats for all filters and imaging modes used e Spectroscopy arcs and flats for each spectroscopic mode and slit combination e Polarimetry flats 4 7 At the End of t
82. ds e Airmass 1 2 e Backgrounds of 16 0 16 0 14 2 and 13 0 in J Js H and Ks respectively The extended source detection limits were made for an aperture with a diameter of 3 arc seconds and for a S N of 4 The values of Table 2 5 can be re scaled to different S N ratios fluxes F and integration times t keeping in mind that for background limited performances S NxF x vt Background limited performances are reached when ADU gt 30 assuming read out noise 12 e in the double correlated read out mode and a gain of about 5 4 ADU e NOTA BENE The detection limits given above vary with the background level which is strongly sensitive to the humidity and and air temperature The background can easily change within 50 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 15 2 8 Instrumental Overheads The fraction of time spent not collecting photons is defined as the instrumental overhead A good conservative rule of thumbs states that it is typically about 30 of the integration time There are several sources of telescope and instrument overhead e acquisition overhead which depends on the instrument mode the selected acquisition template the brightness of the target the angular distance from the previous target in the simplest case of imaging it takes on average 3 5 min the more challenging spectroscopic acquisitions can require up to 5 10 minutes e the time necessary to offset the telescope between different jitter positions
83. ds the user can improve the position of the target on the array without full telescope preset This option is particularly useful if acquisition was aborted for some reason or if multiple OBs to observe the same target are executed one after another The parameter SEQ SAVE allows to save an acquisition image at the prize of some additional overhead for file transfer and saving The interactive pop up windows are usually displayed before new images have arrived on the RTD Therefore operators are strongly advised to carefully check that a new image has arrived before clicking on these windows e g for storing a fixed pattern for changing the DIT NDIT configuration The arriving of a new image on the RTD is marked by a flashing green dot in the middle of the upper part of the RTD window SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 71 C 2 2 SOFI Imaging Science Templates SOFT_img obs_AutoJitter This observation template Table C 7 offsets the telescope between exposures according to a random pattern of offsets automatically generated within the template It is ideal for long integrations on empty fields or fields containing isolated point sources and does not require a long list of offsets to be defined by the observer The offsets are distributed randomly within a box whose size is defined by SEQ JITTER WIDTH in arc seconds with the condition that the distance between any two points in a series of ten values is greater than
84. e IR spectra but rather it is written to illustrate the problems involved and various methods for their solution As the IR window is dominated by time varying atmospheric absorption features that depend non linearly with airmass it is necessary to divide object spectra with the spectrum of what we shall call an telluric standard The standard should be at an airmass that is as close as possible to that of the science target usually Asecz lt 0 05 0 1 it should be observed immediately before or after the target and it should be observed with the same instrument set up grism slit objective etc From technical point of view the telluric standards are observed in exactly the same way as the science targets Since they are usually bright stars with no nearby companions of comparable magnitude the template SOFI_spec_obs_AutoNodOnSlit is perfectly suited for this It is very important that object and standard star spectra are accurately aligned A small misalign ment will result in poor cancellation of the atmospheric absorption features Misalignment could be caused by slit misalignment in between the observations by instrument flexure or when the seeing is smaller that the slit width by inaccurate centering of objects on the slit In general this problem can be remedied later during the data reduction by re aligning of the spectra first by using the telluric features to determine the offset in wavelength direction and second by geometric
85. e achieved in a few steps i block average the image along the horizontal lines note that the IRAF task blkavg is not well suited for this purpose because it does not allow to use median averaging or to impose rejection and lower threshold limits Instead it is better to use imcombine feeding it with a list that contains each row of the image as individual entry finalimage 1 final image 2 finalimage 3 etc The use median averaging suitable rejection algorithm and upper threshold to exclude from the averaging any sources The result from this step will be a 1 dimensional vector image ii expand the vector image to the full size of the final image by replicating it the IRAF task blkrep is well suited for this purpose The result from this step is a 2 dimensional image with a size matching the size of the final image iii subtract the image produced in the previous step from the final image ideally this should remove the residuals from the horizontal lines However one should exercise caution because any imperfect flat fielding may affect the procedure leaving artificial gradients across the image As explained above in many cases the cross talk effect can not be removed completely but sometimes the field of view can be rotated so the ghost image can be relocated to an area of the field away from the target It is enough to relocate the bright source on a different raw or column than the target and on a different raw than the
86. e constraints section can be ignored by the visitor mode observers Templates are the simplest unit of observation They are split into three categories acquisition observing and calibration The templates are described fully in Appendix C The P2PP tool is a browser editor that enables observers to create lists of targets and observing descriptions and then associate them with an acquisition template to form an OB Your support astronomer will start you up on the P2PP and will help you to optimize the observations but we highly recommend the users to read carefully the P2PP Users Manual available the User Support Division USD web page at http www eso org org dmd usg Once the OBs are created they can be stored either in the local cache which resides on the same machine where the OBs were created p2pp cache ID or they can be exported as ASCII files The exported format of the files allows to ftp them from one machine to another NOTA BENE Under no circumstances edit the OBs by hand with a text editor or with scripts The software is sensitive to the format of the OB and extra or not enough spaces tabs often cause crashes In visitor mode P2PP is used to select OBs for execution Observing blocks are selected by highlight ing them with the left mouse button in the P2PP window Once the OB is selected the instrument operator will transfer it to the BOB Broker of Observing Blocks where it will be executed Note that once a
87. e following examples Table C 35 and C 36 demonstrate a typical usage of the generic templates Six images will be obtained 1 the object at the acquisition position 2 the object in an offset position 15 arcsec 3 the sky the object is off the slit 4 the object at another offset position 5 the sky again and 6 the object at the original position A figure with the object positions is also shown Each of the obtained 6 files is the average of 2 NDIT exposures of 40sec DIT The guiding option asks to put the star to the box SOFT User s Manual 2 0 Parameter signature Exposure Name DIT NDIT NSAMP NSAMPPIX Number of columns Number of rows First column of window First row of window Number of exposures Spectro Mode Which slit Return to origin T F X offset list arcsec Y offset list arcsec Obs Type O or S Guiding N B S LSO MAN ESO 40100 0004 Value NGC6118 40 2 4 10 1024 1024 1 1 6 LONG_SLIT_BLUE long slit_1 T 0 15 0 30 0 15 0 0 10 10 10 10 OOSOSO0 S Table C 36 SOFI_spec_obs_GenSpecNonDestr Example St gt O O Figure C 1 Positions of the offsets in the slit SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 95 C 3 7 SOFT Spectroscopic Calibration Templates SOFT_img cal_Darks The template Table C 20 for imaging darks can be used for obtaining spectroscopic darks if neces sary However notice that in practice this is never n
88. e median value of the upper field is different from the median value of the lower field However this problem can be overcome by an independent normalization of the flat field in each field A further artificial polarization is introduced due to the deviation of the transmission ratio of the Wollaston prism ideal 50 50 intensity upper lower beam Three alternatives for the correction of the wrong transmission ratio are proposed to see them in detail please read the technical report available in http www 1s eso org lasilla sciops ntt sofi e Use the transmission ratios between the two beams of the Wollaston of Cg 0 968 and Cy 0 954 respectively for Ks and J Ratios for the other filters have not been determined as of yet Since the sky is also affected by this effect a good measure for the intensity ratio of both beams can be derived from the median of the lower upper image This method may not be applicable if the polarized object covers a large fraction of the lower upper image because in this case the median value may be influenced by possibly polarized object and moreover this method only works properly if the background radiation of the sky is unpolarized so the images are not affected by scattered moonlight Fitting a cos 20 function to the intensity of the object on the image as a function of the orien tation 0 of the Wollaston prism see Ageorges 2000 http www eso org nageorge Pola sofipola html The next st
89. e sure that the offsets are at least 2 3 times bigger than the size of the object A pseudo random offsetting scheme see sec 3 6 is used in the template SOFI_img_obs_AutoJitter The offsets are restricted to be within a square box centered on the object The dimension of the box is defined by the parameter Jitter Box Width This and other parameters of the template are listed in Table 3 1 If the value of the Return to Origin parameter is true T the telescope at the end of the template moves back to where it was at the start of the template In general auto guiding is not required because of the frequent telescope offsets For a more detailed discussion about guiding options and the algorithm used to compute the offsets please to refer to section 3 6 If you wish to enter the offsets individually use the template SOFI img_obs_Jitter This template is discussed fully in Appendix C 3 2 3 Large Objects or Crowded Fields For objects larger than 40 of the field or for very crowded fields it is necessary to image the sky and object separately Unfortunately it is common that the sky frames will contain other objects and it is not uncommon that one of these objects will be in the same region of the array as the science object To avoid this it is standard to obtain several sky images on different locations usually in the context of object sky pairs This technique assumes that the sky fields are sufficiently uncrowded SO
90. e usage of the SOFI_img_cal_StandardStar template Parameter signature Value Exposure Name sj9104 DIT 5 NDIT 10 Number of exposures 5 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Filter wheel 1 J Filter wheel 2 open Instrument Mode LARGE _FIELD IMAGING Combined Offset T F F Return to Origin T F T X offset list arcsec 0 75 150 0 150 Y offset list arcsec 0 75 0 150 0 Table 3 6 Parameters of the standard star template with commonly used values In this template the offsets are along array columns and rows and the units are arc seconds un like other templates where the offsets are given along the RA and Dec The offsets are relative This template is very similar to the template SOFI_img_obs_Jitter Indeed one can use the SOFI_img_obs_Jitter template to do the same observation however we strongly encourage ob servers to use the SOFI_img_cal_StandardStar template as it is both easier to use and it tags the resulting images as standard star frames for archiving purposes and for easier identification by the zero point calculation tool available at the telescope In general is not required for standard star observations and it is useful to set the Return to Origin parameter to true 3 3 Polarimetry In Polarimetry a Wollaston prism and a mask wheel consisting of alternating opaque and transmitting strips are inserted into the beam The widths of the transmitting section
91. ec 20 Return to origin T F T Table C 8 SOFIimg_obs_AutoJitter Example SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 73 Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix DET DIT DET DIT NODEFAULT Detector Integration Time individual exposure sec DET NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Instrument Mode INS IMODE NODEFAULT Instrument Mode Combined offset F T SEQ COMBINED OFFSET F T guiding ON F guiding OFF Jitter Box Width arcsec SEQ JITTER WIDTH 40 Jitter box size Return to Origin T F SEQ RETURN T Returns the telescope to the original pointing if True Sky Offset Throw arcsec SEQ SKYTHROW 300 Radius of the sky offsets region arcsec Pupil Rotation SEQ ROTPUPIL T T rotate N not rotate Table C 9 SOFIimg_obs_AutoJtterOffset If the number of exposures is even SEQ NEXPO 2 pairs of object sky frames are produced If the number of exposures is odd then an extra frame is ta
92. ecessary because i in case of observations the dark including the bias is subtracted together with the sky and ii in case of the calibrations the respective templates take frames with lamps off to be used as darks SOFI_spec_cal_Arcs This calibration template Table C 37 takes arc spectra with the calibration unit The calibration mirror is automatically inserted at the beginning of the template and is automatically removed at the end The number of arc frames is defined by SEQ NEXPO Arc frames can be taken with different DIT and NDIT values defined as lists with SEQ DIT LIST and SEQ NDIT LIST SEQ SPECTROMODELIST defines the spectroscopic mode entered as a list with either B R Z J H K or NB_1 061 standing for the available spectroscopic modes In addition different slits can be called sequentially with SEQ SLIT LIST The allowed values are 1 2 or 0 6 for long _slit_l long _slit_2 and long slit_0 6 respectively Finally the arc lamps are entered into SEQ LAMP LIST Valid values are N Xe Ne or B N stands for None this is why there is no need to take separate dark frames but make sure to obtain at least one image with no lamp at each configuration Xe stands for Xenon Ne stands for Neon and B stands for Both i e Xenon and Neon lamps simultaneously Nota Bene Taking images with no lamp the option N has to be specified explicitely in the SEQ LAMP LIST This is responsibility of the user Remember that th
93. ed J filter top panel and the typical broad K filter Persson et al 1998 AJ 116 2475 see their Table 10 The dotted blue line is the atmospheric transmission model for Mauna Kea for Am 1 0 and water vapor column of 1mm Lord S D 1992 NASA Technical Memor 103957 courtesy of Gemini Observatory The data for the plot and a SuperMongo script are available from the SOFT web page e e Quantum Efficiency of the SOFT detector at T 78K The peak Q E is at 1 970 um and the long wavelength cut off is at 2 579 um o e o Example of 4 point observation scheme of a semi extended objects without extra over head for observation of clear sky Remember than although the target is moving on Example of 6 point observation of semi extended objects alternating between two tar get field and one of two different skies e e Examples of Special Dome Flat images From left to right lamp off lamp off with mask lamp on with mask lamp on e Stability of the Special Dome Flat images accuracy of the photometry as a function ofthe flat Held Sage ui A A a A A ee ae Sag OS DESOBFE ranica a ii Ee AE AS il a Real Time Display with the Store Fixed Pattern option enabled Note the offset between the negative and the positive star pattern This offset is necessary to avoid self cancellation of the objects in the field The green dot on the top of the image flashes ev
94. ep is the sky subtraction since the intensity of the background radiation is an additive component to the intensity of the object the subtraction of the sky will also result in the removal of SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 the contribution of the sky polarization to the net polarization of the object Therefore the sky has to be estimated and subtracted independently for both the upper and the lower field Finally the remaining instrumental polarization is expected to be mainly caused by the reflection on the tertiary mirror M3 It should therefore depend on the altitude angle of the telescope However the remaining instrument polarization was found to be lt 0 3 for the K band and lt 0 4 for the J band see Appendix B of the technical report for more details Since the statistical error of the results from which these limits have been derived is in the same order of magnitude a possible contribution by the mirror M83 could not be extracted If you pan to do polarimetric observations with SOFI we recommend to read carefully the three following reports http www 1s eso org lasilla sciops ntt sofi archive pol report ps http www 1s eso org lasilla sciops ntt sofi archive pol tech rep polarimetry ps http www eso org nageorge Pola sofipola html Please note that this is just a short description consult the polarimetry reports from the SOFI web page for more details Appendix A Calibration Arcs The adapter
95. er is already familiar with the capabilities of the instrument Information about SOFI may be found on line at the NTT Web site http www ls eso org Note that undocumented and unreleased prototype templates may be available at the telescope Contact the SOFI support astronomer for further details SOFT has three types of templates observation templates OT for science observations Calibration templates CT for calibration exposures and Acquisition templates AT for target acquisition Tables C 1 C 2 C 3 and C 4 associates templates with observational scenarios Not all parameters have default values In those cases where there is no default value the value is NODEFAULT IMPORTANT For those parameters that require a string to be entered for example the target name or the exposure name spaces and special characters must NOT be used only letters numbers and gt underscore symbol are allowed The SOFI templates are maintained by the SOFI instrument scientist and by the Science Operations team The following sections will review the SOFI templates in order of the instrument modes imaging spectroscopy polarimetry This Appendix supersedes the SOFI Template Manual ESO document LSO MAN ESO 40100 0007 67 68 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Type of Acquisition Template s to use Simple telescope preset point and shoot SOFLimg acq Preset Preset telescope and move an object onto a pi
96. er a large area of the sky with a map the template SOFI_img_obs_AutoJitterArray is available This template allows to define an array of positions NEXPO through a list of offsets in RA and DEC and to randomly jitter number of jitter positions defined by NJIT around each of the offset positions The NJIT images around each offset are completed before moving to the next offset position However in some cases it is preferable first to obtain a single image at each offset position then to shift the entire offset pattern by a small jitter and to repeat it This can be done with the tem plate SOFIimg_obs_AutoJitterArray_1 that has identical parameters takes the same number of images on the same locations as SOFI_img_obs_AutoJitterArray but differs only in the order in which these images are taken The template SOFI_img_obs_AutoJitterArray may be preferable if it is more important to make sure that at least some of the offset positions are observed with the required depth while the template SOFI_img_obs_AutoJitterArray_1 may be preferable if covering the entire mapped area albeit at a shallower depth has higher priority than the depth of the mosaic The parameters of both templates are listed in Table 3 3 3 2 5 Imaging of Moderately Large Object This section describes imaging of moderately large objects i e objects comparable to the Sofl field of view We discuss this case after the mapping of large areas because it uses the same template
97. eral description of the instrument For those who will be observing at the telescope it is sufficient to read up to the end of chapter 4 Chapter 3 discusses how to use observation templates to set up observations and chapter 4 describes how to use the instrument at the telescope For those who will reduce data taken with SOFI it is sufficient to read chapters 2 3 and 5 Chapter 5 discusses how to reduce data taken with SOFI At the end there are appendixes which contain various information useful for observers detailed template descriptions and examples fits headers information calibration data etc 1 1 A First and Final Word The authors of the manual hope that you find this document useful in writing SOFI proposals or for preparing for your SOFT run The manual is continually evolving Some sections still need to be written and some figures still need to be included If you have any suggestions on how to improve the manual please contact the La Silla Science Operations department lasilla eso org SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 3 Nota Bene 1 2 1 The SOFT Template Signature Files hereafter TSF Parameters Reference Guide is suppressed by this manual see Appendix C Those of you who are successful in being awarded SOFI time must read this document and the related P2PP Users Manual before coming to observe with SOFI This document contains a data reduction cook book Chapter 5 with detailed description of
98. ery time when the RTD updates the image The yellow arrow is a remnant from a previous acquisition it indicates the direction and the size of the offset that Was carried Out aw fect aan Seeks eS dy hae ys eB EL Be Bee ee Rees Go ee A Xenon arc spectrum taken with the blue grism The main lines are marked A Xenon arc spectrum taken with the red grism The main lines are marked A Xenon and Neon arc spectrum taken with the medium resolution grism at the Z atmospheric window The main lines are marked 0004 A Xenon and Neon arc spectrum taken with the medium resolution grism at the J atmospheric window The main lines are marked 0 00004 A Xenon and Neon arc spectrum taken with the medium resolution grism at the H atmospheric window The main lines are marked 0004 vill SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 ix A 6 A Xenon and Neon arc spectrum taken with the medium resolution grism at the K B 1 C 1 E 1 E 2 E 3 E 4 E 5 atmospheric window The main lines are marked 64 The atmospheric transmission at a resolution of 8A Most of the SOFT filters plus some additional ones from ISAAC are marked o a 66 Positions of the offsets in the sSlit 0000000000 00002 0G 94 Finding charts for the photometric standards of Persson et al Finding charts for the photometric standards of Persson et al Finding charts for the
99. es List of DITs List of NDIT Spectral Mode List Slit List Spectral Lamp List Parameter signature Exposure Name Number of columns Number of rows First column of window First row of window Number of exposures List of DITs List of NDIT Spectral Mode List Slit List Header Keyword DET EXP NAME DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO SEQ DIT LIST SEQ NDIT LIST SEQ SPECTROMODELIST SEQ SLIT LIST SEQ LAMP LIST Value SOFI 1024 1024 1 1 1 N N ODEFAULT NODEFAULT NODEFAULT NODEFAULT Table C 37 SOFI_spec_cal_Arcs Header Keyword DET EXP NAME DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO SEQ DIT LIST SEQ NDIT LIST SEQ SPECTROMODELIST SEQ SLIT LIST Table C 38 SOFLspec_cal_DomeFlats Value SOFI 1024 1024 1 1 1 NODEFAU NODEFAU NODEFAU NODEFAU LSO MAN ESO 40100 0004 Description File name prefix Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures Detector Integration Time individual exposure sec Number of DITs averaged into an individual image List of Spectroscopic Modes BRZJNB_1 061 H K Slit list 0 6 1 2 Lamp list N none B both Xe Xenon Ne Neon Description File name prefix Number of columns in the window Number of rows in the window First column of window First row of window Number
100. es 12 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Table 3 11 Parameters in the SOFI_img_cal_Darks template with commonly used values In this template one can enter a list of DITs and NDITs If the number of exposures is greater that the number of elements in either of these lists the list is repeated until the correct number of exposures have been completed The structure seen in the darks is quite complicated In general it is not a linear function of time The signal is made of several components shading a component which depends on the DIT and on the incident flux heat from the readout amplifiers commonly referred to as amplifier glow and classical dark current from the random generation of electron hole pairs The heat status from the readout amplifiers is a function of the number of reads only whereas the dark current is a linear function of the time Other than depending on the DIT the shading pattern seen in darks frames also depends on the incident flux Thus subtracting a dark frame from a science frame with the same DIT will not remove the shading pattern perfectly This issue is discussed in more detail in the next section The experience has shown that the best way to remove the dark current is by taking data in exactly the same way and with exactly the same exposure time as the science observations This is routinely done for the removal of the sky emission so the remova
101. ese images are necessary to remove the bias dark scattered light contribution Note that there is no need to take the arc calibration with non destructive readout because the DIT can always be increased to ensure that the arc spectrum has a sufficient signal to noise ratio There is also no reason to match the readout mode of the arc calibration with the readout mode of the data SOFI_spec_cal_DomeFlats and SOFI_spec_cal_NonDestrDomeF lats Spectroscopic dome flats are taken with these two calibration templates Table C 38 and C 39 The only difference between the two templates is the readout mode it has to match the readout mode used to obtain the scientific observations For each element in SEQ DIT LIST and SEQ NDIT LIST the templates take four images the first with the dome lamp off the next two with the dome lamp on and the fourth one with the dome lamp off The intensity of the dome lamp is controlled manually Note that in case of SOFI_spec_cal_NonDestrDomeFlats it is not crucial that NSAMP and NSAMPPIX are the same for the science frames and calibrations In fact this is sometimes impossible because the DIT of the scientific exposure is rather long often in order of 5 10 min Good values for the parameters of the NonDestrDomeFlat templates are DIT 10 NSAMP 6 NSAMPPIX 4 SOFT User s Manual 2 0 Parameter signature Exposure Name Number of columns Number of rows First column of window First row of window Number of exposur
102. f from the image with the lamp on to form the flat field Small IRAF and MIDAS procedures performing these steps are available at the telescope and from the SOFI web page in the Data Reduction section named special_flat cl and midas specialflat pre respectively The illumination corrections removes the difference between the illumination pattern of the dome flat screen and the sky It is derived from a grid of 9 16 observations of a star preferably a standard across the field of view The illumination correction is created by fitting a plane to the fluxes not magnitudes of the star after it has been flat fielded Therefore each illumination correction surface refers to a particular special dome flat and it can not be used with another special dome flat Special Dome Flats and Illumination Correction Surfaces for all broad band filters for Large Field mode are prepared by the observatory staff monthly The latest versions can be downloaded from the Web Page of SOFT http www 1ls eso org lasilla sciops ntt sofi reduction flat fielding html A data base with older calibrations is maintained as well http www ls eso org lasilla sciops ntt sofi images fits Archive An IRAF script for producing Illumination Correction Surfaces illumination cl is offered to the users It can be downloaded from the SOFI web page http www 1s eso org lasilla sciops ntt sofi reduction sofi scripts Twilight sky flats and flats created from the ob
103. first and the last principle of data reduction is to look at the data The data do contain all answers you seek We urge the users to experiment with differnet techniques combination and rejection algorithms and to look at the quality of the final data product We also warn the users that pipelines as good as they might be are dangerous tools because they separate the observer from the data Therefore the products from any automatic data reduction tools must be treated with caution First and foremost one must understand the data the specific instrumental effects the stability of the weather conditions during the observations etc 5 1 Basic Concepts Here we list some basic concepts for the SOFI data e There are two read out methods available with SOFI NDR non destructive and DCR double correlated Data taken in one readout method should NOT be used to calibrate data taken in another an exception are the spectroscopic arcs e The exposure time keyword represents the total integration time for a single image i e DITXNDIT As the image you receive is the average of NDIT exposures of DIT seconds the correct number to use for flux calibration is DIT and not the value given in the exposure time keyword 5 2 Imaging 5 2 1 Inter quadrant Row Cross Talk This is the first step of the data reduction but some specific cases described bellow require post processing of the final image to remove cross talk residuals A bright source
104. flat field out In addition there are some variable features i e dark regions caused by dust particles that have fallen inside the instrument Rotation of the instrument can remove them Occasionally the detector array shows elevated noise pattern The effect is probably caused by in terference and despite the replacement of most electronic components it remained unreproducible Usually it is confined to a single quadrant The noise amplitude is only a few ADU and it adds negligible error in imaging or low resolution spectroscopy mode However in high resolution spec troscopy mode especially when observing faint targets it is advisable to select nodding and jittering parameters placing the target in the unaffected quadrants 2 5 3 Windowed Reading The SOFT detector can be windowed Each window is defined by the starting pixel coordinates and the size of the windowed region The entire array is still read out by the IRACE controller however only the windowed section is transferred to the workstation This leads to a slight overhead decrease For position angle PA 0 deg starting coordinates of 1 1 correspond to the North East corner and they increase toward South West respectively 14 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 2 6 Calibration Unit The SOFT calibration unit is located inside the telescope adapter It contains a halogen lamp for internal spectroscopic flat fielding Xenon and Neon lamps for wavelength calibration
105. he NTT tracks very well over most of the sky For IR imaging and some spectroscopic observa tions the exposures are short enough that there is no need to guide Guiding is necessary only for spectroscopy of faint targets with exposures of 10 15 min or longer SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Observing Mode Description LARGE_FIELD_IMAGING Imaging with the large field objective LARGE_FIELD_SO_IMAGING Imaging with the large field objective SMALL_FIELD_IMAGING Imaging with the small field objective POLARIMETRY Polarimetric mode LONG_SLIT_RED Long slit spectroscopy with the red grism LONG_SLIT_BLUE Long slit spectroscopy with the blue grism LONG_SLIT_K 2 0 2 3 micron long slit spectroscopy with the medium resolution grism LONG_SLIT_H 1 5 1 8 micron long slit spectroscopy with the medium resolution grism DARK Dark exposures UNDEFINED Undefined Mode Table 3 14 The currently supported SOFI observing modes Nota Bene If you choose to guide you should be aware that the guiding mode is currently fixed to star2box as opposed to box2star and this option has to be chosen in templates that allow to specify the guiding mode 3 6 3 SOFT Observing Modes For SOFI there are several standard observing modes For example there are modes for wide field imaging and long slit spectroscopy The observing mode defines which optical elements are inserted into the beam As an example in the large field imaging mode the grism wheel
106. he Night and at the End of Your Run We will provide you with one set of CDs DVDs tapes as requested in the Data Backup Request Form that the visitors must fill in at the beginning of their run with all the raw data that were acquired during your run If you need more than one set you should make the copies yourself we can provide blank media and we will help you to start with the copying If you want to save reduced data you must do it yourself In that case you should save a copy of your data at the end of the night Do not do it at the end of your run It is common to generate over 2Gb of data per night with SOFT Leaving the back up until the end of the run mz mean that you could miss your plane The raw data is stored in the directories data raw Y Y Y Y MM DD on the machine wgJdoff At the end of your run you should fill out the End of Mission Report This can be done via any web browser You will find the form on the La Silla home page http www 1s eso org lasilla index html and click on End of Mission Report Chapter 5 Data Reduction This chapter does not aim to be the ultimate guide to reducing SOFI data but it does outline the general steps it does provides useful tips and in effect it can be used as a mini cook book It represents the experience the NTT team has gained in reducing data from SOFI This section of the manual will evolve with time and we are very keen to hear of any suggestions that people may have The
107. he first The usual practice is to take two sets of observations with a rotational offset of 45 degrees This can be done with the Rotator Offset parameter The approximate rotation centers for the imaging modes are listed in Table 3 8 Objective Rotation Center x y Large Field 517 504 Small Field 501 505 Spectroscopic Objective 537 502 Table 3 8 Approximate mechanical rotation centers for the imaging modes Note that these values change after each instrument intervention There are two ways to do this In the first method one includes two observation templates for each polarimetric observation one with the Rotator Offset angle set to zero and the second with the Rotator Offset angle set to 45 Alternatively one can set the rotator angle through the acquisition template See Appendix C and keep the Rotator Offset angle zero in both case Since the rotator axis does not correspond to the center of the array the later method is preferable when one is trying to determine the linear polarization of a single object For those who wish to map the polarization over a large field either method can be used 28 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 3 4 Spectroscopy 3 4 1 Small Objects and Uncrowded Fields In spectroscopy like imaging accurate sky cancellation is important Ifthe object is small enough the object can be observed at different slit positions Sky cancellation is t
108. hen achieved by subtracting one frame from another This is a very efficient method because it is not necessary to spend extra time in tegrating off the target This technique is embodied in the template SOFI_spec_obs_AutoNodOnsSlit Typical parameters for this template are listed in Table 3 9 Parameter signature Value Exposure Name Hubble_Deep_Field DIT 60 NDIT 3 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Spectro Mode LONG_SLIT_RED Which Slit long slit_1 Combined Offset T F T Jitter Box Width arcsec 10 Return to Origin T F T Nod Throw Along Slit arcsec 60 NINT 3 Number of Cycles 4 Table 3 9 Parameters of the template SOFI_spec_obs_AutoNodOnSlit with commonly used values In this template the telescope nods the object between two positions along the slit that are Nod Throw Along Slit arc seconds apart For convenience we will call one of these positions position A and the other position B At position A the object is observed for 9 minutes and the observer will receive 3 frames specified by NINT each the average of 3 specified by NDIT exposures of 60 second specified by DIT The telescope then moves to position B where it again integrates for nine minutes producing three frames each the average of three 60 second exposures This completes one cycle The number of A B pairs also called cycles is defined by the parameter Number of AB or BA cycles
109. here a is the intensity of the source which transmits light that is polarized at angle a We have assumed that the rotator is at a position angle of O degrees for the first measurement This need not be the case The degree of linear polarization and the polarization angle are given by JOR P I U 0 0 5 x tan t Q To derive the correct value of 0 attention needs to be paid to the signs of U and Q This algorithm neglects instrumental polarization Preliminary measurements indicate that the in strument polarization is 2 As it is caused by the tertiary mirror the vector defining the instrument SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 11 induced polarization will rotate relative to the sky A method to eliminate the instrumental polariza tion is outlined by Sperello di Serego Alighieri 1989 Proceedings of 1st ESO ST ECF Data Analysis Workshop Technical reports have been released which describe in details this mode and its operation They are available on the SOFI web page Wolf Vanzi amp Ageorges 2002 http www 1s eso org lasilla sciops ntt so 2 5 The DCS Detector Control System The DCS is made up of the detector the front end electronics and the controller IRACE InfraRed Array Control Electronics The IRACE controller controls the detector front end electronics and manages pre processing of the data before sending it to the SOFI workstation It includes an embedded Sparc and two sets of transp
110. imetric Dome Flat Fields SOFIimg cal_PolarimDomeF lats Arcs spectroscopy SOFI spec cal_Arcs Spectroscopic Dome Flats SOFLspec cal_DomeFlats or for different readout modes SOFI spec_cal_DomeF latsNonDestr Spectroscopic Adapter Flats SOFT spec cal_Flats Table C 4 Short guide for calibration templates SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 C 2 SOFI Imaging Templates C 2 1 SOFI Imaging Acquisition Templates Acquisition Templates determine how a target field is acquired by the telescope In case of Sofl some instrument parameters can be set up in the Acquisition Templates such as the instrument mode There are two acquisition template for simple imaging SOFI_img_acq_Preset This acquisition template Table C 5 does a simple telescope preset i e pointing in case of imaging observations to the coordinates of the Target associated with the Observing Block The Differential tracking rates can be set using TEL TARG ADDVELALPHA and TEL TARG ADDVELDELTA There is no operator intervention no acquisition image is saved on disk To save time the instrument and detector parameters most importantly the instrument mode and the filters can be set to values that will be used in the first Observation Template attached with this Observing Block An exception is the NDIT which should be set to 1 to save time because even after the telescope points at the target the next template will not start until the NDITxDIT integrations
111. imilarly the Js and J filters differ mostly in the long wavelength edge the transmission of the broader J filter is in practice limited by the atmosphere and therefore the photometry in the J filter 8 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Filter Filter Central Wavelength Width Peak Transmission Name Wheel um um Z 1 0 9 0 140 J 1 1 247 0 290 Js 1 1 240 0 160 H 1 1 653 0 297 83 K 1 2 162 0 275 88 NB 1 061 1 1 061 0 010 NB 1 083 Hel J 2 1 083 0 016 61 NB 1 187 1 1 187 0 010 NB 1 215 2 1 215 0 018 NB 1 257 Fell J 2 1 257 0 019 NB 1 282 PG 2 1 282 0 019 NB 1 644 Fell H 2 1 644 0 025 NB 1 710 2 1 710 0 026 NB 2 059 Hel K 2 2 059 0 028 81 NB 2 090 1 2 090 0 020 NB 2 124 H S1 2 2 124 0 028 78 NB 2 170 Bry 2 2 167 0 028 71 NB 2 195 1 2 195 0 030 NB 2 248 2 2 248 0 030 NB 2 280 2 2 280 0 030 69 NB 2 336 CO 2 2 336 0 031 GBF 1 0 925 cut on GRF 1 1 424 cut on Open l and 2 Closed 1 and 2 Table 2 2 The broad and narrow band filters available with SOFI is less stable than with the narrower J one However the J filter allows about 20 less photons to reach the detector than the J filter 2 3 Long Slit Spectroscopy SOFI offers low and medium resolution long slit spectroscopy The large field objective is used in this mode instead of the spectroscopic one because the former is achromatic There are three grisms available with SOFI two low resolution grisms and
112. inal position making it especially conve nient for obtaining a sequence of images in different filters on the same field The parameter Rotate Pupil is an option which allows you to rotate the instrument in between the sky an object positions so that the one obtains better sky cancellation For imaging in filters with high backgrounds that is the K and narrow band filters with central wavelengths greater than 2 2 microns we recommend that you set this option to T true For a more detailed discussion about guiding options the algorithm used to compute the offsets and the reasoning behind the option to rotate the pupil please to refer to section 3 6 If you wish to enter the offsets manually use the template SOFI_img_obs_JitterOffset If one wishes to use a more complex pattern that does not involve observing the object and the sky alterna tively use the either SOFI_img_obs_Jitter or SOFI_img_obs_GenericImaging These templates are discussed fully in The SOFI TSF Parameters Reference Guide NOTA BENE The offset to clear sky field introduces extra 130 150 overhead because we have to spend on the sky the same amount of time as on the object sky plus the usual 30 50 overheads for readout telescope offsets etc Therefore it is recommended if possible to take advantage of the SOFI Large Field mode and to observe such targets in the same way as compact targets see section 3 2 2 3 2 4 Maps of Large Fields To cov
113. ion Time individual exposure sec Number of DITs averaged into an individual image Filter wheel 1 position Filter wheel 2 position Alpha Offset for sky subtraction arcsec Delta Offset for sky subtraction arcsec Additional tracking velocity in RA arcsec sec Additional tracking velocity in DEC arcsec sec Rotation offset on the sky degrees T guiding ON F guiding OFF T full preset F fine tunning of the pointing T preserve the acq image F no Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures in the sequence Filter wheel 1 position Filter wheel 2 position T guiding ON F guiding OFF Returns the telescope to the original pointing if True list of offsets along rows list of offsets along columns Rotator offset degrees 85 86 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name NGC6118 DIT 20 NDIT 3 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposures 5 Filter wheel 1 Ks Filter wheel 2 open Instrument Mode LARGE_FIELD_IMAGING Combined offset F T T Return to origin T F T X offset list arcsec 0 50 0 100 0 Y offset list arcsec 0 50 100 0 1
114. is section is available through the manual but here it is concentrated for the convenience of the user 3 6 1 Choosing DIT NDIT and NINT The appropriate value of DIT depends on the intensity of the source and the background First and foremost the DIT must be kept short enough that moderately bright objects of scientific interest do not saturate On the other hand in order to maximize the S N ratio one would like to work in background limited instrument performance i e the sky intensity in a single frame should be 36 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 sufficiently high that the sky shot noise will dominate over the detector read out noise In the K band where the sky is bright this can be reached with DITs of one second Through the spectroscopic modes where the background is lower longer DITs are necessary For most observations this will mean a DIT between 10 and 60 seconds In addition shorter DIT means larger overheads because every DIT is associated with certain fixed overheads i e for reset and read out and the shorter the DIT the more DITs it will take to complete the desired total integration time Once the DIT is chosen the NDIT can be determined from the desired offsetting frequency During good weather conditions one can stay on the object as long as 2 min for broad band imaging 5 min for narrow band imaging and 15 min for spectroscopy before switching to the sky and in this case NDIT 2min DIT i
115. is set to open the mask wheel is set to the large field mask and the objective wheel is set to the wide field objective In addition the collimator used for focusing the instrument is moved to a predefined position depending on the mode and the filter The standard modes available with SOFI are listed in the following Table 3 14 3 6 4 Template Parameters Signatures and Keywords Template parameters can be described either by their signature which are aimed at being self explanatory or by their keyword which are more meaningful to the OS Observing Software When a template is created with P2PP the signature form of the parameter is displayed When the OB is passed to the BOB Broker of Observing Blocks for execution signatures are translated into keywords Although users will mostly be faced with signatures they will later see the corresponding keywords either when BOB is executing the template or in the FIT S image headers where some of these keywords are stored As an example setting the filter on filter wheel 1 is done with the signature Filter wheel 1 whereas the corresponding keyword is INS FILT1 ID 3 6 5 File Naming and Exposure Number The parameter Exposure Name determines the name the output files will be given on the instru ment workstation The raw data become available to the user under different names consisting of an abbreviation of the instrument and a time stamp indicating the time the file was c
116. ith the telescope at an altitude where the bulk of the observations will be done iii Good focus is very important Poor focus can result in distorted images if any aberrations are present In fact distorted images are a good sign that either the telescope needs to be refocused or another image analysis needs to be done The focus depends linearly with the temperature of the four serurriers The coefficient is 0 079 mm of M2 movement per degree In good seeing a displacement in M2 of 0 03 mm induces a noticeable degradation in the image quality So if the temperature has changed by 0 5 degrees or more since the last focus one should refocus On La Silla the temperature changes quickest during the first hour of the night iv During long integrations greater that five minutes it is possible to run the NTT in closed loop SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 mode that is the optics are adjusted during the observations For SOFI integrations are usually shorter and this is usually not needed NOTA BENE The best way to find out if it is time for another image analysis is to monitor the image quality of the incoming raw data and to request an image analysis if you notice degradation As a general guideline however you should trust the TiO s experience 4 3 2 Focusing The telescope and the instrument are focused separately The instrument is focused automatically to a pre defined collimator positions depending on the i
117. ition of the array to another the operator has two options 1 If the target must be centered on the slit one can use the build in feature by clicking on the center button on the RTD dialog box The software will draw an arrow from the location of the selected object to the center of the slit and the operator will be asked to confirm the offset This option is faster and it works well if the science observation is done via simple nodding along the slit 2 The operator can simply draw an arrow on the screen holding the left hand side button of the mouse At this point a window which lists the pixel co ordinates at the start and the end of the arrow will appear The operator can accept the offsets cancel or edit the co ordinates manually If the offsets are accepted the telescope offsets by the desired amount Finally the operator is given the possibility to redraw the arrow for refining the position of the target if necessary Once the operator is satisfied the template finishes Nota Bene There is no way of checking if the slit that is chosen in the Acquisition Template and the slit that will be used subsequently in the Observation Description Templates are the same It is therefore of utmost importance that the astronomer ensures that they are the same Since the spectroscopic modes use the large field objective by default the instrument mode is not a parameter of the template Guiding is possible defaulted to the Telescope Co
118. ity of the lamps is slightly different 3 5 4 Arcs Wavelength calibration can be done with the Xenon and Neon lamps in the adapter The Xenon lamp has a better distribution of lines and is by itself sufficient to calibrate the wavelength scale of data taken with the low resolution grisms Both the Neon and Xenon lamps should be used to calibrate the medium resolution grisms Arcs are taken with the template SOFI_spec_cal_Arcs The parameters in this template are dis played in Table 3 13 In this template one can enter a list of DITs NDITs grisms slits and lamps SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 35 If the number of exposures is greater that the number of elements in either of these lists the list is repeated until the the correct number of exposures have been completed The choices for the lamps are Xe Xenon Ne Neon B Both and N None The choices for the slit are 0 6 1 and 2 The choices for the spectral mode are B and R for low resolution grisms H and K for the high resolution grism Although the template enables one to do this it is best to keep things simple i e just takes arcs with one slit and if another slit is required run the template again with that slit The Xenon lamp warms up quickly and produces high background level in the arcs notably when calibrating the red grism To minimize the background the user may want to split the time when the lamp is on giving the lamp some time to cool To do thi
119. ken and SEQ NEXPO 1 2 pairs of object sky frames are taken Users are encouraged to give the parameter SEQ NEXPO an even value Guiding is only possible for the object frames if SEQ COMBINED OFFSET is set to True T must be done by the observed during the preparation of the Observing Block and guiding is started before the template starts done by the operator after warning from the observer By default there is no guiding for the sky frames This template can also be used to observe small extended objects by setting the throw small enough so that the object is always within the field of view In such a case however SEQ ROTPUPIL should be set to False F Using this template with a jitter box size parameter SEQ JITTER WIDTH set to zero is equivalent to staring on the object For the pupil rotation see the discussion in Sec 3 6 Consider the following example Table C 10 the sky is taken at 600 arcsec of the object and a small jitter of 20 arcsec around the target is done The number of exposures corresponds to the TOTAL number of exposures that means SCIENCE exposures SKY exposures So here you will have 5 images of your objects and 5 images of the sky Each image is the average of 10 NDIT exposures of 6 sec DIT No guiding is used SOFI_img_obs_Jitter This observation template Table C 11 allows the user to offset the telescope between exposures according to a list of predefined offsets SEQ OFFSETALPHA LIST and
120. l of the dark is naturally removed when the sky is removed The only case when the removal of the dark remains an issue is the flat fields see the next section for discussion Darks appear to be stable over the period of a typical observing run Thus it is sufficient to take darks only once during the run or more often as a verification of the detector array status 32 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 3 5 2 Flat Fields As in the case of visible observations one must correct for differences in pixel sensitivities The method for creating flat fields is identical in both the spectroscopic and imaging modes They are created by exposing alternatively an illuminated and an unilluminated dome panel The flat field images are constructed from the difference of the two normalized to one This technique is especially important when working beyond 2 3 microns where it removes the thermal component of the signal which does not depend on the intensity of the flat field lamp Spectroscopic flats must be taken with the same slit as the observations Imaging flat fields can also be obtained from the twilight sky or from the observations themselves However dome flats represent better the low frequency sensitivity variations of the array Twilight spectroscopic flats cannot be used to flat field your data although they can be used to determine the slit transmition function for spectroscopy of extended sources The slit transmition f
121. lters a focus pyramid an open position and a fully closed position The grism wheel contains three grisms for long slit spectroscopy a Wollaston prism for imaging polarimetry an open position and a fully closed position The objective wheel contains two objectives for imaging at 0288 large field and 0 144 small field per pixel a spectroscopic objective an open position and a fully closed position 2 2 Imaging SOFT offers imaging at several different pixel scales The pixel scales and the fields of view are summarized in Table 2 1 The large and small field objectives are used with the corresponding mask in the mask wheel that covers the same field of view The masks reduce the amount of stray light entering the instrument For a normal use of SOFT rotator angle 0 deg the orientation is showed in the Figure 2 2 You could modify the orientation simply using a rotator angle in the template This option could be useful in spectroscopy if you want to align two objects in the slit the positive angle is from the north to the east or in imaging if you want to map some asymmetric target i e an elongated galaxy A simple rule of thumb says to align the North South axis of the field of view or the slit in case of spectroscopy along an axis on the sky with a certain Position Angle one has to apply in the acquisition template a rotaton angle equal to this Position Angle 6 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 COLLIMATO
122. mage is a function of the flux incident on the array so it is not always useful to subtract separately the dark frame In fact the bias and the dark contributions to the total signal are almost impossible to separate without special efforts In cases where the mean count level is low for example narrow band images the dark frame is a good approximation to the underlying bias pattern In cases where the mean flux level is high for example broad band images the dark frame is a poor approximation to the underlying bias pattern so subtracting it serves no real purpose Therefore we suggest to avoid separate dark subtraction Instead the user can subtract the darks together with the sky as described in the next section The advantage of this approach is that the overall illumination is nearly constant so the varying bias level problem mentioned above will be avoided The only case when the dark bias subtraction is still necessary is the reduction of the flat field calibration images See the description of the special procedure for that in Section 5 2 5 5 2 4 Sky Subtraction This is a critical operation which depends on how many frames are available to create a sky image The experience shows that at least 5 7 images are necessary to achieve acceptable sky subtraction and this number depends strongly on the nature of the sky field if the field is crowded with stars i e the Galactic plane this may not be enough In such cases we recommend
123. mg obs_AutoJitterOffset template the images are now taken in a user defined sequence instead of in a random jittering pattern The first offset 0 0 is executed before taking the first image the second offset 5 5 is executed before taking the second image and so on The number of exposures corresponds to the TOTAL number of exposures Each image is the average of 10 NDIT exposures of 6 sec DIT No guiding is used SOFT_img obs_AutoJitterArray and SOFI_img_obs_AutoJitterArray 1 These templates Table C 16 are very similar to SOFIimg _obs_Jitter They define a sequence of offsets in RA and DEC but in addition to that they allow to jitter randomly around each of the defined offset array positions They are useful to build maps of large areas of the sky SEQ OFFSETALPHA LIST and SEQ OFFSETDELTA LIST are used to define the map array on the sky The offsets between different positions are relative and they do not have to be equidistant i e irregularly shaper maps arrays are acceptable If you have less offsets than number of exposures SOFT User s Manual 2 0 Parameter signature Exposure Name DIT NDIT Number of columns Number of rows First column of window First row of window Number of Exposures Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset T F Return to origin T F RA offset list arcsec DEC offset list arcsec Rotate Pupil LSO MAN ESO 40100 0004 77 Value NGC6118 6 10 1024 10
124. mn of window First row of window Spectroscopic Mode Which Slit T guiding ON F guiding OFF Jitter box size Returns the telescope to the original pointing if True Nod throw arcsec Number of exposures in each A or B position Number of AB cycles 90 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 position along the slit In addition to nodding random typically small a few tens of arcsec or smaller offsets can be added in the middle of a cycle to locate the spectra at different positions on the array improving the bad pixel removal and the flat fielding A sequence of 4 cycles with jittering will result in the following sequence A B e1 B e1 A e2 A e2 B e3 B e5 A e4 where ey are the random jittering offsets They are generated withing the interval defined by SEQ JITTER WIDTH in arcsec If SEQ JITTER WIDTH is set to zero then the template will just nod between A and B It is recommended to use a jitter box width smaller or equal to the nodding length to avoid any chance of overlapping of the spectra on sequential images If SEQ COMBINED OFSET is set to T and if guiding was started before the start of the template the telescope will guide with Star To Box Guiding is recommended for observations longer than 15 min but close to the meridian or the zenith it may be required even for shorter sequences The templates have been optimized to minimize overheads However after a telescope offset one or more DIT
125. n OB is transfered to BOB any changes that the observer make to the OB in the P2PP have no effect on the observations unless the OB is loaded into BOB again In general the highlighting of the OBs selected for execution is all the observer needs to do Some observer intervention may be required during the execution of an observing block For example the user may need to select the correct object to be put in the slit or to indicate an offset during the acquisition Everything else is either automatic or it is done by the support staff of the observatory The users are advised to use the available time for inspection of the raw data BOB is a very versatile tool It can be used to display the contents of an OB it can skip templates within an observing block it can be used to pause at a template and it even allows to edit the parameters of an OB but only in the templates that have not been started yet Thanks to this facility 1t is possible to fine tune the execution of an OB in real time However with this freedom comes the danger to make a mistake especially during a long and stressful observing night Therefore we recommend to make changes within the P2PP and then to re load the edited OB into BOB This provides an extra layer of verification because the P2PP has some built in tools for verification of the OBs BOB runs a Tel script which sends commands to the OS Observing Software which then sends commands to the TCS Telescope Control Sy
126. ncomplete headers The OS GUI is used only for trouble shooting 4 5 RTD The SOFI RTD Real Time Display Figure 4 2 is a versatile tool of frequent use As the array is continuously read out images are continuously displayed on the RTD During acquisition or when the instrument is idle the data is still taken with the last used DIT and NDIT but it is not written to disk A very useful feature of the RTD is the ability to store a frame into memory buffer and to subtract that frame from incoming frames In effect this feature allows a simple sky subtraction This is essential for recognizing faint objects on backgrounds with significant structure The stored frame is called a fixed pattern and to activate this feature first click on the button labeled Store Fixed Pattern on the RTD screen and the current image will be stored Next click on ON OFF to subtract the stored images from the following ones You ll probably have to click on the button labeled Auto Cuts or edit the cut levels directly to see the image Clicking on Store Fixed Pattern again will load a new fixed pattern Naturally it is best to store a fixed pattern before the telescope is centered on your science target SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 NA Cars TTP CRU PVE MA AA IATA te prm a pe rem eae Cabas cnm tgo Fie Bow TES STESA l onserer gt SOFI radi DARK sort_esaz ONLINE a n tems WAC Mew Dats
127. need to log in e Ina workspace of your choice you can start either an IRAF or MIDAS Both reduction packages are supported Your data is stored as FITS files in the directory data raw Y YY Y MM DD where YY YY MM DD is the date at the beginning of the observing night We suggest that you do all your reductions in this directory e At the beginning of your first observing night fill in the Data Backup Request Form available from the La Silla web page Note that you are requested to press the submit button twice first after you have filled in the form and the second time after you have verified the data Based upon the information in this form the observatory staff will prepare a data package for you at the morning after your last observing night It is important to fill in the correct postal address we may need it in case we have to contact you or mail you some data later The data package includes only the raw data Observers are responsible for saving their own reduced data CD and DVD writers are available for this purpose e This machine runs the important dataSuscriber program responsible for transferring the data from the instrument work station to the archive and to the off line machine Please make sure NOT to stop it e There is a number of quick look data reductions tools available on wgdoff for imaging and spectroscopic observations for photometric calibration Your support astronomer will introduce to them SOFT User s Man
128. nstrument configuration This process is fully transparent to the user The telescope focusing can be done with a focus pyramid that produces five separate images There is a specific template to do the focusing Note that the image analysis normally corrects for the telescope focus so the focus pyramid is used mostly for health check and the only change of the focus that actually has to be done manually is the correction for the temperature of the primary mirror All this is done by the TIO The role of the visitor is to monitor the quality of the incoming raw data for any degradation 4 4 The SOFT OS GUI Panel The SOFI OS GUI panel Figure 4 1 is used by the instrument operator to show the status of the instrument It can also be used to setup the instrument the detector execute exposures and even point the telescope A detailed description of this panel is given in the OS manual VLT MAN ESO 14100 1510 In addition to the OS GUI panel there are various panels related to ICS DCS and IRACE and a large number panels that show the status of the instrument and the detector These panels are not of direct interest to the observer We do not allow observers to use this panel to start exposures as it is far more efficient and safer to observe with OBs running on BOB Furthermore data archiving a fundamental requirement at the NTT and the VLT is only possible if frames are accurately classified because the images taken via the OS panel have i
129. ntrol System s option Star To Box only if SEQ COMBINED OFFSET is set to T this must be done by the observed during the preparation of the Observing Block and if guiding is on before the start of the science templates this will be done by the operator after warning by the observer before starting the execution of the Observing block The experience has shown that the guiding is necessary if the observer plans to stay at one pointing more than 15 min This is rarely the case in imaging mode but can often happen during spectroscopic observations The SEQ PRESET parameter allows the observer to use the fine tunning pointing functionality of the 88 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Header Keyword Value Description DIT DET DIT NODEFAULT Detector Integration Time individual exposure sec NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Which Slit INS WHICHSLIT NODEFAULT Slit Alpha Offset arcsec TEL TARG OFFSETALPHA 0 0 Alpha Offset for sky subtraction arcsec Delta Offset arcsec TEL TARG OFFSETDELTA 0 0 Delta Offset for sky subtraction arcsec Add Velocity Alpha TEL TARG ADDVELALPHA 0 0 Additional tracking velocity in RA arcsec sec Add Delta Velocity TEL TARG ADDVELDELTA 0 0 Additional tracking velocity in DEC arcsec sec Rotation Off
130. nuity of a few percent in the ZP across the center of the array For most programs this is not a major problem Nevertheless we have developed a technique to remove this residual and this technique is embodied in the template SOFI_img_cal_SpecialDomeFlats This template in addition to the four frames taken with the template SOFI_spec_cal_DomeFlats takes frames with the mask partially obscuring the array Therefore each exposure generates a sequence of 8 frames In addition to them 1 2 or more images are taken at the beginning in order to adjust and check the illumination level set to 4000 6000 ADU so the total number of the obtained images by this template may vary Note that only the last 8 images are actually the flat field Half of this sequence is shown in Figure 3 3 If the parameter Number of Exposure is gt 1 a corresponding sequence of frames will be generated so generally use 1 These frames are used to estimate the shading Further details are given in the section describing data reduction Figure 3 3 Examples of Special Dome Flat images From left to right lamp off lamp off with mask lamp on with mask lamp on Flat Field frames for Large Field imaging with broad band filters are taken by the observatory staff on average once a months and they can be downloaded from the SOFI Web page The stability of the flats was studied over a period of about a year and it appears the degradation over time is minimal as can be
131. o the clear portion of the polarimetric mask in polari metric mode or to specific position on the array in imaging mode The interaction between the RTD and the acquisition templates is done by the TIO after the user specifies his hers requirements 4 6 The Data Flow Path The raw data passes from the detector to the IRACE controller where pre processing of the data occurs Exactly what the pre processing does depends on the readout method For example in non destructive readout mode the pre processing involves fitting a line through the individual readouts and multiplying the slope by the DIT Next the data is written to the disk of the instrument workstation wsofi as FITS files and it is displayed on the RTD Finally copies of the data are sent to the archive machine wgddhs and to the off line computer wgJdoff SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 During this transfer the file names are changed from exposure name plus number to instrument name plus time stamp The archival machine is used to produce the data package the user is given at noon after the last night of the observing run The offline machine is where the user can access the data in a directory data raw YYYY MM DD The data transfer processes are transparent to the user 4 6 1 Quick Look Data Reduction Tools and Pipeline It is foreseen to install a pipeline at the NTT in the future A new Sofl VLT compliant pipeline is available at the NTT
132. object S sky Guiding N B 8 SEQ GUIDING LIST NODEFAULT N no guiding B Box To Star S Star To Box Table C 18 SOFI_img_obs_GenericImaging do any sequence of telescope offsets whether they be combined offsets offsetting the telescope and the guide probe necessary in case of observations with guiding or non combined offsets offsetting the telescope alone when guiding is not used i e the telescope pointing changes on average at least once every 15 minutes However with this flexibility comes complexity This is a complicated template and it is meant to be used in situations in which none of the other templates is suitable Telescope offsets and guiding options are defined as lists in SEQ OFFSETALPHA LIST SEQ OFFSETDELTA LIST and SEQ GUIDING LIST The offsets are relative to the previous position they are in RA and DEC and they are in arcsec There are three guiding options N B or S For option N the offsets are non combined Option B stands for Box To Star but this option has been disabled and currently the default guiding mode is Star To Box Please always use option S that stands for Star To Boz Note With large combined offsets the guide probe may not be able to follow the one guide star In such a case the guiding system will automatically find another star and resume guiding If any of the entries in SEQ GUIDING LIST are different from N and if guiding is off at the beginning of the template a pop up windo
133. of the template After the last exposure the instrument is rotated back to the original position With this scheme it is possible for the user to sample the object and the sky as desired for one rotator position and then restart the template with another orientation on the sky for another series of exposures At least two different orientations separated by 45 degrees are required for computing the Stokes parameters This implies that the template must be called at least twice within an Observation Block or with two different Observation Blocks both with two different rotator positions The most likely situation will be to set SEQ ROT OFFANGLE to O degrees in the first template and then to 45 degrees in the second template Guiding is possible defaulted to Star To Box if SEQ COMBINED OFSET is set to T and if guiding was started before the start of the template If SEQ RETURN is set to True T the telescope slews back to its original position at the end of the template If not the telescope is not moved Note that the instrument mode is not a parameter of this template because all polarimetric obser vations are taken in Large Field mode This example Table C 26 gives the typical values of the parameters Note that to determine the Stokes parameters the template will have to be executed twice with a rotator offset of 45 degree C 3 3 SOFI Polarimetric Calibration Template SOFI_img_cal_PolarimDomeF lats This template Table C 27
134. on Frames hi hn goa A ADP eap Bo elas GP eles Sil Darks Biases ne sitios Bes bop eh A EA ts nok ee de 39 2 Flat RIGIAS ant co Se gh Ho aS leek dln oes en hes en ete Be ee ee Petes Les 3 5 3 Illumination Corrections OA ATEOS 24 BAO ea hoe 2 A A ea ep So ee ee a A oe 3 6 Finer Pots ite Sa a os ck OE ee a a ce Ve e 3 6 1 Choosing DIT NDIT and NINT 2 0 0 e i A 3 02 AUTOS su E ee A AAA ee ee 3 6 3 SOFI Observing Modes aaa 3 6 4 Template Parameters Signatures and Keywords vi 4 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 3 6 5 File Naming and Exposure Number 2 0220004 3 6 6 Detector Window g s e angoni eri Ba a a eG Phase 2 Preparation and Observing with SOFI 4 1 General Issues oca hath a we se Bw A ee BAR eS a da eas 4 2 The VLT environment P2PP BOB OS TCS DCS ICS 4 3 Arriving at the Telescope ee 4 3 1 Image Analysis 2 h a lo ina a ee AvS 2 HOCUSINGS serui Been A A eos Sty ee A Bk eS 44 TheSOFI OS GUI Panel ee ee es A RED i e Minis aes tetera MR ees hls caine ath ae pa Mean ma a iam eae okt Bs 4 6 The Data How Rabbit a aed et enh te eat eee ee A a As SR 4 6 1 Quick Look Data Reduction Tools and Pipeline 4 6 2 Fhe Archive cy ah a ee oe eA we oe ee ee E 4 6 3 The Calibration Plan aaa 4 7 At the End of the Night and at the End of Your Run
135. ool for sky subtracting of imaging data works for the spectroscopy as well 5 3 3 Flat Fields Spectroscopic flats are taken with the dome flat field screen Like imaging flats one subtracts an image with the dome lamp off from an image with the dome lamp on These flats also suffer from the shortcoming that the shade pattern is not perfectly removed However the slits do not cover the entire chip there is a region of approximately 50 pixels wide which is free of direct illumination and it may be possible to use this region to estimate the residual shade pattern and to correct the flat 5 3 4 Removing Slit Curvature Spectra taken with all SOFI grisms show slit curvature For the red and blue grisms the curvature amounts to a few pixels from the middle of the slit to the edge and is well fitted with a quadratic For point source observations removing the slit curvature is an unnecessary step However extended objects may require to correct it in order to avoid degradation of the spectral resolution The correction is done by doing a 2 dimensional wavelength solution to arc spectra and both MIDAS and IRAF have tasks to do this For the red and blue grisms a quadratic in the slit direction and a cubic or quartic in the wavelength direction is adequate There are no cross terms An excellent description of the method and the IRAF tools to do it can be find in the cookbook 2D Frutti Reductions with IRAF by Mario Hamuy and Lisa Wells The relevant
136. p panel and the typical broad K filter Persson et al 1998 AJ 116 2475 see their Table 10 The dotted blue line is the atmospheric transmission model for Mauna Kea for Am 1 0 and water vapor column of 1mm Lord S D 1992 NASA Technical Memor 103957 courtesy of Gemini Observatory The data for the plot and a SuperMongo script are available from the SOFI web page The grism can also be used in the higher orders with the J and Z filters However there is significant overlap between these orders so the useful wavelength range is limited Furthermore the line profile degrades in the blue so that the resolution is not significantly better than that obtained with the low resolution blue grism Three slits of different fixed widths 0 6 1 0 and 2 0 arc seconds are available The slit length is 290 arc seconds 2 4 Polarimetry SOFT offers imaging polarimetry A Wollaston prism in the grism wheel splits the incoming parallel beam into two beams that are perpendicularly polarized The beams are separated by 48 arc seconds Thus an image taken with the Wollaston prism will contain two images of every object To avoid sources overlapping a special mask consisting of alternating opaque and transmitting strips can be inserted at the focal plane Therefore in a single exposure at least half the field will be missing So three exposures with telescope offsets in between are required to cover one field 10 SOFT User s Manual 2 0 LSO M
137. plates Table C 29 and C 30 nod the telescope between two positions A and B along the slit A cycle is a pair of two observations AB or BA Cycles are repeated in ABBA se quences E g 3 cycles corresponds to an ABBAAB sequence 4 cycles correspond to an ABBAABBA sequence etc Each observation consist of one or more exposures as defined in the template by the NINT parameter The total number of frames corresponds to SEQ NABCYCLES x SEQ NINTx2 The mean amplitude of the nod is defined by SEQ NODTHROW in arcsec The first exposure A is done after offsetting the object along the slit by SEQ NODTHROW 2 arcsec which can be either negative or positive The second exposure B is therefore SEQ NODTHROW 2 arcsec from the initial SOFT User s Manual 2 0 Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Spectro Mode Which Slit Combined offset F T Jitter Box Width arcsec Return to Origin T F Nod Throw arcsec NINT Number of AB or BA cycles Header Keyword DET EXP NAME DET DIT DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY INS SMODE INS WHICHSLIT SEQ COMBINED OFFSET SEQ JITTER WIDTH SEQ RETURN SEQ NODTHROW SEQ NINT SEQ NABCYCLES Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 NODEFAULT NODEFAULT False 1 Table C 29 SOFI_spec_obs_AutoNodOnSlit Parameter signature
138. posures RA offset list arcsec DEC offset list arcsec Value NGC6118 20 3 1024 1024 1 1 5 J open 75 Description File name prefix Detector Integration Time individual exposure sec Number of DITs averaged into an individual image Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures in the sequence Filter wheel 1 position Filter wheel 2 position Instrument Mode T guiding ON F guiding OFF Returns the telescope to the original pointing if True list of offsets along RA list of offsets along Dec LARGE_FIELDIMAGING T T 0 50 0 100 0 0 50 100 0 100 Value 2 10 0 5 10 2 5 5 0 5 10 2 5 5 Table C 13 SOFLimg_obs_Jitter Offset Example SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix DET DIT DET DIT NODEFAULT Detector Integration Time individual exposure sec DET NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence Filter wheel 1 INS FILT1 ID NODEFAULT
139. r To Box and the offsets are handled as for B except that guiding is resumed with Star To Box With large combined offsets the guide probe may not be able to follow the one guide star In such a case the guiding system will automatically find another star and resume guiding If any of the entries in SEQ GUIDING LIST are different from N and if guiding is off at the beginning of the template a pop up window prompts the user to start guiding The observation type can be defined for each image and entered as a list in SEQ OBSTYPE LIST O stands for Object and S stands for Sky In addition to observations that require large sky offsets these templates are very useful for slit scanning across an object i e simulation of 3D spectroscopy by defining a list of offsets in the y direction perpendicular to the slit direction A third case when these templates can be used is when the user wants to obtain spectra at more than two positions across the slit then the offsets will be only in the x direction These templates cannot by nature be easily optimized After each exposure one or more DIT s are skipped At the end of the templates the telescope is returned to the original position if SEQ RETURN is set to true T If not the telescope is not moved The lists can have any length however having lists of different lengths can become extremely con fusing It is good practice to use either lists of one value or lists of equal length Th
140. r of the slit Since each slit has a different position on the detector the slit name i e the 0 6 1 or 2 arcsec slits is explicitly stated in the template via the INS WHICHSLIT parameter This acquisition has two parts First it presets the telescope to the coordinates of the Target associated with the Observation Block Next it takes and displays an image on the RTD Finally it requests the operator to interact with the RTD so that the user specified object is moved to either a center of the slit or to a user specified location on the slit To make the target clearly visible the telescope can do a small offset after the preset to obtain a sky image The size and direction of the offset are template parameters TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA The operator is then prompted to store a fixed pattern that is subsequently subtracted from the incoming images via the real time display RTD features After storing the fixed pattern the telescope returns to the original position At this point in time the operator is offered the possibility to change DIT and NDIT This is useful if the target can not be identify securely because of the low signal to noise If the operator changes the values the telescope offsets again and the operator is required to store another fixed pattern before the telescope returns to the nominal position This loop can continue until the operator has identified the target To move the target from one pos
141. ration Time individual exposure sec Number of DITs averaged into an individual image Number of Samples Sample Number per Reading Number of columns in the window Number of rows in the window First column of window First row of window Number of exposures Spectroscopic Mode Which Slit Returns the telescope to the original pointing if True List of offsets in X arcsec List of offsets in Y arcsec Observation type O object S sky Guiding B Box To Star S Star To Box N none Table C 34 SOFI_spec_obs_GenSpecNonDestr SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name NGC6118 DIT 40 NDIT 2 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposure 6 Spectro Mode LONG_SLIT_BLUE Which slit long slit_1 Return to origin T F T X offset list arcsec 0 15 0 30 0 15 Y offset list arcsec 0 0 10 10 10 10 Obs Type O or S OOSOSO Guiding N B S S Table C 35 SOFI_spec_obs_GenericSpectro Example TEL ROT OFFANGLEPA 0 deg There are three guiding options N B or S For option N the offsets are non combined Option B stands for Box To Star In this case the telescope offsets by first moving with a non combined offset to the position where guiding was last enabled and from there with a combined offset to the new position Guiding is then started with Box To Star Option S stands for Sta
142. reated i e SOFT 2005 02 03T10 17 22 354 fits where the time is UT However the exposure name is stored in 38 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 the fits header and some users may find it convenient to use it in order to rename the files to somewhat more intuitive names for easier data reduction The exposure names consist of a base name defined by the parameter Exposure Name plus an extension of the type _0003 fits The extension is automatically set If the base file name is found on disk the sequential number is automatically incremented For example if there exists a file fileName_0024 fits on disk the next file generated by the template is fileName_0025 fits If no file with the same base name is found on disk the sequential number is automatically set to 0001 see for an example Table 3 15 NOTA BENE Do not use spaces slashes or other non alphanumeric characters in file names and target names otherwise the SOFT OS will report an error However the underscore symbol 2 is acceptable Signature Keyword Default Description Input Value Exposure Name DET EXP NAME Exposure Base filename ngc1068 Number of Exposures SEQ NEXPO Si Number of exposures for 10 the template Table 3 15 File naming signatures and keywords The first filename will be nec1068_0001 fits if no file with the nec1068 string was found on disk and the last file will be nec1068_0010 fits 3 6 6 Detector Window
143. rs and the necessity of integrating sufficient photons to achieve background limited performance are such that the frequency is of the order of once per minute This sky subtraction technique has the additional advantage that it automatically removes offsets due to fixed electronic patterns bias and dark current NOTA BENE The sky and the object sky have to be sampled equally integrating more on the object sky than on the sky will not improve the overall signal to noise ratio because the noise will be dominated by the sky There are two standard techniques to estimate the sky The first is appropriate for angularly large objects or crowded fields and the second is appropriate for angularly small in comparison with the field of view objects or uncrowded fields 3 2 1 Selecting the best DIT and NDIT Selecting the best DIT and NDIT is a complex optimization problem and it depends on the nature of the program targets required signal to noise frequency of sky sampling etc Therefore it is hard to give general suggestions and the users should exercise their judgment and discuss their choices with the support astronomer in charge of their run The first constraint is to keep to signal from the target on the linear part of the detector array dynamic range which is bellow 10 000 ADU The minimum DIT of 1 183 sec allows to observe without problem stars of 10 mag under average seeing humidity conditions Keeping in the linear regime any bright
144. rument Mode INS IMODE NODEFAULT Instrument Mode Table C 21 SOFIimg_cal_DomeFlats and SOFI_img cal_SpecialDomeF lats unlike the other templates where the offsets are defined along the RA and Dec here they are defined along detector rows and columns and are entered into SEQ OFFSETX LIST and SEQ OFFSETY LIST Offsets are in arcsec Guiding is possible defaulted to Star To Bozx if SEQ COMB OFFSET is set to True T and if guiding was started before the start of the template If SEQ RETURN is set to True T the telescope slews back to its original position at the end of the template If not the telescope is not moved Although it is possible to observe standard stars with any of the imaging science templates using this one is recommended because it sets up properly the header keywords necessary for the pipeline data reduction of the standards In the next example Table C 23 SOFI_img cal_StandardStar is used to obtain 5 images the first on the acquisition position and the others defined by the list of offsets At the end you have an x pattern SOFT User s Manual 2 0 Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset F T Return to Origin T F X offsets list arcsec Y offsets list arcsec Header Keyword DET EXP NAME DET DIT
145. rvations Consider the following example Table C 8 the template will produce 6 fits files Each image is jittered regarding to the previous one and the jitter offsets are chosen randomly inside a box of 20 arcsec Jitter Box Width around the central position Each of them corresponds to the average of 6 exposures NDIT of 10sec DIT At the end of the exposures the telescope moves back to the preset position Return to origin True SOFI_img_obs_AutoJtterOffset This observation template Table C 9 allows the user to move the telescope alternatively between the object and a nearby patches of sky However when pointing to the object the position of the telescope is still randomly distributed within a box of size SEQ JITTER WIDTH in arcsec The offset skies are at a constant distance defined by SEQ SKYTHROW in arcsec from the original telescope position but at an angle randomly distributed between 0 and 360 degrees i e the offset skies are distributed on a circle surrounding the initial telescope position By default there is no telescope offset before the first exposure If SEQ RETURN is set to True T the telescope slews back to its original position at the end of the template If not the telescope is not moved It is assumed that odd numbered exposures are on the object and are consequently identified as SCIENCE frames Even numbered exposures are assumed to be sky frames and are consequently identified as OTHER
146. ry raw data frame has to be multiplied by this mask ensuring that all bad pixels will have the same value The second mask will have zero values at the good pixels and a large negative value i e 1e6 at the bad values This mask will have to be added to the row frames after the multiplication It will set the bad pixels to a value outside the good pixel values for SOFI Of course the fractional offset during the image alignment may move some of this artificial negative flux into the acceptable values but the large value makes sure that this will happen only rarely The final step of the application of the bad pixel mask is to set an appropriate lower threshold during the combination of the aligned images Indeed some data reduction tools like IRAF has the capability to incorporate a bad pixel mask and the shifting of the images into the combination step However the imcombine task can carry out only integer shifts and this will degrade the spatial resolution of the image This is the reason we recommend a 2 step shifting fist with fractional offsets and then with integer offsets see Section 5 2 6 The only possibility to apply the bad pixel mask in this case is before any of the offsets as described here 5 2 3 Subtracting the Dark Bias Frame First and foremost a SOFI dark frame changes with the DIT so if a dark frame is subtracted from other frames the DIT must be identical Secondly the underlying bias pattern to any i
147. s Therefore these template can execute sequences of the type target1l sky target2 target1 sky target2 saving from the time spent on the sky Unlike some other templates such as the SOFI_img_obs_AutoJitterArray the freedom to choose the offsets means that the targets and the sky can form an irregular pattern Parameter signature Exposure Name DET DIT DET NDIT Number of columns Number of rows First column of window First row of window Number of exposures Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset F T Return to Origin T F RA offsets list arcsec Dec offsets list arcsec SOFI User s Manual 2 0 Header Keyword DET EXP NAME DET DIT DET NDIT DET WIN NX DET WIN NY DET WIN STARTX DET WIN STARTY SEQ NEXPO INS FILT1 ID INS FILT2 ID INS IMODE SEQ RETURN SEQ OFFSETALPHA LIST SEQ OFFSETDELTA LIST LSO MAN ESO 40100 0004 Value SOFI NODEFAULT NODEFAULT 1024 1024 1 1 1 NODEFAULT NODEFAULT NODEFAULT SEQ COMBINED OFFSET F T NODEFAULT NODEFAULT Table C 11 SOFI_img_obs_Jitter Parameter signature Exposure Name DIT NDIT Number of columns Number of rows First column of window First row of window Number of Exposures Filter wheel 1 Filter wheel 2 Instrument Mode Combined offset F T Return to origin T F RA offset list arcsec DEC offset list arcsec Table C 12 SOFIimg_obs_Jitter Example Parameter signature Number of Ex
148. s just decrease the number of exposures and repeat the OBs with a few minutes delay as many times as the NEXP was reduced Parameter signature Value Exposure Name Arc_Calibration DIT LIST 2222 NDIT LIST 5555 Number of exposures 4 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Spectro Mode BR Slit List 1 Lamp List N Xe N Ne Table 3 13 Parameters in the SOFIimg_cal_Arcs template with commonly used values Nota Bene Remember to take images with the lamps off to remove the pattern due to dark bias scattered light and thermal background This makes it easier to identify the weaker emission lines In general you should avoid taking arcs during the night since the high illumination levels of the strongest emission lines in the arc images may cause persistence problems It is therefore advisable in such cases that you use the arcs in the morning after your run to do the wavelength calibration and then use the atmosphere to check the zero point of the calibration during the night Both OH emission and sharp atmospheric absorption lines can be used The P1 branch lines of OH are very good for wavelength calibration Avoid the blended Q branch lines Sample spectra for Xenon Neon and the main OH emission lines are displayed in appendix A Predefined OBs for wavelength calibration with the correct exposure times are available at the tele scope 3 6 Finer Points Some of the information in th
149. s appropriate Note that in case of short DITs the NDIT may have to be reduced further by a factor of 1 2 1 3 to take into account the overheads If conditions are not so good e g the sky intensity is fluctuating rapidly or if one simply does not wish to integrate that long then a lower value of NDIT can be used As a general rule the switching frequency should be higher for larger fields of view or longer wavelengths Another consideration that has to be taken into account in choosing the NDIT is the need to have a sufficiently high number of jittered images 1 to create a good sky image without stellar residuals and 2 to remove well the array cosmetic defects and the cosmic rays albeit there are few cosmic rays because of the short integrations for individual images in the IR Three jittered images is the lowest meaningful number to ensure the array cosmetics removes well but having 5 7 or more images is preferable especially if they have to be used for creating a sky frame An additional restriction on the offsetting frequency applies to imaging observations Unlike equa torial telescopes the pupil plane of the NTT rotates relative to the image plane and for reasons which may be due to internal reflections within the objectives this causes pupil ghosts images of the telescope support structure near the primary mirror to appear in sky subtracted images In other words the traces of the spider supporting the secondary mirror do not full
150. s are about 40 arc seconds whereas the widths of the opaque sections are slightly larger Thus to cover the whole field one SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 needs to take three separate images shifted by 30 arc seconds from each other This techniques is embodied in the template SOFI_img_obs_Polarimetry The parameters for this template are listed in Table 3 7 Parameter signature Value Exposure Name Planetary_Disk DIT 1 8 NDIT 10 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of exposures 3 Filter wheel 1 J Filter wheel 2 open Combined Offset T F F Return to Origin T F T X offset list arcsec 000 Y offset list arcsec 64 32 32 Rotator Offset 0 Table 3 7 Parameters of the polarimetric template with commonly used values This example takes three exposures in the J band By default the polarimetric mode uses the large field objective Offsets are relative to array rows and columns are independent of the rotator angle and are in arc seconds In this example three exposures with offsets along the Y direction are taken In this way an entire field can be covered Observers may wish to include additional exposures with offsets in the X direction to help with sky subtraction To measure the Stokes parameters and hence the degree and direction of linear polarization one needs at least one additional set of observations with a position angle different from t
151. s are skipped to make sure the telescope has stabilized after the move Otherwise when the telescope is not moved between frames i e when NINT gt 1 or at the beginning of a new cycle all DIT s are kept At the end of the templates the telescope returns to the original position if SEQ RETURN is true T If not the telescope is not moved at the end of the template SOFI_spec_obs_AutoNodOnSlit and SOFI_spec_obs_AutoNodNonDestr differ only in the control over the readout mode The second template allows to define NSAMP and NSAMPPIX as explained the section 3 4 1 Otherwise they are identical The only two additional parameters with respect to the SOFI_spec_obs_AutoNodOnSlit are DET NSAMP and DET NSAMPIX respectively the number of samples and the sample number per reading During the integration the signal is sampled a number of times defined by DET NSAMP The largest the number of samples the lowest the read out noise However DET NSAMP must not exceed 60 above this value in fact the glowing of the shift registers become the dominant source of noise For long integrations DET NSAMP between 20 and 40 is advisable A way to further low the read out noise keeping DET NSAMP small enough is to sample the video signal more than once The number of sampling of the video signal is DET NSAMPIX 4 is a good number for this parameter For long integrations DIT should not exceed 300 sec The following examples Table C 31 and C 32 show typical parameters
152. sci ence target http www 1s eso org lasilla sciops ntt sofi IRstandards index html A similar tool with extra capability to create automatically OBs for observing of telluric standards is available on the SOFI instrument workstation but we recommend that the user creates their own OBs with the P2PP tool Nota Bene Until recently IR spectrophotometric standards were not available How SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 31 ever Cohen et al 2003 AJ 125 2645 published supertemplates for a series of 33 stars that span wavelength range from the UV to mid IR 40 um The preliminary tests show excellent agreement between the synthetic photometry derived from these spectra and the observed magnitudes of the stars Pending further tests this spectral library may prove extremely useful for flux calibration of IR spectra 3 5 Calibration Frames 3 5 1 Darks Biases The concept of bias frames with IR arrays does not have the same meaning as CCD bias frames With IR arrays a zero second exposure is not possible It is better to think of all exposures without direct illumination as dark frames Note that in this context the biases are integral and indistinguishable part of the darks Dark frames are taken with the template SOFI_img_cal_Darks The parameters for this template are listed in Table 3 11 Parameter signature Value Exposure Name Dark_Frames DIT LIST 10 10 20 20 10 10 NDIT LIST 663636 Number of exposur
153. ser to obtain several consecutive exposures of 12 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 60 F an 50 F 5 Q E 40 E A um Figure 2 4 Quantum Efficiency of the SOFI detector at T 78 K The peak Q E is at 1 970 ym and the long wavelength cut off is at 2 579 um NDITxDIT as defined by NINT and NJIT parameters NOTA BENE The counts in a raw data file always correspond to DIT seconds However a single raw data file represents total integration time of NDITxDIT because the counts in the file are the average of NDIT sub integrations each of DIT seconds 2 5 1 Readout Modes Unlike optical CCDs the charges in individual pixels of infrared detector arrays are not shifted from pixel to pixel during the read out process Instead each pixel is independently read and each column is individually reset This enables one to develop several readout methods For SOFI two readout methods are available DCR Double Correlated Read and NDR Non Destructive Read DCR incurs less overheads and is suitable for modes where the dominant source of errors is the Poisson noise of the sky emission imaging and low resolution spectroscopy NDR has a lower read out noise and is suitable for modes for which the read out noise is comparable to the other sources of error low and medium resolution spectroscopy e Double Correlated Read Here the voltage is sampled twice once at the beginning of the integration and a second
154. servations themselves can also be used to flat field the data However these frames suffer from the same problems as the uncorrected dome flats and in the case of the shade pattern it is not clear how they can be corrected Last but not least the Special Dome Flats and the Illumination Correction Surfaces are mode and filter specific i e they have to be prepared separately for each instrument mode and filter combination 5 2 6 Image Alignment and Combination Most data reduction package have tools for alignment of images and for their combination For exam ple in IRAF one can use mezam imalign and or imcombine We refer the user to the corresponding user manuals Note that in case of IRAF the task imcombine can not handle fractional offsets The workaround is to use a 2 step shifting procedure First shift the images by the fractional part only with the task imshift If you intend to do photometry on the final image set the interpolation parameter of imshift to linear in order to conserve the flux during the shifting In addition set the boundary value that will be given to pixels with no value to constant and set that constant to a large negative value i e le6 Later during the combination these pixels can be excluded with an appropriate lower cut off threshold If you use any of the outer options for the boundary pixels you will end up creating data The second step of the shift can be carried with imcombine using the
155. set on Sky TEL ROT OFFANGLE 0 0 Rotation offset on the sky degrees Combined offset F T SEQ COMBINED OFFSET False T guiding ON F guiding OFF Preset Telescope F T SEQ PRESET True T full preset F fine tunning of the pointing Save Image F T SEQ SAVE False T preserve the acq image F no Table C 28 SOFIimg_acq_MoveToSlit template without presetting the telescope In other words the user can change the position of the target on the array without full telescope preset This option is particularly useful if acquisition was aborted for some reason or if multiple OBs to observe the same target are executed one after another In this case it is advisable to re acquire the target with a false preset every 1 2 hours to ensure it has not drifted away from the slit The parameter SEQ SAVE allows to save an acquisition image at the cost of small additional overhead for file transfer and saving The interactive pop up windows are usually displayed before new images have arrived on the RTD Therefore operators are strongly advised to carefully check that a new image has arrived before clicking on these windows e g for storing a fixed pattern for changing the DIT NDIT The arriving of a new image on the RTD is marked by a flashing green dot in the middle of the upper part of the RTD window C 3 6 SOFT Spectroscopic Science Templates SOFI_spec_obs_AutoNodOnSlit and SOFI_spec_obs_AutoNodNonDestr These observation tem
156. st in time is sufficient The advantage of this technique is that it takes into account sky variation on time scale a few times NDIT x DIT In the specific case of SOFT it also removes very well the pupil ghosts usually observed as diagonal stripes in the South East corner of the array discussed earlier Both these advantages are pronounced better if the number of images we use to subtract the sky is kept small ideally just the preceding the the succeeding images While the reason for this is obvious for sky variations it is not the case with the pupil ghost until one remembers that the pupil rotates and the ghost moves across the field of view Therefore it is best to sky subtract using images that have the ghost nearly at the same position as on the target image The problem with this method is that some negative residuals from stellar images will remain on the sky subtracted images Indeed with only the preceding and the succeeding images to subtract the sky it is impossible to remove the negative stars This difference with the previous method makes us to consider it separately Note that the negative stars can be dealt with later on the stage of combining the shifted object images Given enough object images and sufficiently low crowding this technique works well This method is applicable for observations taken in both simple jittering more and with offsets on a clear sky region A special attention is required to o
157. stars that may have randomly fallen into the filed is also desirable because these stars will become a source of horizontal lines due to the cross talk The cross talk is correctable well only if the sources that cause it are not saturated Unfortunately it is not always possible to use small DIT because the smaller DIT increases greatly the overhead up to 50 in the cases of 1 2 2 sec DIT For comparison observations with DIT 8 sec and NDIT 8 or 64sec of integration in total require about 80sec so the overhead is 25 The user may choose to accept the large overheads or the presence of increased lines in the image that are hard but not completely impossible to correct The sky background is another factor that has to be accounted when selecting a DIT It is the strongest in K band when it amount to 400 800 ADU per second depending strongly on the humidity The sky background can easily saturate the array by itself if the user selects a big DIT of 15 30 sec or more depending on the filter Furthermore this background is not constant It varies on a time scale of a 1 3 minutes becoming a source of systematic uncertainties To account for them the user must monitor these variations on the same time scale This is done by alternatively observing the target and a clear sky field next to the target the next sections contain some useful tips how to optimize these sampling in case of different types of targets The frequency of the sky s
158. stem the DCS Detector Control System and the ICS Instrument Control System The status of the instrument is displayed in the OS GUI Graphical User interface and the obtained raw images are displayed on the RTD Real Time Display The results are also sent to the archive machine and the off line workstation wg5off This will be discussed in more detail later When creating OBs it is useful to keep them as simple as possible Do not create complex OBs which switch from one mode to another OBs with multiple science templates with different filters imaging the same target are acceptable SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 41 4 3 Arriving at the Telescope The NTT is run by a section of the joint control room the Ritz Consult the map on the La Silla web page to find out where it is located Please be aware that you will be sharing the Ritz with the observers at two other telescopes and some understanding and patience may be required When you arrive at the Ritz you will be confronted by a vast number of terminals Most of these are of no concern to you You will be primarily interested in the terminals at the right end of the room NTT section Starting from right to left there is an empty table with a network connection that can be used to plug in a laptop The first desktop computer next to it is wgdoff where you can run RAF MIDAS eclipse IDL web browsers and other useful programs This is a dual moni
159. t is not rea sonable to the usually faint science targets The IR Solar spectrum is available from the National Solar Observatory An atlas of the solar spectrum in the infrared from 1850 to 9000 cm 1 Livingston W amp Wallace L N S O Technical Report 91 001 July 1991 A more detailed description how to use solar analogs as telluric standards and an IRAF based tool can be obtained from Maiolino Rieke amp Rieke 1996 AJ 111 537 web sites http www arcetri astro it maiolino solar solar html and http nicmos2 as arizona edu marcia solar This page contains lists of stars with re liable spectral class determinations that can be used as telluric standards The final step is to flux calibrate the object spectrum The flux calibration is done by integrating the object spectrum over one or more broadband filter band passes and scaling the result with the respective broadband magnitudes This method typically gives accuracy of order of 5 10 It is very different to what is done to flux calibrate spectra in the optical In the optical atmospheric absorption is a relatively smooth function of wavelength so it is sufficient to divide extracted spectra by a smoothed standard star spectrum and then multiply by the smoothed absolute energy distribution of the standard In the IR accurate spectroscopic standards do not exist For an example of IR flux calibration in the more general case of extended objects see Ivanov et al 2000 ApJ
160. telescope points back to the original position This means that the sum of the offsets along the RA is zero same as the sum of the offsets along the 22 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 arcsec 148 oO oO n Es lt a East 100 Dec arcsec Figure 3 1 Example of 4 point observation scheme of a semi extended objects without extra overhead for observation of clear sky Remember than although the target is moving on the RTD it is really the telescope that is moving DEC Fortunately the autojitter feature of the template ensures that the the next batches of four images will not be taken at exactly the same positions as the previous ones In the example the jitter box is 20 arcsec and for simplicity it is shown only on the first of the four positions Four dashed circles show where the target would be if the random jitter offsets move the target center to the edges of the jitter box This has to be remembered well because too large a jitter box may move the target out of the field of view or it may lead to overlapping of the target edges in sequention images The overlap is better to be avoided because it may affect the sky subtraction The user should try to keep the target as close to the center as possible without the allowing the overlap mentioned above In fact this example allows some modification the offsets can be reduced from 148 to 110 120 arcsec and the jitter from 20 to 10
161. time at the end of the integration This method is called Double Correlated Read or DCR for short It is commonly used in high background situations where integrations are forced to be short In principle this method is susceptible to 1 f noise however the Poisson noise from the background will be by far the most important noise source The SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 13 minimum integration time for SOFI in this mode is 1 183 seconds e Non Destructive Read In Non Destructive Read or NDR the array is sampled several times after the reset We will call the number of samples NSAMP In other words the array is read NSAMP times during each individual DIT The flux in each pixel is computed by fitting a linear function to the voltage as a function of time The fitted slope equivalent to the photon rate is then multiplied by the integration time There are several ways the signal can be read in the NDR mode The one used in SOFI concentrates the read out at the beginning and at the end of the integration in this way the noise is minimized This mode is called Fowler sampling Unfortunately with the number of readings the glowing produced by the heat from the shift registers at the border of the array increases For NSAMP 60 the photon noise of the glowing starts to compete with the read out noise and above 60 becomes the dominant source of noise For this reason the NSAMP must not exceed 60 To further minimize the noise
162. tor machine From here you can access and examine your data Note that the system clocks of these computers are set to the UT To the left of the wg5off is the terminal whddhs where you run the P2PP Immediately to the left of wgddhs is a sequence of three vertically placed dual screen terminal for the SUSI2 EMMI and SOFI workstations in this order The bottom screens show their OSs BOBs etc while the upper screens show their RTDs For convenience we can run some applications on the screens of the instruments that are not in use at the moment To the left of the SOFI workstation are the computers of the telescope The user will have no interaction with them When you arrive at the telescope the entire system will have been started for you by the daytime operator During the entire night you will have a telescope instrument operator TIO It is important to emphasize that the telescope instrument operator will be the one controlling BOB and SOFT You as an observer will be selecting OBs for execution via P2PP and inspecting the incoming data NOTA BENE The TIO is responsible for the operation and safety of the telescope and instrument he has the authority to refuse an action that would jeopardize this safety this include closing the telescope in case of dangerous weather conditions On the terminal wg5off do the following e The machine will be started by the support staff together with all the necessary applications so there is no
163. tracted image As usual the global average is preferable if the sky background is stable and the floating sky subtraction is preferable if the sky varies NOTA BENE The sky subtraction of every observation has to be considered individually The recipes provided here refer to the most general cases Every observation is different and covering all possibilities is out of the scope of this manual We provide some tools that help to carry out the sky subtraction Check the Data Reduction section of the SOFI web page and ask your support astronomer for their latest version 5 2 5 Flat Fields and Illumination Corrections Creating a good flat field for SOFT data is difficult The simplest way of creating flat fields is to subtract an image of the dome flat field screen with the dome lamp off from an image with the dome lamp on This flat field has two short comings First the shade pattern of the array is a function of the overall flux so the shade pattern in the image with the lamp on is different from that in the image with the lamp off Thus the difference of the two will contain a residual shade pattern Second the illumination of the dome panel is slightly different from that of the sky Furthermore it changes with time because of the aging of the lamps used to illuminate the screen and in the case of observations with A gt 2 3 micron due to the variations of the dome temperature Both these effects are at the 1 2 level and both c
164. tt mn a LO OD or O oO me a QQ o lt Q Q tid O ASA Ne oO a QQ NA QQ ca IDA Cr lav nN fae QQ Q Na q Q Na QQ gt E y o D o wo o oo wo o o o D 2 6x10 3 i o E f A O gt E a f eat a j i A i S f E A oe D i i a i E E CTE 4 DG E pS 4 Loa s4x10 z A i i 5 f i E A i i o A i o oi E L o 1 1 1 1 1 1 L 1 2x10 2 1x10 2 2x10 2 3 T10 Wavelength A Figure A 6 A Xenon and Neon arc spectrum taken with the medium resolution grism at the K atmospheric window The main lines are marked Appendix B Atmospheric Absorption In Figure B 1 the atmospheric transmission in the 0 8 to 2 5 micron region is plotted as a function of wavelength Also plotted are the pass bands for the SOFI filters 65 66 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 1 T Transmission 0 5 T 0 Transmission pea Li 4 1 45 1 3 1 55 1 6 1 65 Transmission Transmission Wavelength microns Figure B 1 The atmospheric transmission at a resolution of 8A Most of the SOFI filters plus some additional ones from ISAAC are marked Appendix C SOFI Templates A Reference Guide C 1 General Points This section provides a summary of the currently released SOFI templates It assumes that the read
165. ual 2 0 LSO MAN ESO 40100 0004 On the terminal wg dhs do the following e If required login to this terminal as visitor Your support astronomer will provide you with the password e In a workspace of your choice click the left mouse button on the background window free area of the screen to start a terminal and from the prompt type p2pp to start the P2PP tool Next you will need to login with your username and password They are the same as the ones you were given to login in the web letter service where you had found the notification for the approval of your program If you do not have the userid and the password ask your support astronomer You are now ready to observe and even to obtain some calibrations before the beginning of your observations You can use them to reduce the data during the night 4 3 1 Image Analysis The optics of the NTT are actively controlled and to get the best out of the NTT frequent correction to the primary are necessary The telescope aberrations are determined by examining images of a guide star recorded through a Hartmann mask The process is called image analysis Its outcome is used to compute corrections to the optical configuration which are applied as deformations to the primary mirror and as displacements of the secondary along three axis Preferably the image analysis should be done with the telescope pointing in the direction of your next target As most exposures in the IR are relati
166. uction has allowed some users to reach with SOFT relative photometry of 0 3 To get good photometry you should choose standard stars that are as close as possible to your program objects and you should observe them before and after you observe your program objects The classical optical method of determining extinction coefficients by observing standards over a wide range of airmasses works less well in the infra red as the extinction co efficients are generally not a linear function of airmass and may vary with the conditions i e with the humidity During the night you should observe at least three different standards and you should observe at least one standard every two hours On nights where the humidity which is a rough measure of the water vapor column is varying considerably let s say by 40 in one hour you will need to observe standards more frequently When observing standards two or more images typically five one in the center of the array and four in the centers of each quadrant of the standard star are obtained with a telescope shift s in between In this way one image can be used as the reference sky of the other s If several images are obtained the uniformity of the flat field illumination can be checked Several standard star lists are currently available but it should be noted that each list was created using detectors and filters that differ from the detectors and filters used with SOFI Note also that most standards
167. unction describes the transmission variation along the slit It is usually affected by magnetized or sticky dust particles that attach to the slit edges These particles can make the slit effectively narrower Normally steps are taken to clean the slit during interventions so the slit transfer function is highly uniform This effect can be neglected for spectroscopy of point sources Spectroscopic flat fields can be taken with the halogen lamp in the calibration unit Nasmyth flat Again one takes an image with the lamp on and off The flat is the difference between the two normalized to unity The optical path length between the Nasmyth flat field lamp and the front window of SOFI is considerably smaller that the path length between the incandescent lamp on the floor of the dome and the front window of SOFI Thus one will see less of the strong atmospheric absorption features in the Nasmyth flats There are four templates to create dome flat fields two for imaging SOFI_img cal_DomeFlats and SOFT_img cal_SpecialDomeFlats and two for spectroscopy SOFI_spec_cal_DomeFlats and SOFI_spec_cal_DomeFlatNonDestr Please note that the last template must be used ONLY to flat frames taken with the NDR Mode In Table 3 12 is an example for imaging dome flats Parameter signature Value Exposure Name Imaging Dome Flats Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of exposures 1 DIT LIST 3 NDIT L
168. uters Further details on the IRACE system can be found at http www eso org projects iridt irace The amount of data pre processing depends on the readout mode The readout modes available with SOFI are discussed below The detector used by SOFT is a Rockwell Hg Cd Te 1024x1024 Hawaii array manufactured by Rock well Scientific with 18 5 micron pixels The array is read out in four quadrants The average quantum efficiency is 65 Figure 2 4 The dark current is very low 20 e hour and the readout noise with the IRACE controller in DCR Double Correlated Read mode is 12e In NDR Non Destructive Read values as low as 3e7 have been reached with integrations of one minute About 0 1 of the pixels are bad An updated mask of bad pixels is available in the SOFI web page The gain of the array 5 4 e ADU The well depth of the array is around 170 000 electrons 32 000 ADU Although the array non linearity is limited to less than 1 5 for a signal up to 10 000 ADU we recommend that observers keep the exposure short enough so that the background does not exceed 6 000 ADU This is due to the bias of the array which has a complicated dependence on the flux when the flux is above 6 000 ADU NOTA BENE A star of K 10 mag produces 9 000 10 000 ADU at the central pixel under seeing 0 6 0 7 arcsec and average conditions in Large Field imaging mode This is the case with many of the photometric standards and since these values are close to
169. values According to the NINT and number of cycles they produce 3x3x2 18 files each of them corresponding to the average of the DIT x NDIT SOFI_spec_obs_GenericSpectro and SOFI_spec_obs_GenSpecNonDestr These observation templates Table C 33 and C 34 are for spectroscopy and they have the flexibility to do any sequence of telescope offsets whether they be combined offsets offset the telescope and guide probe or non combined offsets offset the telescope alone SOFT spec obs_GenSpecNonDestr is identical to SOFI_spec_obs_GenericSpectro except for the non destructive readout more With this flexibility of these templates comes complexity They are meant to be used in situations that cannot be accommodated by the nodding templates described above i e when the target is too extended and to obtain sky spectrum one has to move further than the slit length allows Telescope offsets and guiding options are defined as lists in and SEQ OFFSETX LIST SEQ OFFSETY LIST and SEQ GUIDING LIST Telescope offsets are relative to the previous position they are defined along detector lines X and columns Y and they are in arcsec The slit is oriented along the N S direction i e along the X axis N to the left negative and S to the right positive and E W along the Y axis E down negative and W up positive for Rotation Offset on Sky SOFT User s Manual 2 0 Parameter signature Exposure Name DIT NDIT Number of columns Number of rows First
170. vely short the image analysis is done off line Time spent doing image analysis is not spent on your source thus image analysis should be used wisely It usually takes 10 to 20 minutes to perform an image analysis and verify the result Once done the telescope will be at the correct focus It is no longer necessary to check the focus after image analysis The image analysis is done by the TIO and the only action a visitor can take is to request an image analysis of the image quality is degrading How frequently one does an image analysis depends on the required image quality Here are some hints on how frequently it should be done i If image quality needs to be better than 1 0 image analysis should be done if the altitude changes by 20 degrees some times 30 degree trust the advice of the telescope operator For long integrations where the source is changing altitude one should perform the image analysis at an altitude that is 20 degrees higher if the source is rising or 20 degrees lower if the source is setting The analysis should be repeated once the altitude of the object differs from the altitude where the previous image analysis was done by more than 20 degrees In this way the object is never observed more that 10 degrees from the last image analysis ii If the image quality is not critical this is usually the case for spectroscopic observations then perform the image analysis once at the beginning of the night It should be done w
171. vered The template SOFI_img_obs_AutoJitterArray 1 will instead take one image at each 4 offset positions then it will repeat the entire offset pattern NJITT 3 times adding before each cycle a small jitter offset SOFI_img_obs_GenericImaging This template Table C 18 has some similarity with SOFI_img _obs_Jitter but it allows the user to SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 79 Parameter signature Header Keyword Value Description Exposure Name DET EXP NAME SOFI File name prefix DET DIT DET DIT NODEFAULT Detector Integration Time individual exposure sec Number of columns DET WIN NX 1024 Number of columns in the window Number of rows DET WIN NY 1024 Number of rows in the window First column of window DET WIN STARTX 1 First column of window First row of window DET WIN STARTY 1 First row of window Number of exposures SEQ NEXPO 1 Number of exposures in the sequence List if NDIT DET NDIT NODEFAULT Number of DITs averaged into an individual image Filter wheel 1 INS FILT1 ID NODEFAULT Filter wheel 1 position Filter wheel 2 INS FILT2 ID NODEFAULT Filter wheel 2 position Instrument Mode INS IMODE NODEFAULT Instrument Mode Return to Origin T F SEQ RETURN T Returns the telescope to the original pointing if True RA offsets list arcsec SEQ OFFSETALPHA LIST NODEFAULT list of offsets along RA Dec offsets list arcsec SEQ OFFSETDELTA LIST NODEFAULT list of offsets along Dec Obs Type O or 8 SEQ OBSTYPE LIST NODEFAULT O
172. w prompts the operator to start guiding A list is also provided for NDIT SEQ NDIT LIST but not for DIT The observation type can be defined for each image and is entered as a list in SEQ OBSTYPE LIST O stands for Object and S stands for Sky This is to make it essayer for the user to classify the images during the data reduction The total number of exposures is defined in SEQ NEXPO This number can differ from the number of elements in the aforementioned lists Lists do not need to have the same length If the number of exposures is larger than the number of elements in a list the list is restarted from the beginning as many times as needed until the correct number of frames have been acquired The lists can have any 80 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 Parameter signature Value Exposure Name NGC6118 DIT 20 Number of columns 1024 Number of rows 1024 First column of window 1 First row of window 1 Number of Exposures 7 List of NDIT 3232323 Filter wheel 1 Ks Filter wheel 2 open Instrument Mode LARGE FTIELD IMAGING Return to origin T F T RA offset list arcsec DEC offset list arcsec Obs Type O or S Guiding N B S 0 150 140 150 140 250 250 0 150 140 150 140 250 250 OSOSOSO S Table C 19 SOFIimg obs GenericImaging Example length however having lists of different lengths can become extremely confusing It is good practice to use either lists of one value and or lists of equal
173. were observed with single channel photometers with very wide apertures Thus 26 SOFT User s Manual 2 0 LSO MAN ESO 40100 0004 close companion stars were probably included The standards listed in Carter and Meadows 1995 MNRAS 276 734 have proved to be very useful although they may prove to be too bright for SOFT and to require defocussing the telescope The more recent NICMOS standards by Person et al 1998 AJ 116 2475 are more suitable as one does not need to defocus the telescope except for seeing better than 0 6 arcsec They are the standards of choice for most observers Predefined OBs are available at the telescope for most of these stars A list of references to these and other standard star lists as well as useful papers regarding the transformations between one photometric system and another are available from the SOFI web page It is important that you take care where the standard star lies on the array Use the RTD Real Time Display to find areas that are clear of bad pixels Furthermore check if other objects are in the field The offset between exposures should not be such that other objects in the field interfere with flux of the standard when one frame is subtracted from another Alternatively the standard can be observed in several different parts of the array A pattern of five exposures with the standard observed once in the center of the array and once in each of the quadrants is one example Table 3 6 demonstrated th
174. xel SOFT_img acq MoveToPixel Preset telescope and center an object in a slit SOFT_img acq MoveToSlit Preset telescope and position an object for polarimetry SOFIimg_acq_Polarimetry Table C 1 Short guide for acquisition templates Type of Imaging Template s to use Imaging of uncrowded fields SOFIimg_obs_AutoJitter or or point like objects SOFIimg_obs_Jitter Imaging of crowded fields SOFIimg_obs_AutoJitterOffset or or extended objects SOFIimg_obs_Jitter Offset Map of extended fields SOFIimg_obs_AutoJitterArray or SOFIimg _obs_AutoJitterArray_1 Imaging requiring complex telescope offsets and or guiding options SOFIimg_obs_GenericImaging Imaging Polarimetry SOFI_img_obs_Polarimetry Table C 2 Short guide for imaging and polarimetry templates Type of Spectroscopy Template s to use Spectroscopy of point like or moderately extended objects SOFT spec obs_AutoNodOnSlit As above but in Non Destructive read out mode SOFT spec obs_AutoNodNonDestr Spectroscopy of extended objects i e wider than 2 arc minutes or complex sequences of slit positions SOFI_spec_obs_GenericSpectro As above but in Non Destructive read out mode SOFI_spec_obs_GenSpecNonDestr Table C 3 Short guide for spectroscopic templates Type of calibration Template s to use Darks SOFT_img cal_Darks Imaging Dome Flat Fields SOFIima_cal_DomeF lats Special Imaging Dome Flats SOFT ima cal_SpecialDomeFlats Standard Star imaging SOFLimg_cal_StandardStar Polar
175. y subtract on images taken more than 5 10 min apart The effect is greatest when the paral lactic angle changes quickest and this occurs when the telescope is near the meridian The effect is worst for images taken in K and the large field objective To minimize this effect the beam switching frequency should be such that the parallactic angle does not change by more than 0 5 degrees between exposures For jittered images the residual effect is at or below the shot noise level when applying standard reduction techniques which involve running sky measurements over 10 frames For applications requiring large object sky offsets it is advisable to use a strategy which automatically equalizes the parallactic angle of the two This strategy can be employed in two SOFT templates SOFI_img_obs_AutoJitterOffset and SOFI img_obs_JitterOffset just set the parameter rotate pupil to true It is possible to reduce the effect of the ghosts during the data reduction by running sky measurements over only 2 4 frames but this makes the data reduction somewhat more complicated Finally in the spectroscopic mode only one can accumulate a number of NINT exposures in one nodding position A or B before moving the telescope to other side of the nod This feature allows to minimize the overheads for moving the telescope between the two nodding positions These overheads are bigger than the ones related to the small jitter offsets 3 6 2 Autoguiding T

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