Home

The LIRIS@WHT cookbook - Isaac Newton Group of Telescopes

1. taka d doe Sg dero Yer 2 9 9 Calibrations s 5 e244 donum xo RR wa ee 2 4 Multi object spectroscopy 2 4 1 Number of available mask positions 2 4 2 MOS mask preparation 3 Getting started at the telescope 3 1 Introduction by your support astronomer 3 2 The WHT LIRIS observing 3 2 1 Basic observing commands 3 2 2 The A amp G box and 4MS 4_ III IV CONTENTS 3 23 The camera server Window 18 3 2 4 The real time display 18 3 2 5 The three status mimics 22er 22 3 2 6 The night log and TCS info 22 4 Before you start observing with LIRIS 25 4 1 Features GU oru VELIM uhr Ea RE 25 4 1 1 Scrambled pixel mapping 25 4 1 2 Very bright sources and detector remanence 25 4 1 8 Detector reset anomaly 26 4 2 Checklist before the observing run 26 43 Calibrations ORAE EL a RONDE EORR EDU 28 BlaseS nd DArk8 comode a Ian 28 43 2 5 s dann EL UIN Prax uan cs eve TM X EE 29 4 3 3 Spectroscopic flats seco oca 30 ABA AROS BRR ers
2. 16441 1 16524 4 16744 6 16945 2 17449 7 17919 6 18434 5 19822 9 19971 2 20322 6 20621 9 20991 8 21338 7 21540 1 22083 2 23139 5 8954 7 9047 9 9165 2 9516 0 9802 4 9925 9 10530 8 10841 3 11088 3 11745 5 12238 6 12626 8 13660 8 14146 2 14736 8 9125 5 9227 0 9356 8 9660 4 9787 2 10472 9 10676 5 11081 9 11491 2 11671 9 12115 6 124062 124427 112491 1 12705 7 12806 2 12960 2 13217 6 13276 3 1338168 33708 13507 9 13626 4 13682 3 37223 13911 3 14097 5 15050 6 79 SV LLV IVULOHAS XIUNHAAV 015118 OY sour ore pue g q amS cn Mt aum orm uh mone ed i ar me anii 19822 9 19971 2 20192 7 20267 8 20322 6 20621 9 20991 8 21338 7 21475 9 21540 1 22083 2 23139 5 23199 6 23851 5 23973 1 99 66 APPENDIX D SPECTRAL ATLAS Appendix E Imaging data reduction with THELI On both the WHTDRPC1 and WHTDRPC2 machines in the control room there is THELI 1 installed which can be used for a full reduction of your imaging data ob tained with LIRIS The full documentation of THELI can be found in the menu under Help gt GUI user manual It contains a more detailed processing example in the ap pendix To start THELI run export theli th
3. 30 5 Imaging 33 Dal AA a a a A A A ds 33 5 2 Seripts asy RR Ue erg 34 5 2 1 Imaging without blank sky fields 34 5 2 2 Imaging with blank sky fields les 34 5 3 Choosing the correct values for andnruns 35 9 9 12 Example ds at e 35 5 82 Ex nmple Zo 42 S EON ace 35 5 4 Integration times and co averaging 36 55 Polarimetry 1 36 5 6 COrOBOBSTADDNY c iude ew sra tede oe EIOS eae 38 zu lex oN ekg eas x ee ae tas 38 5 8 Filter change overheads 39 6 Spectroscopy 41 6 1 Long slit object acquisition 41 6 2 Spectroscopy scripts sca 43 6 2 1 Spectroscopy without blank sky fields 43 6 2 2 Spectroscopy with blank sky fields 43 6 3 Wavelength 44 6 3 1 Integration 44 6 3 2 General advices 44 6 4 Spectropolarimetry ee 44 6 5 Multi object spectroscopy 46 6 5 1 ACQUISITION 46 6 5 2 Calibration standard star in a few slitlets
4. 47 6 5 3 Calibration standard star in all or a few slitlets 47 CONTENTS A Support astronomer notes go Y 1 A 2 A3 4 5 Rotation centre twilight Alignment with respect to the sky twilight Check the alignment of slitmasks afternoon Alignment of Long Slit masks A 4 1 Update 489 masks A 4 2 Update the slit mask data base Alignment of MOS masks A 5 1 Update 489 masks and TCS mask A 5 2 Update slit mask data base A 5 3 Update 489 slit mask buttons A 5 4 Standard star calibrations with MOS Troubleshooting Filter transmission Spectral atlas Imaging data reduction with THELI E 1 E 2 E 3 E 4 5 E 6 E 7 Organising the data and initialsing THELI Preparation adi nan b Ux LOMAS m Calibrations Ir eu deer p ctr Ble iR sa pee ug Dr iid Supertlatting no t et REESE wur ni oet as Ent ERST Weighting n aine Ee Mec Ae a boe Astron Photii d 5 oa lasts Bala sal A Oe Rete Ee oho Coadditlon is 49 49 49 49 50 50 50 51 51 52 52 53 55 59 63 0 1 DOCUMENT
5. 6 3 2 General advices Once an object is acquired and the guider running the object will stay well centred on the slit for times longer than an hour It is usually not necessary to re check the acquisition unless you notice a drop in flux beware that could also just be due to clouds If you observe very faint targets which do not show a recognisable signal in one exposure check the acquisition after two hours or as frequent as you feel comfortable For very faint targets that do not leave a clearly traceble signal after one exposure it is a good idea to orient the position angle of LIRIS such that a bright source falls elsewhere on the slit This source can then be used as a reference when the spectra are combined taking into account the offsets Aligning the slit along the parallactic angle is usually not necessary but of course feasible if desired 6 4 Spectropolarimetry This mode hasn t been commissioned yet 6 4 SPECTROPOLARIMETRY 45 1 mu onn ug LL gcn EU iti TU MI 7 1 T T TI m TUTO LT mui Jj ihi debis n m amm My 3 unt MAT D n T HUN A Figure 6 1 A 600s exposure through a MOS mask The spectra are dominated by sky lines hence it is important that the slitlets are chosen long e
6. run liris exp time title To take an exposure without saving the file is called s1 fit and will be overwritten glance liris exp time title To take N exposures multrun liris exp time title To take an autoguided series of 9 dithered exposures ag mdither 9 lt exptime gt title Move the Acq Comp mirror in agcomp Move the Acq Comp mirror out agmirror out Make a telescope offset tcsslowoff lt xoffset gt lt yoffset gt 10 The offsets are given in arcseconds the numeric value specifies the speed by which the telescope moves 3 2 2 The A amp G box and 4MS display The Acq Comp mirror Between the telescope and LIRIS is the so called A amp G box which amongst others contains the autoguider a set of neutral density filters a tungsten lamp and two arc lamps for wavelength calibration In addition there are a couple of mirrors which lead the light into the currently mounted Cassegrain instrument in our case LIRIS The current status of the A amp G box is displayed in the A amp G box status window Fig 3 1 and 3 2 The configuration of the A amp G box is handled by means of the 4MS console Fig 3 3 If you want to take spectroscopic flats with the tungsten lamp or arc spectra with the arc lamps then you have to move in the Acq Comp mirror This can be achieved from within the first or the second menu tab of the 4MS console A amp G A a
7. Once you are done switch off the lamp and move the acq Comp mirror out of the beam so that LIRIS can see the sky again Do not forget to remove the ND filter Table 4 2 shows how long to expose in order to achieve an exposure level of about 20 kADU Of course you can also take dome flats with the zJ grism see next paragraph For spectroscopic flats with the HK or the K grism you should not use the tungsten lamp due to the presence of a broader absorption feature Instead observe the illuminated dome Switch on all the dome flat lamps but leave the dome illumination itself switched off since the latter features lots of emission lines You must take out the Acq Comp mirror Even though by observing the dome means looking through just a few meters of air you will see the very prominent absorption lines due to water vapour forming the natural separation between the H and the Ks bands 4 3 4 Arcs For wavelength calibration LIRIS offers an Argon lamp lamp 1 and a Xenon lamp lamp 2 Bring LIRIS into spectroscopic mode and do not forget to insert your desired slit mask Switch on one or both calibration lamps This can take a short while since the lamps have 4 3 CALIBRATIONS 31 Table 4 3 Exposure times for arcs with the 1 wide slit Grism slit exp time sec exp time sec Argon lamp 1 Xenon lamp 2 lr zj 11 2 low 4 low MH 10 high 40 high lr hk 11 60 60 mr_k 11 600 600 to warm up Move the Acq Comp mirror i
8. Transmission eo ERR a TEUER 1 4 1 5 1 6 427 1 8 Wavelength microns Transmission TS 2 0 2 5 Lambda microns Tronsmission APPENDIX C FILTER TRANSMISSION Transmission 0 0 1 1 1 1 1 1 10 1 15 1 20 1 25 1 50 1 35 1 40 Lambda microns Of e NAAA TEA VETE 72 1 0 8 0 6 0 2r ook y 1 9 2 0 2 1 2 2 2 3 2 4 Wavelength microns Figure C 1 Cold transmission of the z J H Ks broad band filters and the spectroscopic bandpass for the low resolution grism 1 07 TITTTTTTTTT TTTTTTTTTTTTTTTTTTT TTTTTTTTT TTTTTTTTTTTT m c 0 61 1 04t E L 1 0 2 f 3 1 21 1 22 1 23 1 24 1 25 1 26 Wavelength microns ME SS S S S S S S S S I 1 07 7 aal Heont BOK j 5 06 y 1 5 04 E 02 doa a ag 1 52 1 54 1 56 1 58 1 60 1 62 1 64 Lambda microns LAA AAA CH4 at 72 oak 5 0 6 5 0 4p al 0 2r 4 o 0L 1 50 1 55 1 60 1 65 1 70 1 75 1 80 1 85 Wavelength microns Tronsmission Tronsmission 61 at 80 1 26 1 28 1 30 1 32 1 34 1 36 Wavelength microns A A a _ _ _Q 1 0 Fell BOK 1 0 8 5 0 6 E 0 4 0 2 0 0 A AAA AAA 1 55 1 60 1 65 1 70 1 75 Lambda microns 1 0
9. the 9 display Fig 3 8 Therein you find the RA and DEC coordinates azimuthal and elevation angles airmass etc You can bring up the T CS display yourself by typing http www ing iac es Astronomy observing inglogs php 3 2 THE WHT LIRIS OBSERVING SYSTEM 23 Figure 3 7 The night log Figure 3 8 The TCS display tcsinfo amp 24 CHAPTER 3 GETTING STARTED AT THE TELESCOPE 4 Before you start observing with LIRIS 4 1 Features 4 1 1 Scrambled pixel mapping Due to some issues which are not further explained here some of the pixels in the LIRIS image are not in the places where they should be At some point in the digitisation process the LIRIS data is in the form of a one dimensional array and part of this array is shifted by one index When the two dimensional image gets reconstructed some of the pixels get dislocated In particular this affects the lower left quadrant which is entirely shifted by one pixel to the right easily recognisable in an arc image and in the sky lines of long slit spectra Its rightmost column gets wrapped around and appears on the left edge few isolated pixels in the other read out quadrants are also dislocated When you use the THELI reduction package Sect E all LIRIS images get automatically descrambled 4 1 2 Very bright sources and detector remanence Very bright sources will leave a ghost image imprinted on subsequent exposures which becomes visi
10. 2 596 within 10000 20000 ADU Charge persistency lt 196 disappears after three clearing reads automatic Saturation limit 45000 ADU before Dec 2006 35000 ADU Dark current 0 065 s at 83 Readout time 0 9s Minimum exposure time 0 85 e the grism wheel with low and medium resolution grisms and a Wollaston prism for imaging polarimetry Depending on the individual observations the observer selects from a series of pre defined instrument setups which then automatically configure LIRIS For example by typing limage ks one selects Ks band imaging as an observing mode which will au tomatically clear the slit and the grism wheel and choose the optimal pupil for the Ks filter The observer does not have to deal with individual wheel movements even though such are possible if desired 1 1 3 Overheads Any change of the LIRIS wheel configuration takes about 1 minute on average That includes filter changes or switching from imaging to spectroscopy or polarimetry mode The LIRIS wheel movements are made in parallel in order to minimise the overall overhead Please be aware of the fact that the overhead for centering very faint objects mag 17 18 or fainter on a slit for spectroscopy can easily amount to 15 minutes depending on the integration time needed to make the object visible More overheads are given in Table 1 2 1 1 INSTRUMENT PROPERTIES 10 TTFITITTTT box d 4E EFECTE L Nonli
11. AAA AAA at 72 0 8 0 6 0 4 0 2 4 o o L 200 2 02 204 206 208 210 Wavelength microns Figure C 2 Cold transmission of the jc pabeta hc fell ch4 and hel narrow band filters 62 Tronsmissicn Transmission 1 0 0 8 0 6 0 4 0 2 0 0 Figure C 3 H2 1 0 O 80K 0 8F 0 6 0 2 0 0 PEE 2 08 2 7 2 16 AE 2 18 PSS 2 10 2 12 2 14 Lambda microns q He A 75551 gt GE AA AH 5755 H2 2 1 2 18 2 20 2 22 2 24 2 26 228 2 30 Lambda microns Transmission Transmission APPENDIX C FILTER TRANSMISSION 0 8 0 6 0 4 0 8 0 6 0 4 0 2 0 0 Br gamma at 72 K A a ta 10 2 12 2 14 2 16 2 18 2 20 2 22 2 24 Wavelength microns at 72K TT AAA AAA AS 2 22 2 24 2 26 2 28 2 30 2 32 Wavelength microns Cold transmission of the h2v10 brg h2v21 and kc narrow band filters D Spectral atlas 63 USUS MY AT pue oy sour 918 pue 8 14146 2 14244 8 14738 8 15422 6 16057 7 16732 6 17330 5 20267 8 21475 9 23199 6 14097 5 14260 8 14654 3 15050 6 15176 8 15306 1 1890460 15993 9 161274 16184 4
12. I have centred the object on the slit but I do not see any spectrum 57 A Switch back to imaging mode Be autoguiding Re acquire the object Take through slit exposure by moving in only the slit wheel 13114 slit name Can you see the object now in the slit If it is well centred then continue Overplot the corresponding slit mask over the through slit exposure Are they properly aligned If not do a 1sethome slit then limage lt filter gt and start over again Q I got an alarm that the temperature pressure of LIRIS is out of lim its Inform the TO and the duty engineer DE Likely one of the temperature or pres sure sensors has just failed The DE can switch to a spare sensor Or LIRIS ran out of nitrogen unlikely though in which case the TO can refill the cryostat Pressure and temperature alarms should always be followed up and clarified by either the TO or the DE before you can continue observing Q I don t get any light through LIRIS or I observe a strange vignetting shadowing in the detector edges Check that the Acq Comp mirror is moved out of the light path and that you didn t forget to move out the neutral density filter after e g taking spectroscopic flats See Sect 3 2 2 for details 58 APPENDIX B TROUBLESHOOTING Appendix Filter transmission 59 60 80 Tronsmission 0 95 1 09 1 05 1 19 Wavelength microns 1 0 H at 72 K
13. Note that very faint objects will hardly be visible in this image due to the lack of exposed pixels in its surroundings lowered contrast If the object appears centred on the overplotted slit it can in general be considered as being acquired Through slit exposures are usually not necessary 8 Switch to spectroscopy mode using lspec lt 1 gt slit then start your observing script see Sect 6 2 below Please note that the use of the autoguider is strongly recommended in the acquisition process even for objects that require only very short exposure times such as standard stars The reason is that the acquisition process usually takes 3 5 minutes including switching from imaging to spectroscopy mode During this time the source can have slightly wandered off the slit and then you have to re acquire it One such iteration takes already much longer than just using the autoguider right away from the beginning Blind offsets from a known source to a very faint invisible source are in principle possible However you will have no guarantee that your object actually ended up on the slit If you can not make your object visible with LIRIS in a 2 or 3 minute exposure then you should ask yourself why you have not applied for spectroscopic time at a larger telescope exception is multi object spectroscopy where you have three reference stars in an individually taylored slit mask and thus can be sure that the target even t
14. THELI documentation for more details and when to modify the numeric parameters E 5 Weighting Here THELI calculates indiviual weight maps for each exposure You want to do the Create global weights and the Create WEIGHTs tasks Upon execution you will find a WEIGHTS directory on the same directory level as SCIENCE 70 APPENDIX E IMAGING DATA REDUCTION WITH THELI E 6 Astrom Photom In this PG THELI downloads an astrometric reference catalogue from the web and tracts an object catalogue for each image taken The object catalogues are then compared to the reference catalogue to correct for inaccurate CRPIX1 2 header keywords 1 zero order astrometric solution Thereafter a full distortion correction is performed and relative photometric zeropoints obtained Remaining sky background residuals can be modelled and subtracted at the end For the reference catalogue choose Web CDS and USNO B1 as a source enter a search radius of 3 and a magnitude limit Leave the RA and DEC fields empty If you observed an empty field and don t see many stars say less than 20 then enter 21 for the magnitude limit If you see numerous stars try 17 or 18 If your field is extremely crowded lower the magnitude limit to 12 Then click Get catalogue and wait until you get a response in the yellow message window telling how many stars there are found This can take a while and THELI will appear unresponsive during that wait You should adjust t
15. depending on the main target or unaware of the fact that certain standard types do not exist in the vicinity of their targets Thus any standards required must be defined in the proposal by the applicant e BIASes FLATS spectroscopic FLATs and ARCs These will be taken automatically at the beginning or the end of the night and do not have to be requested Note that the use of BIASes is rather limited for the same reasons as for the they are provided for convenience only 2 4 Multi object spectroscopy 2 4 1 Number of available mask positions LIRIS can accomodate about 6 8 MOS masks at the same time change masks the LIRIS cryostat has to be warmed up which is preferentially done only once per semester since there is some risk involved for the sensible IR detector array If the number of applications for MOS mode is very high also two warming ups can be scheduled per semester but this depends on different factors and should not be taken as granted Rather the one or other long slit or the coronographic mask will be traded in to accomodate more MOS masks 12 CHAPTER 2 BEFORE YOUR TIME APPLICATION OBSERVING RUN 2 4 2 MOS mask preparation The MOS masks are produced by an external company using an electromagnetic discharge process The slitlets are 0 8 wide and by default 10 long About 20 such slitlets can be put per mask The length of the slitlets is user definable They can be made shorter but should opti
16. wht var liris_rotcent_pos dat If the rotator centre is severly off the middle of the detector ask the TO to do 7 star pointing model A 2 Alignment with respect to the sky twilight Check for any net rotation of LIRIS That is ask the for a sky position angle of 0 degrees Then position a star at the far left side of the detector at a y coordinate of around 500 pixels to minimise effects of optical distortion and measure its position Then move the star to the far right of the field of view If you observe any net angle larger than about 0 1 0 15 degrees 2 3 pixels in y over 1000 pixels in x then ask the TO to update the TCS database and retry Otherwise it will happen that during spectroscopic nodding the target will be decentred or even driven off the slit when larger nodding offsets are made A 3 Check the alignment of slitmasks afternoon This should be done once every time LIRIS goes back to the telescope and in particular if changes to the slit wheel have been done Point the telescope to the zenit and take the AG mirror out agmirror out The primary mirror cover can remain closed as the ambient thermal radiation is suffi 49 50 APPENDIX A SUPPORT ASTRONOMER NOTES ciently bright to give you enough signal Run the following script which takes through slit exposures through all slit masks takes about 10 minutes home whtobs liris reg liris_thruslit sh If necessary adjust the exposure time
17. It is not an instrument handbook describing all the technical details but focusses on those issues relevant for a successful observation This includes the preparation of the observations and the observing strategy the observations themselves and the data reduction In Section 2 an overview of the most important characteristics of the instrument is given Section 3 helps you planning your observations with LIRIS and Section 4 gives you an overview of the WHT observing system In Section 5 we outline some important detector features you should be aware of and show you how you can obtain calibration frames The next two chapters focus on LIRIS in imaging and spectroscopy mode Lastly in the Appendix you find a troubleshooting section the filter transmission curves and a wavelength atlas as well as an example of how to reduce imaging data with a programme called THELI Chapter 1 LIRIS an overview 1 1 Instrument properties LIRIS is a near IR multi mode instrument mounted at the Cassegrain focus of the 4 2m WHT telescope Its main observing modes are imaging in broad and narrow band filters and spectroscopy with low and medium resolution grisms Both long slit and multi object spectroscopy are available Furthermore offered are imaging polarimetry and spectro polarimetry as well as a coronographic mode All of which are described below 1 1 1 Detector LIRIS uses a HAWAII detector with 1024x 1024 pixels Its basic characteristics ar
18. Shows the directory where the data is being stored e Store mode The store mode of the data e Status Status of the RTD Usually this will say that the RTD is waiting for a new image to arrive e File detection If you activate this box the new images are automatically loaded into the RTD e Scale auto Automatic or manual image scaling e Star profile When you activate this task you can measure the image seeing Switch File detection off then point the cursor on the star you want to measure Press b on the keyboard to move the little green box onto the star then q to perform the measurement and to quit The results will be displayed in the Star parameters section e Display state This section offers several options to display the data OBS REF Displays the difference between the current image and the reference image for sky subtraction OBS pre Displays the pre read of the current image OBS post Displays the post read of the current image OBS diff Displays the post minus pre of the current image e Refresh Refreshes the image shown in ds9 e ObsToRef Loads the current observation data as reference data Load OBS REF Loads any fits files of the data directory either as Observation data or as Reference data Just type the file name of the image in the corresponding field In the example of Fig 3 5 the difference of the images r919862 fit and r919838 fit is selected to be displayed in DS9
19. are significantly tilted or any other filter wheel did not move fully in place Likely the corresponding wheel lost its position Do lsethome slit fwheel1 wheel2 grism pupil all gt then try again You need to bring LIRIS back into imaging mode after this to be able to continue Q I get a red error message in the black camera server window Fig 3 4 for LIRIS and I can not take any images more A You have lost connection to LIRIS Do startobssys in the yellow DAS13 console In any case ask the TO or SA for assistance Q The LIRIS images show very strange features or a negative imprint of previous images in particular in dark frames It does not go away even after hours A That can happen You can fix this with a dasreset in the yellow DAS13 can change into DASzx console In any case ask the TO or SA for assistance I just can t get this object on the long slit it is always slightly off af ter using the lobject_inslit command Or I get it on the slit but once I take the spectrum the source has wandered off the slit A Use the autoguider And give the autoguider 30 seconds after the command has fin ished to settle down then take another image The seeing might also fool you and drive the source around In that case increase the exposure time Even if you do not use the autoguider wait for at least 10 seconds before the next imge to make sure the telescope has settled into its final position Q
20. do per dither position the smaller the total execution time will be This is because the telescope makes less dither offsets and if autoguiding is selected it has to re acquire the guide star less frequently In particular in Ks band these overheads can become comparable to the typical integration time of 10s and therefore increase the execution time very significantly e The less nruns and the more you do with jittering the better your spatial and temporal sampling of the sky background will be If the sky conditions vary rapidly you must not use too many nruns since otherwise you will not be able to correct properly for the sky any more e If you set nruns 1 one exposure per dither point then all your exposures have the same sky background but still suffer from the reset anomaly see also Sect 4 1 3 This makes the data reduction more easy If you set nruns to a value larger than 1 be aware of the fact that some 5 20 of the first exposures at each dither point can have uncorrectably bad sky backgrounds and must be discarded Thus your number of nruns should preferably be equal or larger than 3 in order to minimise the loss e The sky varies faster with increasing wavelength The speed of this variation is also different from night to night and can not be determined in advance Make sure that you fit enough dither points into the characteristic time scale i e do not make nruns too large For Ks band this is as short a
21. more images are being taken and in the talker window can see the following error message insufficient space left in buffer A Look up on which DAS machine LIRIS is running e g DAS13 In the corresponding orange DAS window do obssys shutdownobssys startobssys In the other window where you issue the observing commands do online LIRIS whtdas13 bias liris startudasrtd Then check that the RTD is updated after a glance Q I get a red alert window about the OCSCorbaDrama interface pointing me to fault report 17037 A In the observing window do ps Aef grep OCSCor which will say something like whtobs 23960 tclsh wht release bin OCSCorbaDramaInterface In that case 59 56 APPENDIX TROUBLESHOOTING kill 9 23960 OCSCorbaDramaInterface amp Q The Real time display does not update the image anymore A Check if you have switched the file detection on Sect 3 2 4 If yes and the RTD does not update close the RTD control window and the DS9 display Then from the system console restart the RTD with startudasrtd amp The multrun series in a mdither script stops after the 4th exposure and 1 get an insufficient space left in buffer error message in the talker window A Do a shutdownobssys in the yellow DAS13 window followed by an obssys and a startobssys Possibly do a shutdownobssys and a startobssys in the system console as well Q The sky lines slit image in spectroscopy
22. proper alignment of the stacked exposures Do not make the co averaging too large in order to still be able to sample the sky background frequently enough with dithered exposures see also Sect 5 3 5 5 Polarimetry Polarimetry works essentially as normal imaging i e one uses the same scripts Switch LIRIS into polarimetry mode using lipol lt filter name gt 5 5 POLARIMETRY 37 7 7 Figure 5 1 An image taken in polarimetry mode Four polarisation vectors are mapped simultaneously their orientations are given in this figure 38 CHAPTER 5 IMAGING You then get in one exposure simultaneously four image slices Fig 5 1 of 4 x 1 in size These correspond to polarisation angles of 0 90 135 and 45 degrees respectively counted from the top to the bottom of the image other polarisation angles can be achieved by means of instrument rotation a half wave plate will be installed soon though Just ask the for a new sky position angle Since the flux is divided by four you may want to adjust the exposure times but see Sect 5 3 and 5 4 When using the mdither scripts for imaging you want to decrease the default value of 12 for yoffset since the field of view is only 1 wide in the y direction Alternatively nod5_pol int time title ncyc int nruns int offset float jitter float does a five point dither pattern where the images are offset along the x axis This is identical to the mdit
23. set to 1 second in the script The name of the corresponding slit mask used will be shown in the Object column of the night log window see also Sect 3 2 6 It is also contained in the LIRSLNAM FITS header keyword Now overplot the corresponding LIRIS slitmask region from the ds9 display If you see any deviations follow the instructions given in Sections A 4 and A 5 below for the long slit masks and the MOS masks respectively If you see in the first pass that all masks line up nicely with the defined masks in ds9 then you can safely assume that the slit wheel is working perfectly and you do not need to to run a second pass If you see any odd behaviour try lsethome all to re initialise all wheels before running the task a second time A misaligned ds9 mask can either mean that the slit wheel is not working properly or as likely that the mask positions simply have to be re measured see below The latter will be the case when physical changes to the slitwheel have been done A 4 Alignment of Long Slit masks afternoon Using the through slit exposures Sect A 3 measure the x centre of all long slits at y position of y 400 For the broken long slit lOp75extr measure it at 300 and y 750 A 4 1 Update the ds9 masks In wht var you will find the LIRIS longslit masks for ds9 They are named liris_ lt longslitname gt reg and have the format image box xcentre ycentre width height 0 Adjust all 4 paramete
24. time of the entire exposure series i e within 5 20 minutes If your exposure series is longer either split it into different SCIENCE directories or choose a dynamic superflat e g Window size 4 See the THELI documentation for more details E 4 Superflatting In the Superflatting PG select Subtract SUPERFLAT and Merge sequence IR These tasks will scale and subtract the sky background model from the data in directories SCIENCE_Si and then recollect the corrected images in the SCIENCE directory They have now the extension OFCU in their file name indicating that the sky background has been subtracted The old OFC images are found in the OFC_IMAGES subdirectory A certain fraction of images will still suffer from the reset anomaly 1 they look like in Fig 4 1 To correct for this effect you can calculate and subtract an average column from the data This process is called Collapse correction Enter 2 0 and 5 for DT and DMIN and specify to indicate that the feature you want to correct for runs horizontally Once run the file names change into OFCUC It is now the time to look at quite some of them to make sure that they are satisfying They should largely be flat and in the ideal case not show any features anymore in the sky background apart from possible slow large scale variations one or two images still look very uneven most likely the very first image of the entire sequence then feel free to just delete them Check the
25. you can t accomodate this within the field of view of LIRIS you must go to a blank field Keep this in mind when writing your proposal At the telescope the chopping to a blank sky field is handled automatically by means of dedicated observing scripts 2 2 2 Individual exposure times The individual exposure times are difficult to estimate Most observers want to stay below 20 25 kADU for their science target in order be in the linear regime lt 2 nonlinearity of the detector see also Fig 1 1 This of course depends on the sky background dust level and extinction and the current seeing The individual integration times are therefore best determined by means of test exposures just before the actual observations 2 3 Service mode 2 3 1 Available observing modes Up to 4 hours of time can be allocated through ING s service programmet INCLUDED observing modes for service mode are e imaging and imaging polarimetry using all available filters e spectroscopy and spectro polarimetry using all available long slits and grisms Spectro polarimetry is limited to the use of a dedicated 0775 long slit EXCLUDED observing modes for service mode are e multi object spectroscopy e coronography 2 3 2 Restrictions The LIRIS detector and read out mode can be configured in several ways However in service mode the following restrictions apply e the read out speed is set to slow for all observations e the pre read will be subtra
26. COPY 6 3 Wavelength coverage With LIRIS spectra in the range 0 9 2 5um can be taken The low resolution grisms cover the zJ and H K bands whereas the medium resolution grism is so far only available for The low resolution grisms have a sufficient overlap i e the zJ grism covers the onset of whereas the H K grism still shows the red end of J Thus the spectra can be tied together The blue end of the spectrum is left the red end on the right side in the exposures If extended wavelength coverage is desired the l0p75ext slit mask can be used This long slit is divided in the middle with the upper half shifted to the left and the lower half to the right Thus if an object is positioned in the upper half of the slit the wavelength coverage is extended to the right red and vice versa As compared to the normal long slits the wavelength range covered is shifted by about 1090 of its normal extent in the corresponding direction position an object in the upper or the lower half of this slit mask do lobject_inslit 10 7 x lt y gt noyoff lobject_inslit 10p75extb x y noyoff 6 3 1 Integration times Spectroscopy is much more sensitive to sky background variations than imaging since the various night sky emission lines can change rapidly and independently from each other Only for the very faintest targets should you expose for 600s We recommend to keep exposure times down to 300s or less
27. HELI is fully parallised but LIRIS has only one detector Hence select 1 CPU for the number of CPUs used Then from the instrument list on the right select LIRIS WHT as the instrument In the data directories part enter export images whtguest myrun the main path and FLAT and SCIENCE in the corresponding sections If the path names are identified by THELI to be correct i e existing the background colour of the fields will change from red to green E 2 Preparation Activate the Split FITS correct header task and set the file suffix to fit Two com mands will appear in the command window at the bottom of THELI Click the green Start button to execute this step The task gets green background colour indicating that it has been done already When finished you will get a Done message printed in the yellow message window What happens at this step is that a standardised FITS header is written that T HELI can understand The LIRIS image is also descrambled see Sect 4 1 1 In the FLAT and SCIENCE directories you will now find files ending in 1 fits and a ORIGINALS directory which contains the unprocessed raw data E 3 Calibration The bias pre read Sect 1 1 1 is automatically subtracted from the image before it is stored by the observing system software Hence mark the Do not apply BIAS DARK box at the top Several tasks can be done in one go which we will do here e Mark the the Process flats task Upon execut
28. HISTORY 1 0 1 Document history Version 1 0 5 2007 09 26 e Updated the SA notes concerning MOS handling e Updated the description of how to obtain calibrations in MOS mode e Smaller clarifications throughout the text Version 1 0 4 2007 07 08 e Changed the tex style from article to book Version 1 0 3 2007 06 05 e Explained the warning message one gets with the lobject_inslit command at zenith distances larger than 75 degrees and what to do in that case e The section in the Appendix about the slit mask alignment measurement was ex panded in particular for the MOS masks Version 1 0 2 2007 05 26 e Added a section with various checks to be done in the afternoon and at twilight when LIRIS comes back to the telescope for a new run e Expanded the trouble shooting section in the Appendix e Fixed some minor items throughout the text Version 1 0 1 2007 03 05 e Section 3 has now a display of the TCS info e Fixed Table 4 3 in Sect 4 and Table 5 1 in Sect 5 e The description and name of the ag spec ext script in Sect 6 2 2 was wrong e Added the spectral atlas for the medium resolution K band grism in Appendix D e Included the nonlinearity plot shown in Fig 1 1 Version 1 0 0 2007 01 17 The LIRIS WHT cookbook has its first light 2 CONTENTS 0 2 Scope of this document The present manual is thought as a guide line for visiting and supporting astronomers and based on the author s own experiences with LIRIS at the WHT
29. In a first step point the telescope to your standard star and then bring it close to the first reference hole in the MOS mask lacq_mask mask name gt Then measure the reference star centroid see e g Sect 6 1 lobject inslit mask name gt lt xcentre gt lt ycentre gt 48 CHAPTER 6 SPECTROSCOPY The telescope will calculate and show you the corresponding offsets and asks for confirmation to execute the move At this point DO NOT allow the tele scope to move answer n Instead pass the offset shown to the TO and ask him to execute the move in APOFF mode The reason for this is that the tweak command sent by the lobject_inslit script would change the aperture 0 definition of the TCS which was set previously to a different value by lacq_mask which you executed in first place Once the telescope has moved re check the pointing overplot your slitmask in the RTD If you are not satisfied repeat this step Now you can bring LIRIS into spectroscopy mode and run a dedicated script that will cycle through all slitlets using a 3 point nodding pattern for each slitlet lspec lt grism gt mask name gt lpos mult mask name gt int time offset float nruns int Note that offset should not be larger than about 1 3 of the length of your slitlets other wise the star will be driven outside the slitlet At each nodding position nruns exposures are taken Due to the large offsets from slitlet to slitlet no autoguiding is
30. S filters Filter name type cut on um cut off um wheel 7 broad 0 996 1 069 2 broad 1 170 1 330 2 h broad 1 490 1 780 2 ks broad 1 990 2 310 2 ucm narrow 1 177 1 186 1 narrow 1 216 1 244 1 pabeta narrow 1 277 1 310 1 he narrow 1 558 1 582 1 fe2 narrow 1 632 1 656 1 ch4 narrow 1 640 1 740 1 hel narrow 2 043 2 073 2 h2v10 narrow 2 106 2 138 1 brg narrow 2 150 2 182 2 h2v21 narrow 2 231 2 265 1 ke narrow 2 253 2 287 2 1 2 Observing modes In the following an overview of the various observing modes is given Details are contained in Sect 5 and 6 where we explain their usage in detail 1 2 1 Imaging The imaging mode is offered with a large selection of broad and narrow band filters which are listed in Table 1 4 Apart from the standard configurations filters in wheel 1 can be arbitrarily combined with filters in wheel 2 1 2 2 Long slit and multi object spectroscopy LIRIS supports long slit spectroscopy using a series of slits with different widths Multi object spectroscopy is supported as well The number of slitlets per mask is 20 the slitlets have a length of 10 user definable and are 0 8 wide Three reference sources must be included in each MOS masks for proper mask alignment Further information about possible spectroscopic configurations are given in Table 1 5 1 2 3 Polarimetry LIRIS offers polarimetry in imaging as well as in spectroscopy All filters or grisms used for normal imaging and spectro
31. The LIRISOGOWHT cookbook for visiting and support astronomers Mischa Schirmer Isaac Newton Group of Telescopes Version 1 0 5 September 25 2007 II The cover image shows the cluster ID66 of massive stars taken with LIRIS by the MASGOMAS project image credit Annique Lenorzer Mischa Schirmer Contents 01 Document history ls Poe ee Gy es be ete ANE 0 2 Scope of this document 1 LIRIS an overview 1 1 Instrument properties Detectors ua da ia doe pe ERES Ten 1 1 2 Instrument layout ELS Overheads sia a a a 1 1 4 Detector windowing and maximum frame rate 1 1 5 1 2 Observimgm des u eek exa A a e L23L O 1 2 2 Long slit and multi object spectroscopy 1 2 3 Polarimetry em eee unt er ER 1 2 4 Goronography A We Dd 2 Before your time application observing run 2 1 Exposure time 2 2 Observing strategy 2 come eles Re Ene 2 24 Blank sky arro Ree PR RIDE eta E PUR pn 2 2 2 Individual exposure times 2 3 Bervwice mode wu Saris ee Be te eee Paty eee EG 2 3 1 Available observing 2 9 2
32. To restart the RTD i e if it crashed do the following startudasrtd This command kills the current ds9 session and the control panel before relaunching them Should the old windows not get terminated close them manually 22 CHAPTER 3 GETTING STARTED THE TELESCOPE Th About STOP STOP STOP Cold Finger 71 56 OLD_READING Calib LAHP 1 Argon OFF Fanout 69 5 OLD_READING Calib LAHP 2 Xenon OFF Stagel 66 48 WORKING_Ok StagelT 12 95 WORKING_Ok uit SetPoint Temp 71 97 wit Control loop OFF Press I mbar 3 46e 05 WORKING Status Refresh Quit Figure 3 6 The LIRIS status mimics the current filter slit grism configuration lower left the temperature display lower right the status of the two arc lamps 3 2 5 The three status mimics Three windows Fig 3 6 tell you about the current status of the LIRIS instrument the current configuration of LIRIS the temperatures inside the cryostat and if the arc lamps are switched on off 3 2 6 The night log and TCS info You do not need to take notes about all the exposures you are taking with LIRIS At the end of the night a night log is created which contains all relevant information You can see the night log at any time during the night and put comments into it Fig 3 7 The night log will be put online the next day You can ask the TO to send you a window showing the current status of the telescope
33. _pos dat faking long slits For example you could do maskname left 1 xcentre ycentre maskname middle 1 xcentre ycentre maskname right 1 xcentre ycentre where x ycentre are again the centre coordinates of the slitlet visiting astronomer can then acquire the standard star onto the slitlets with the lobject inslit command like it was a normal long slit mask see Sect 6 5 2 Another way of achieving this in a more automatic manner is to use the approach outlined in Sect 6 5 3 which normally cycles through all slitlets The 1 mult script will put the standard star in all slitlet position it finds in wht var liris maskname mask If you make a backup copy of this file e g cp liris maskname mask liris maskname mask allslitlets then you can remove in the original file all those slitlets which you do not want for the standard star observations This change has no other effect on any other LIRIS MOS operation Which way is less time consuming depends on how many slitlets your observer wants to be done If it is just three or less the first approach is the safer bet However it is more manual work and requires several switches between imaging and spectroscopy mode 54 APPENDIX A SUPPORT ASTRONOMER NOTES Appendix Troubleshooting In arbitrary order Q Taking a series of exposures with multrun or using the nruns param eter in a dither script the exposure series hangs with or after the fourth exposure No
34. background and form the third group After the three sky models have been subtracted from their corresponding images the exposures can be merged again and treated together for the remaining reduction steps See also Sect E 3 and E 4 for how to do that with the THELI software package Please note that a certain fraction of images in the first group which suffers most from the reset anomaly and the poorer temporal sampling can still have bad image backgrounds after sky subtraction These images should be discarded When you plan your observing strategy take this into account and increase e g the number of exposures nruns taken per dither point by one 4 2 Checklist before the observing run Before you actually start observing with LIRIS there are a few settings you have to check Normally LIRIS will be ready to observe when you arrive at the telescope However it can happen that during the afternoon checks the setup has been slightly modified and is 4 2 CHECKLIST BEFORE THE OBSERVING RUN 27 Figure 4 1 The reset anomaly for LIRIS after subtraction of an average sky background model The amplitude and sign of the discontinuous jump seen varies from exposure to exposure It is usually less pronounced than in this example and often absent after sky subtraction It can be corrected for by subtracting an average column from the image 28 CHAPTER 4 BEFORE YOU START OBSERVING WITH LIRIS not as you want it Therefore take 5 minutes an
35. band filter limage ks and ask the TO to point the telescope somewhere close to zenith Integrate between 5 10 seconds to average out seeing effects and then select a non saturated star from the display a peak count of 25 kADU is fine but much fainter stars do as well In the RTD control see Fig 3 5 switch the automatic file detection off and select Star profile Point the cursor onto the desired star press b and then q on the keyboard Under Star parameters you will then find the current FWHM image seeing Increase the focus in steps of 0 05 and repeat the process until you have identified the optimal focus In the latter the stars are perfectly round whereas defocused images show degraded PSFs which makes the recognition of the best focus easy You do not need to interpolate between the focus steps of 0 05 unless the image seeing is extremely good below 0 5 and you want absolute best performance performance is still very good if you are 0 025 steps away from the best focus even in very good seeing If the seeing is good better than 078 you will find the focus very easily If the seeing is bad e g you see noticeable changes in FWHM in two subsequent exposures without changing focus or no difference in two images with different focus then don t spend too much time in focusing If you can t tell the difference between two or three different focus images just take the value in the middle Even if this is not the tru
36. ble when a dither offset has been applied in between default of 3 clearing reads is automatically applied before a new exposure is taken which lowers the amplitude of the ghost to about 0 1 0 2 of the brightness of the original source If this is still too much you can increase the number of clearing reads by cmd LIRIS SETCLEARREADS n Using values of 6 or larger for n should largely eliminate the ghosting for the expense of a increased overhead additional 2s for 6 This holds for imaging and spec troscopy 25 26 CHAPTER 4 BEFORE YOU START OBSERVING WITH LIRIS 4 1 3 Detector reset anomaly Near IR detectors have very different properties than CCDs used in optical instru ments While they are not exposed they are continuously resetted and rapidly reach a resetting equilibrium Contrary when a series of exposures is made the detector reaches an imaging equilibriwm In the ideal world there is no difference between these two states In the real world it takes about three exposures to go from the resetting equilibrium to the exposure equilibrium This is recognised in a varying sky background that stabilises for the rest of the exposure series However even after the sky backgrounds have been calculated and subtracted corre spondingly there is an instable component showing up as a discontinuous jump Fig 4 1 between the upper and the lower two read out quadrants The amplitude of this jump depends amongst othe
37. cted automatically from the images store mode set to DIFF http www ing iac es ds service serviceform php instrument LIRIS 2 4 MULTI OBJECT SPECTROSCOPY 11 e single read out correlated double sampling is used for imaging e four non destructive read outs are used for spectroscopy 2 3 3 Calibrations ING takes only a limited number of calibration images in service mode Please take into account the following when submitting a service proposal e DARKS Dark exposures are not taken by default due to changes of the corresponding in strument characteristics during the night If you require dark exposures they must be taken immediately before or after your observations at night time and thus have to be included in the total time you request e STANDARD STARS Since the sky in the near IR changes rapidly observations of standard stars will not be taken unless explicitly asked for in the proposal and accounted for in the total time requested photometry can often be done by means of 2MASS If you request standard star observations of any type photometric spectrophotometric telluric then please explicitly state 1 which RA DEC specific standard star you wish for each object 2 which instrumental configuration you wish filters and grisms The reason for this is that applicants are often very unspecific concerning their standard stars or unaware of the need for particular standards various spectral types for example
38. d If you observe many targets in different filters you can easily save half an hour per night by selecting the proper filter order For example observing in the order H J Ks is optimal as it takes only 82s for the filter change whereas J H Ks is worst with 126s Doing H J Ks brg hel takes 132s as compared to 296s for J brg hel H Ks Table 5 2 lists the overheads for some filter changes If you use other filters not mentioned in this table and plan frequent filter changes then it is worth making a few timing measurements in the afternoon before your run and work out the optimal strategy Note that filters can be changed while the telescope moves to the next target 40 CHAPTER 5 IMAGING 6 Spectroscopy 6 1 Long slit object acquisition In order to acquire an object on one of the long slits the following steps must be done 1 Obtain an image of your object preferably in the filter where it appears brightest If necessary subtract a dithered exposure from it to remove the uneven sky background Sect 3 2 4 Overplot the desired slit from the LIRIS slits button in the DS9 display 2 The object should be brought to within 10 15 of the slit Usually the default pointing will be good enough for this If not ask the TO to move the object closer to the slit Ideally you want the object at a detector y coordinate of 400 430 3 Ask the TO for a guide star 4 Once the guider has started and stabilised s
39. d flatfields can still be applied correctly To define enable or disable a window you type in the system console Sect 3 2 1 window liris n xmin xmax ymin ymax enable window liris n enable window liris n disable where n is the number of the corresponding window Windowing makes only sense if a source has to be covered with higher time resolution However the gain is much less than expected as Table 1 3 shows By using a window as small as 64 x 64 pixels and the shortest possible exposure time of 0 8s a factor of 2 3 can be gained as compared to a full frame read out The maximum frame rate is then about 0 85 images s 1 1 5 Data store modes The bias and dark level of near IR detectors is usually not stable over time Hence one bias image the so called pre read is taken immediately before the actual exposure post read The default setting is to store only the difference between the pre read and the post read store mode DIFF Alternatively the difference and the pre read DIFF PRE or the pre read and the post read PRE POST can be stored For the latter two a multi extension FITS file is generated For data reduction there is nothing else one can do with the pre read apart from subtracting it from the post read Therefore the recommended setting for the store mode is DIFF To set the storemode from the system console do storemode liris diff diff pre post 1 2 OBSERVING MODES 7 Table 1 4 LIRI
40. d verify the following From within the 4MS window Fig 3 3 and the amp box display Fig 3 1 check that e the Acq Comp mirror is out so that LIRIS can see the sky The latter is indi cated by a continuous line through A amp G box display Unless you want to take spectroscopic flats or arcs move the Acq Comp mirror out e no neutral density or other filter is in the main path e autoguider has the I band filter e the tungsten calibration lamp W is switched off As for LIRIS itself check that LIRIS is responding that the read out speed is set to slow and that you have no windows set All this can be controlled by the LIRIS server window Fig 3 4 To change the read out speed to slow or to disable any windowing type in the system console rspeed liris slow window liris n disable where n is the number of the window you want to disable If the camera server says that LIRIS is not responding which will be indicated in big red letters then this can usually be fixed with dasreset or startobssys in the yellow DAS13 console Ask your TO or SA for help or see the trouble shooting section in Appendix B 4 3 Calibrations 4 3 1 Biases and Darks The bias and dark level of LIRIS is unstable with time as is the case for most near IR detectors bias image pre read is taken and subtracted automatically for every image taken Further calibration with extra biases is thus not needed The dark current o
41. due to uncorrected effects of the reset anomaly or other factors If you want to compensate for the loss increase the exposure time by a small amount or choose e g the 9 point dither pattern over the 8 point dither pattern leaving all other settings unchanged 5 4 Integration times and co averaging The integration times of individual exposures depend on the sky background the maxi mum peak brightness desired and the sampling rate of the sky If you want to stay below 2 non linearity for the object of interest then choose the integration time such that the peak brightness of your object does not exceed 20 25 kADU Typical maximum integration times in the JH Ks filters are given in Table 5 1 In the optimal case you should achieve a sky background level of a few 1000 ADU for optimal results minimised systematics Exposures in narrow band filters can happen to have less than 1000 ADU In particular in Ks band with its high background you may want to require a certain minimum number density of objects per exposure e g when observing high galactic latitude fields You can achieve this by co averaging N subsequent exposures taken at the same dither position coave liris N This is the same as setting nruns N in the mdither scripts but only the combined expo sure will be stored instead of the N individual exposures If the total time required for exposures reaches about 1 minute you should consider using the autoguider to ensure a
42. e corresponding values The format for the TCS mask is SLIT xcentre ycentre REF xcentre ycentre Once you are done with this 11115 maskname reg liris maskname mask wht var A 5 2 Update the slit mask data base In a last step you need to update the slit mask data base so that the lobject_inslit command can work Using the same order as in liris maskname mask insert the first 3 reference coordinates in taurus wht var liris slitdb pos dat using the format maskname 3 xref1 yref1 xref2 yref2 xref3 yref3 If an entry for this mask does not yet exist create it A 5 3 Update ds9 slit mask buttons Once you have done all of the above the only step remaining is to update the RTD so that the visiting astronomer can overplot the MOS masks over the image this end edit home whtobs ds9 ini You must update the names shown on the buttons and further down in the file the actual path This file is self explanatory 5 ALIGNMENT OF MOS MASKS 53 A 5 4 Standard star calibrations with MOS masks Running a standard star through all slitlets of a MOS mask is a very time consuming process it takes easily 30 mins This can be avoided by observing the standard star in only three slitlets the leftmost the right most and the slitlet in the middle This yields the full wavelength coverage To achieve this during night time operation it is necessary to enter three new mask names in taurus wht var liris slitdb
43. e focus the seeing will blur the image so much that the difference is absolutely negligible In bad seeing you can also increase the step size to 0 1 to make focusing a bit easier If the seeing is so bad that you can t find a focus just ask the for the focus value of the previous night You can safely adopt it in such conditions 33 34 CHAPTER 5 IMAGING There is also an IRAF based focus routine available see the LIRIS user s guide on the LIRIS webpage for details However manual focusing is faster more intuitive and less complicated 5 2 Imaging scripts First switch LIRIS into imaging mode see Table 1 4 for the filters available limage lt filter name gt 5 2 1 Imaging without blank sky fields To obtain dithered images with or without autoguider do ag mdither lt ndit gt int time title ncyc int nruns int xoffset float yoffset float jitter float This moves the telescope to ndit 2 3 4 5 8 9 dither positions The dither points are arranged in a regular grid If the autoguider is used take the ag mdither script At each dither position nruns images are taken with the specified integration time The dither pattern is repeated ncyc times step size of the pattern is given by xoffset and yoffset in arcsec defaulted to 12 When the script has finished the telescope returns to the starting point which is the centre of the dither pattern You can set the jitter parameter to a maximu
44. e given in Table 1 1 Note that the BIAS and DARK level are not very stable and change through out the night Therefore the BIAS pre read is determined automatically before every exposure and subtracted right away If you require darks take them immediately before or after your exposures 1 1 2 Instrument layout Most optical and mechanical parts of LIRIS are contained in a cryostat and are therefore unaccessible during observations LIRIS is warmed up and opened usually only once per semester During these sessions usually new multi object slit masks are introduced Smaller reconfigurations take place as well The LIRIS cryostat contains 5 wheels e slit mask wheel containing slit masks and the coronographic mask e the first filter wheel with narrow band filters and Wollaston prism for spectro polarimetry e the second filter wheel with broad and narrow band filters e the pupil wheel for optimised sky backgrounds 3 4 CHAPTER 1 LIRIS AN OVERVIEW Table 1 1 Overview of the LIRIS detector Detector size 1024 x 1024 pixels Pixel size 18 5 um Pixel scale 07251 Field of view 427x ADT Full well capacity 100000 150000 e Spectral range 0 85 2 5 wm Quantum efficiency 90 K 80 J Readout noise CDS lt 10 e7 Readout noise MNDR lt Gain 3 5 3 7 ADU Bad pixels 1 5 most of them near the edges Nonlinearity 296 within 25000 ADU see Fig 1 1 Before Dec 2006 196 within 0 10000 ADU
45. ee the guiding errors displayed in the TCS info display do a Star profile Sect 3 2 4 on your object In the Star parameters field the x and y centroids of the object will be displayed 5 Launch the following command lobject_inslit slit lt x centroid gt lt y centroid gt noyoff This will move the object onto the specified slit If the noyoff option is given the object will be driven only along the x axis onto the slit and no y offset will be made Otherwise the object will be centered about 25 below the middle of the detector at a pixel y coordinate of 400 WARNING The lobject_inslit command will NOT WORK for zenith distances larger than 75 degrees airmass 4 The flexures of LIRIS have not been measured for such angles If you really want to observe at this airmass then ask the TO to bring the object onto the slit 41 42 CHAPTER 6 SPECTROSCOPY 6 When you got the prompt back wait another 20 30 seconds to allow the guider to stabilise Then take another image to verify that the object is centred in the slit overplotted If it is not centred take another exposure to make sure that the seeing is not fooling you If it is still not centred repeat the lobject_inslit command with the new object position as an argument 7 OPTIONAL If you want to make sure that the object is actually in the slit take a through slit exposure using the command lslit slit to move in the desired slit Then take the exposure
46. eli gui theli from anywhere in the console In the following we assume you took a set of dithered exposures in Ks band of NGC 1234 with nruns 6 and ncyc 2 and 10 seconds exposure time The target is compact so that no blank sky fields were required You have taken a set of bright skyflats as well We give here a rough step by step guide for a typical simple imaging session without going deeper into details or exploiting the full functionality of THELI The result obtained can be considered being of science grade quality Intermediate results serve as a quick look step E 1 Organising the data and initialising THELI Create a new directory in export images whtguest let s say myrun and therein sub directories called FLAT and SCIENCE The data taken in your night can be found under obsdata whta 20070125 if January 25th 2007 was the date of your observing run Copy all the flats into FLAT and all exposures of your target into SCIENCE In THELI you find seven processing groups hereafter PG e Initialise e Preparation l Available via anonymous ftp from ftp ing iac es mischa THELI 67 68 APPENDIX E IMAGING DATA REDUCTION WITH THELI Calibration Superflatting Weighting Astrom Photom e Coaddition In the nitialise PG enter a new LOG file name e g 1234 and then clear the processing status This is needed since THELI automatically loads the last LOG file from the previous reduction run when launched T
47. f LIRIS is very small Extra dark calibrations are usually not necessary since the dark contribution is automatically subtracted from the data during the sky subtraction This is considered superior than an extra calibration since the correction is determined from the data itself If you want extra dark calibration then take at least 30 darks immediately before and after your observation and possibly in between if your exposure sequence is long Reject the first 5 darks from each series since they are not representative Before you can take a series of N darks you must first move in some blanks into LIRIS lblanks lt multbias multdark gt liris lt gt int time 4 3 CALIBRATIONS 29 Table 4 1 Exposure times dome flats Filter 2 J H Ks brg hel Exp time s 25 40 16 25 25 30 Since bad pixels usually saturate quickly in detectors a master dark can serve as a very good bad pixel mask 4 3 2 Flats Since the read out time of LIRIS is short you can aim for a large number gt 30 of flats per filter in order to obtain an optimal S N ratio Flat fields are essential for LIRIS since the detector is not homogeneously sensitive and shows some cosmetic features Most of hese are corrected well by flatfields others disappear later on with the sky background subtraction The flatfields can be considered as very stable within few percents over long periods of time months Skyflats are superior over domeflats since they
48. f the offset parameter Each nodding point will then be offset from its nominal position by a random vector of the specified length The number of multiple non destructive reads is automatically set to mndr 4 if the mndr parameter is not set Like that the exposures taken get read out 4 times and averaged in order to suppress the read noise In imaging mode this is not necessary due to the high sky background If you require three nodding positions then use ag spec nod3 It has the same parameters as the normal ag spec nod script One cycle consists of a ABC pattern The first exposure will be taken at position A 0 0 the second one at 0 offset and the third one at C 0 offset 6 2 2 Spectroscopy with blank sky fields If your target is extended then you need to chop to a blank sky field frequently every few minutes so that you can calculate a sky background model from it For this purpose you can use ag spec ext int time lt sky_offset gt title ncyc int nruns int offset float mndr int jitter float invert This moves the telescope in an Obj Sky Obj Sky fashion where the sequence Obj Sky forms one cycle Sky refers to a blank sky position The sky positions are randomly gen erated to lie on a 15 long segment perpendicular to the slit centred at sky_offset 0 No autoguiding is performed at the sky position field Use the invert option to reverse the sky offset 44 CHAPTER 6 SPECTROS
49. fferent for your observing run Polarimetric flats must be taken as low on the sky as possible since the twilight sky is highly polarised at larger elevations Ask the TO explicitly to lower the telescope Even 30 CHAPTER 4 BEFORE YOU START OBSERVING WITH LIRIS Table 4 2 Exposure times for spectroscopic flats with the 1 wide slit Grism slit light source ND filter exp time sec Ir zj 11 tungsten lamp ND 1 8 1 3s lr hk l1 dome flat lamps 5 0s mr_k 1 dome flat lamps 10 05 though skyflats should preferred in case of polarimetry domeflats can be a good alternative for the more complicated skyflats 4 3 3 Spectroscopic flats For spectroscopic flats at zJ wavelengths you can use the tungsten lamp that is integrated in the A amp G box In order to bring the light from the lamp to the detector you need to move the Acq Comp mirror into the light path include a neutral density filter ND 1 8 and switch the tungsten lamp on this can be done from within the 4MS window Fig 3 3 In the A G Observer menu tab select the W for the tungsten lamp then click the gears symbol next to it to switch the lamp on or off In the Mirror Control section select Acq Comp to move the mirror in or out Now bring LIRIS into spectroscopic mode and do not forget to insert the slit mask you want as well Then take N spectroscopic flats lspec lt grism name gt slit mask name gt multflat liris N int time title
50. for the autoguider This is largely irrelevant for imaging since exposure times are very short However for spectroscopy you want to select the band filter The latter is closest to the near IR and therefore differential tracking effects at higher airmasses due to atmospheric dispersion are minimised a default we recommend to use J band filter always unless you can t find a bright enough guide star Neutral density filter If you observe very bright sources you may want to consider a neutral density filter You can choose from broader range of ND filters from the 4MS window However be aware that these filters lead to significant vignetting In addition the density has been evaluated for the optical only The transmission in the near IR can be higher than expected and not be neutral any more also because these filters are warm and not cold Hence instead of using the A amp G box ND filters using a LIRIS narrow band filter is probably a better alternative if high flux levels are a problem Consider the jc hc and kc continuum filters 3 2 3 The camera server window The black camera server window Fig 3 4 shows under the LIRISCASS menu tab the current status of the LIRIS camera Apart from telling if communication with LIRIS has been established successfully this is the e total exposure time of a started exposure e elapsed exposure time of the current exposure e current exposure number of a sequence of exposures e
51. fore unacceptable Also note that the initial acquisition of the guide star is 5 8 FILTER CHANGE OVERHEADS 39 Table 5 2 Some filter change overheads in seconds to z J H Ks brg hel from 2 18 63 47 74 69 J 0 58 43 69 64 39 0 68 23 21 Ks 54 55 0 39 34 brg 55 86 58 0 80 hel 56 79 64 18 0 a bit cumbersome with the WHT and can take 2 3 minutes If you can t decide if you should guide or not shorten the exposure times a bit and go for non guiding You will save a significant amount of overhead For exposure times over 60s the guiding overhead is negligible When you need guiding tell the to centre the guide star in the guide probe so that the offsets of the ag dither script do not drive the guide star off the probe The field of view of the latter is 1 and therefore could accomodate a 9 point dither pattern with a step size of 20 and a jitter of up to 5 Do not forget to tell the TO that you are using a large dither pattern so that he can configure the guider s tracking window accordingly 5 8 Filter change overheads Changing LIRIS filters and other wheels takes on average 1 minute ranging from 18 to almost 90 seconds The filter change overheads are not symmetrical i e changing from e g z to H takes significantly more time than changing from H to z Thus if you observe many targets in several filters and with short integration times the filter change overhead becomes an important factor which can be optimise
52. he magnitude limit such that you obtain between 10 200 reference sources Then select Create source cat For a very empty field enter 3 and 5 for DT and DMIN respectively If you see few dozen stars you may go with 5 5 or 5 10 If the field starts to show hundred sources go with 10 10 and if it is very crowded try 50 10 The reason for this is to keep a reasonable number of objects that will be matched with the reference catalogue In general the tolerance for these parameters is very large so don t think too much about it and just try We recommend you create the catalogues now before proceeding to the next step Then activate Astro photometry and choose Astrometrir 1st choice but slower or 2nd choice but faster Both calculate full astrometric solutions including distortion correction Both will usually run on most data sets If one fails try the other If both fail try changing the detection parameters and possibly the magnitude limit If they still fail just go with the Shift only option The latter does not care about distortions or sky coordinates and matching with the reference catalogue it just determines the offsets of all exposures with respect to the first image This can be useful in very sparse fields where hardly any sources are visible For a more detailed discussion see the T HELI manual The astrometric solution will be stored in SCIENCE headers the object catalogues in SCIENCE cat For the lat
53. her script with the yoffset set to zero However in that case the jitter value can not be set to a value larger than zero hence the existence of the nod5_pol Script 5 6 Coronography To get into coronography mode do lcor filter Two occulting spots 1 2 diameter are available one in the upper left the other in the lower right quadrant The source can be centred behind one of the two masks using lobject inslit lt corup cordown gt If you are not happy with the alignment ask the TO to make small offsets in the de sired direcion Note that you have to be guiding in order to ensure that the source stays behind the mask over longer times You should already be autoguiding before you make fine correcions WARNING The lobject inslit command will NOT WORK for zenith dis tances larger than 75 degrees airmass 4 The flexures of LIRIS have not been measured for such angles If you really want to observe at this airmass then ask the TO to bring the object onto the mask 5 7 Autoguiding The WHT tracks rather well As a rule of thumb one can expose for 60s without noticeable image degradation in the optical In the near IR the seeing is significantly better and therefore tracking errors show up quicker We recommend to guide for exposures longer than 40s For exposures with less than 40 seconds guiding is superfluous for exposures of less than 20 seconds the guiding overheads become comparable to the exposure itself and are there
54. hough very faint is in the slitlet Since the sky background saturates quickly you can achieve long integration times using a co averaging of subsequent exposures For example coave liris 10 run liris 15 gives you an average of ten 15 second exposures ie an integration time of 2 5 min utes This helps in the acquisition of very faint targets Do not forget to switch the co averaging off once you start taking spectra Otherwise you will stay for a very long time on the same dither point and a proper sky background subtraction will become impossible 6 2 SPECTROSCOPY SCRIPTS 43 6 2 Spectroscopy scripts The scripts for spectroscopic observations are very similar to the imaging scripts Sect 5 2 6 2 1 Spectroscopy without blank sky fields To obtain dithered nodded spectra using the autoguider do ag int time title ncyc int nruns int offset float mndr int jitter float This nods the telescope in an fashion with A and being the two nodding positions If the autoguider is not desired use the spec nod script not recommended see Sect 6 1 At each nodding position nruns images are taken with the specified integration time The AB pattern is repeated ncyc times The nodding step size is given by offset in arcsec defaulted to 12 When the script has finished the telescope returns to the starting point You can set the jitter parameter to a maximum value of 3096 o
55. ion this will create a master flat called FLAT_1 fits in the FLAT directory e Activate the Spread sequence IR task As described in the example of Sect 4 1 3 one has to split the series of exposures taken at one dither point into different groups of similar sky background Our example consists of 6 exposures at each dither point and the detector equilibrium will be reached with the third exposure Hence we SUPERFLATTING 69 enter 3 and 6 in the group and length fields respectively This task will create three subdirectories SCIENCE_S1 3 at the same level in the directory tree as the SCIENCE directory and group the corresponding images therein e Activate the Calibrate data task We also need to calculate a superflat or sky background model hence choose the Calculate SUPERFLAT option Enter 6 5 and 256 for the fields DT DMIN and SIZE These are Sextractor parameters that are used in the detection and masking of objects before the images are combined for the sky background For the Window size you enter 0 Then click Start and all tasks will be executed After finishing you will find a file SCIENCE_Si_1 fits in the SCIENCE_Si directory This is the corresponding sky background model for these exposures has not yet been applied The exposures themselves have a string OFC in their file name meaning that they have been flat fielded This procedure assumes that the sky background has not changed significantly within the
56. k name that refers to the leftmost middle and rightmost slitlet It can be just a normal number e g 1 2 or 3 or a string such as left middle or right depending on how the setup was done If this information was not passed onto you by your SA look it up in wht var liris_slitdb_pos dat Once you are fine with the centering overlay your mask from within the RTD switch to spectroscopy mode and get your exposures lspec lt grism gt lt maskname gt lag spec_nod3 int time title ncyc int nruns int offset float mndr int jitter float Note that offset should not be larger than about 1 3 of the length of your slitlets otherwise the star will be driven outside the slitlet The advantage of this method is that is a very controlled way of obtaining your standard spectra as it works like for longslit spectroscopy In addition you can use autoguiding which is not possible with the method described below due to the large offsets made The disadvantage is that it requires several switches between imaging and spectroscopy mode which increases the overhead If you want only three or less slitlets covered this is the method of choice But see the last paragraph of the following section 6 5 3 Calibration standard star in all or a few slitlets If your science requires your standard star to be taken in all slitlets which can take 30 mins or longer the procedure is different from the one outlined in Sect 6 5 2
57. m value of 3096 of the x yoffset parameter If the x yoffset are different then the smaller one serves as the reference Each dither point will then be offset from its nominal position by a random vector of the specified length We strongly recommend to use the jitter option as this increases the sampling of the sky and therefore leads to better results 5 2 2 Imaging with blank sky fields If your target is extended then you need to chop to a blank sky field frequently every few minutes so that you can calculate a sky background model from it For this purpose you can use ag mdither ext lt ndit gt int time lt sky_offset gt pa offset title ncyc int nruns int xoffset float yoffset float jitter float This does essentially the same as the normal mdither script but it goes to a blank sky position after one cycle The offset of the blank position with respect to the target is specified by sky_offset in arcsec and the position angle offset No autoguiding is performed at the sky position field since the guide star is driven off the guider chip It will be re acquired automatically when moving back to the target field l www ing iac es Astronomy instruments liris liris cookbook ch4 html 5 3 CHOOSING THE CORRECT VALUES FOR AND NRUNS 35 5 3 Choosing the correct values for ncyc and nruns The following factors influence your choice for the and nruns parameters e The more nruns you
58. mally still allow for a 3 point dither pattern for optimal sky background subtraction The slitlets of one mask do not need to have the same length The MOS masks are inserted into a portion of the focal plane of the telescope which is free of distortions Therefore it is sufficient that the observer provides a target list with accurate 0 1 or better RA and DEC coordinates The target list must include the coordinates of three reference stars which are used to accurately align the mask on the sky Ideally these three reference stars form a triangle that extends over a large part of the detector area The reference stars must be highlighted in the list since they will not get slitlets assigned in the mask but circular holes The coordinate list must be sent at least two months in advance of the scheduled warming up of the cryostat to IAC The company that produces masks needs one month for the production hence the long lead time It is your responsibility to provide the coordinate lists in due time If you fail you run the risk that by the time of your observing run your masks will not be available in LIRIS Any questions concerning the scheduling and production of MOS masks should be directed to IAC contact Jose Acosta japQiac es 3 Getting started at the telescope 3 1 Introduction by your support astronomer Your support astronomer SA will usually meet you at around 15 00 in the WHT con trol room The basic in
59. mp G observer Alternatively you can use the agcomp command in the system console Moving the mirror into the beam takes about 20 seconds The light path in the A amp G box display will be updated correspondingly Fig 3 2 Note that with the Acq Comp mirror in place LIRIS can not see the sky anymore To move the Acq Comp out again just click the same push button again and confirm the action or use the agmirror out command 3 2 THE WHT LIRIS OBSERVING SYSTEM 15 File Help CAGBFilters A amp G Cassegrain A amp G Box Last Handshake 14 34 11 Port Filter RGC S mm Filter B Lamps Colour Out Door ND Filters Out Figure 3 1 The A amp G box In that state LIRIS can see the sky In this representation the telescope is on top of the figure and LIRIS below 16 CHAPTER 3 GETTING STARTED THE TELESCOPE File Help CAGBFilters A amp G Cassegrain A amp G Box Last H ort Filter Filter R Mirror Lamps Off Colour Filters ND Filters Out MF NDO 3 MF POL PER MF POL CIR ND1 3 Theta 50001 millidegs Figure 3 2 The A amp G box In that case LIRIS can not see the sky but the tungsten and arc lamps for spectroscopic flats and wavelength calibration 3 2 THE WHT LIRIS OBSERVING SYSTEM 17 File Help A amp G A amp G Observer Help Message Console Mirror Control Filter C
60. nearity LIRIS WHT 2007 05 07 lower left 4 normalisation area from 3 to 30 kADU L lower right J L SPEED SLOW FILTER J upper left 4 105 upper right relative sensitivity 0 90 lappa lp lt pt 1 IN 0 10 20 30 40 Figure 1 1 The linearity of the LIRI EN OU S detector Table 1 2 General LIRIS overheads Detector read out time Time until image is displayed and saved after read out Time until you get the prompt back after read out Average time for filter change Average time for grism change Average time for switching between imaging spectroscopy polarimetry coronographic modes Acquisition of a guide star Acquisition of object onto slit for spectroscopy Acquisition of a multi object spectroscopy mask Restarting the DAS if communication with LIRIS is lost Restarting the entire observing system 18 4s 2s 1 min 1 min 1 min 2 min 5 15 min 10 15 min 0 5 min 3 5 min 6 CHAPTER 1 LIRIS AN OVERVIEW Table 1 3 LIRIS maximum frame rate 0 8s exposure time Window size frame rate 5 1 1024x 1024 0 37 512x512 0 45 256x256 0 64 128x128 0 77 64 64 0 85 1 1 4 Detector windowing and maximum frame rate Up to four non overlapping windows can be specified for the LIRIS detector The resulting image is as big as the unwindowed data showing the windowed sections in their proper locations Thus unwindowe
61. nough default 107 to accomodate three dither positions The spectra of three reference stars used for mask alignment can be seen as well 46 CHAPTER 6 SPECTROSCOPY 6 5 Multi object spectroscopy 6 5 1 Acquisition The acquisition of a MOS mask is similar to that for a long slit The only major difference is that you have to specify three reference sources instead of one 1 Ask the TO to rotate the instrument to the required sky position angle Switch the autoguider OFF 2 Take two dithered images with an offset of about 5 for sky subtraction Subtract them from each other in the RTD display 3 Overplot the mask from the DS9 display using the LIRIS MOS button in the menu bar or load it directly in DS9 via Region Load wht var liris_ lt maskname gt reg 4 Measure the three x and y coordinates using the Star profile task in the RTD control see Sect 3 2 4 5 Acquire the mask in a first approximation calling lobject_inslit lt mask name gt lt 1 gt lt 1 gt lt x2 gt lt y2 gt lt x3 gt lt y3 gt This routine will calculate shifts and rotations The rotations are expected to be small but can still lead to large movements of the guide star hence the guider has to be switched off for this step The reference stars must be given in the right order which you can check by means of the overlaid mask in DS9 If you do not remember the order of your reference sources have a look at wht var liris_slitdb_p
62. nto the light path see also Sect 3 2 2 and then take the arc images 2 exposures per setting are sufficient agcomp 1 lt grism name gt lt slit mask gt llampon 1 llampon 2 multarc liris lt N gt int time Arcs llampoff ALL agmirror out Guidelines for arc exposure times are given in Table 4 3 the spectral atlas can be found in Appendix D For the lr_zj grism two exposure times are recommended since there is a small number of bright lines that saturate quickly For the medium resolution K grism the number of arc lines is rather small You may want to switch both lamps on at the same time It is usually sufficient if arcs are taken at the beginning of the night You are free to take more before or after your target However the best wavelength calibration comes from the numerous sky lines themselves since they are tied directly to each individual exposure 32 CHAPTER 4 BEFORE YOU START OBSERVING WITH LIRIS Chapter 5 Imaging 5 1 Focusing Focusing LIRIS is best done manually The focus is usually found at 98 10 0 05 refo cussing during the night is not necessary unless the seeing conditions were bad during the focus run and improve significantly later at night To change the telescope focus type focus lt value gt in the system console To find the optimal focus take into account that the seeing improves significantly with increasing wavelength and decreasing airmass Select the Ks
63. ontrol Move Initialise Colour Main ND Filter Aux Filter Autoguider Filter Csi a Ee laudi mm udi sd Lage Feed EE FU 157 Large Feed Small Feed 2 MF POL PAR 2 MF NDO 3 2 1 2 amp G CLEAR Small Feed Acq Comp 3 MF POL PER 3 MF ND0 3 3 3 AG OPAQUE ACRE 4 MF POL CIR 4 MF ND1 3 4 Nm 5 95 5 MF ND1 8 5 RGOR1 5 JAG Y E MF GG385 6 MF ND3 0 6 HARI Bes Clear ove 2 1 Global Status and Control and Focus Control Comparison System AG Focus microns TV Focus microns Comparison Filter A Lamps 4000 10000 4MS Monitor Mode SMS CF cLEARI B Enabled Disabled coag 000 FeNe oss EIN o A 0 8000 0 2 CF NDOZ Fear Locked Unlocked 2395 8699 3 0 6 Update Filters temes Q 189 Qi 008 AIMgCa Update All Status AG Radial microns AG Theta milli degrees 20000 5 CF ND1 8 NaK Stop All Mechanisms H 60000 120000 10000 30000 6 CF ND3 0 _ Reset Controller 3 7 7 cF GG375 Monitor Mode Level 2 9 40098 Gy 180000 66496 S Last4MS Response 142411 gg Pm O Figure 3 3 This GUI controls the amp box 18 CHAPTER 3 GETTING STARTED THE TELESCOPE Autoguider Filter In the 4MS window you can select the filter used
64. os dat Therein you will find an entry for your MOS mask in the form maskname 3 x1 yl x2 y2 x3 y3 with xi and yi being the pixel coordinates of the reference sources WARNING The lobject_inslit command will not work or give a warning for zenith distances larger than 75 degrees airmass 4 If you really want to observe at this airmass then fine tune the mask acquisition manually or ask the TO to bring the objects onto the mask 6 The mask is now roughly acquired Switch the autoguider ON 7 Repeat steps 4 and 5 for the fine tuning of the mask position It can be less time consuming to centre the mask manually at this point Ask the TO for small offsets and or rotations 6 5 MULTI OBJECT SPECTROSCOPY 47 6 5 2 Calibration standard star in a few slitlets Apart from arcs and spectroscopic flats see Sect 4 3 you might want to observe a standard star in a few or all the slitlets The latter method can easily consume 30 minutes of time We recommend to use it only if really necessary In many cases it will be sufficient to put the standard in the left and rightmost slitlet and in one close to the middle of the mask This gives you full wavelength coverage The method is as follows and essentially identical to long slit spectroscopy First go to the standard source measure its centroid position see Sect 6 1 then lobject_inslit lt maskname_xxxx gt lt xcentre gt lt ycentre gt Here xxxx is an appendix to your normal mas
65. ou know which mask belongs to which image The third one will be used by the TCS to align the mask with respect to the sky Hence the latter is sloppily refered to as the 9 mask and we will deal with it in a moment Using the Regions Load feature in ds9 overplot rXXXXXX reg over the corresponding through slit exposure You can also directly use liris_maskname reg if you know to which image it corresponds LIRSLNAM header keyword Check that all the slitlets and reference positions are properly detected The latter show up as round holes If that is case then just copy liris_maskname reg and liris maskname mask to wht var and you are done Non or ill detected slitlets If a slitlet or reference position falls onto the border between two readout quadrants it probably is not or not properly detected In that case edit liris_maskname reg until the corresponding slitlet is shown in its right position The 52 APPENDIX A SUPPORT ASTRONOMER NOTES format of that file is image box xcentre ycentre xwidth ywidth 0 text Slt index image circle xcentre ycentre 10 text Ref index for the slitlet and the reference positions respectively All values are given in pixels If a slitlet is missing entirely you need to create a new entry for it at the end of the file and assign it a running index number Once you found the proper entries you need to update liris maskname reg if you worked on rXXXXXX reg and the TCS mask with th
66. possible Due to the large number of large offsets made it is possible that due to error prop agation the centering on the last slitlets is not perfect anymore evaluate this or in case of trouble shooting run 1pos_mult in imaging mode and see how the standard star is positioned Once happy do it for real in spectroscopy mode The 1pos_mult script looks up the slitlet positions in the file wht var liris maskname mask It is possible to remove any entries of unwanted slitlets from this file after creating a backup copy and the standard star will then be put only onto the remaining slitlets e g the leftmost middle and rightmost slit Discuss this possibility with your support astronomer The advantage of this method is that as compared to the more manual ap proach in Sect 6 5 2 it reqires only one switch from imaging to spectroscopy mode and hence saves a lot of overhead The disadvantage is that all the offsets will be made in an unsupervised way and it can happen that after a series of offsets the standard star is not properly centred anymore in the slitlets Appendix Support astronomer notes Here is a list of various checks the SA should do after LIRIS came back to the telescope A 1 Rotation centre twilight Take two pictures of a star at sky position angles 0 and 180 degrees and then calculate the centre of the line connecting the two points Ask the TO to update the data base with the new rotation centre in
67. preserve the natural light path and do not suffer from possibly enhanced fringing effects coming from the flatfield lamps Narrow band flats can be taken already before sunset or after sunrise broad band flats just after sunset Ask the TO to either switch off the tracking of the telescope or use one of the dither scripts see Sect 5 2 to make sure that stars do not end up on the same detector pixels The TO will point the telescope to a suitable position on the sky A number of N flat fields can be obtained with the command multflat liris lt N gt lt integration time gt title no dithering or mdither 9 lt integration time gt title Domeflats can be done using a weak flat lamp which is mounted on the top ring of the telescope It can be switched on from the control desk on the left side in the control room your SA or TO will show you the magic button Switch the weak lamp on and turn the intensity to maximum Let the three additional much stronger flat lamps turned off they can be used for spectroscopic flats see below Even in narrow band filters LIRIS is too sensitive for even one of the brighter lamps Individual exposure times for all four broad band filters and two narrow band filters can be found in Table 4 1 However note that in particular for the K band filters around 2um the illumination level mostly depends on the dome temperature rather than the lamp brightness The corresponding exposure times can therefore be di
68. read out speed e read out mode i e CDS correlated double sampling or MNDR multiple non destructive reads e detector windowing 3 2 4 The real time display RTD Any exposure taken with LIRIS will be displayed in the RTD which uses the commonly known 059 programme The behaviour of the RTD is controlled over the RTD control panel Fig 3 5 One of the most frequent applications is to subtract a previously dithered exposure from the current image yielding a sky subtracted image see the OBS REF option below Therein faint objects are better visible Further display options are available if the store mode Sect 1 1 5 is other than DIFF in which case e g the pre read can be shown as well On the RTD control panel amongst others the following items can be found 3 2 THE WHT LIRIS OBSERVING SYSTEM 19 File Help TVCASS AUTOCASS LIRISCASS AUX SUMMARY LIRIS Server Responding mndr idling slow ndow ndow nd D Run No Run Figure 3 4 The LIRIS camera server window 20 CHAPTER 3 GETTING STARTED AT THE TELESCOPE 919862 9198238 obsdata whta 20070105 astRRHP 319852 Mew Target Dir Figure 3 5 The LIRIS RTD control panel 3 2 THE WHT LIRIS OBSERVING SYSTEM 21 e Observation data Shows the file name of the image actually displayed Reference data Shows the file name of the image loaded as reference data see below e Data directory
69. rs on exposure time illumination level and the order of the image in a series of continuous exposures There is also a large erratic component involved hence this discontinuity can not be modelled as a function of the parameters mentioned Each image has to be corrected for this effect individually Since the effect is also present in flat fields the photometry of objects in the corresponding regions can be affected by 5 2096 If you have dithered observations see Sect 5 2 and you take a series of e g 10 exposures per dither point then the detector reaches the imaging equilibrium with the third exposure All subsequent images will have the same image background as this third exposure When you then move the telescope to the next dither point the detector is resetted during the slew and goes into the resetting equilibrium again and thus is in the same state as just before the first exposure sequence Therefore the image backgrounds are the same for the i th exposures in n th sequence The consequence is that the sky background has to be determined separately for each group of images Example You have 12 images per dither point and you know that the detector settled into its equilibrium with the third exposure Then you need to calculate three different sky backgrounds The first is for the group of images 1 13 25 etc in the entire obseving series Images 2 14 26 etc form the second group All the remaining images have the same sky
70. rs such that the slitmask overlaid matches the image A 4 2 Update the slit mask data base In taurus wht var liris_slitdb_pos dat update the third column xcentre for the long slits with the measured x values at the given y position Note that this is different from the values you entered in the ds9 masks 5 ALIGNMENT OF MOS MASKS 51 The and y positition given here is the one onto which the 1object inslit command will move the object Normally the data base has one line entry per mask The format is maskname 1 xcentre ycentre For the long slits for example 1 looks like 10p65 1 531 5 400 10p75 1 531 5 400 11 1 538 5 400 15 1 531 1 400 lOp75extr 1 400 5 750 10p75extb 1 625 5 300 For the l0p75ext slit you have to measure two positions as this mask has two shorter long slits at different x coordinates for extended wavelength coverage A 5 Alignment of MOS masks A 5 1 Update the ds9 masks and the TCS mask Take all the MOS through slit exposures see Sect A 3 and copy them into some directory let s say DIR Then run the following script home whtobs mischa get_mask sh lt DIR gt Three files liris_maskname reg rXXXXXX reg and liris_maskname mask will be cre ated in DIR The previous two are just masks for the overlay in ds9 they have identical content The first one is the one which will be used when you click the corresponding ds9 button the second one is created just for your convenience so that y
71. s 5 minutes in J band it can be 20 30 minutes 5 3 1 Example 1 You want a total integration time of 3600s in Ks band Choosing an integration time of 15 seconds leaves you with 240 exposures Take the 8 point dither pattern hence you must take 30 exposures per dither point Choosing nruns 5 and ncyc 6 gives you exactly this In that case you would spend 75 seconds 85 seconds with overheads per dither point and you would get 8 different sky positions in about 11 minutes This would be a good sampling of the sky variation in Ks Since the sky in Ks is very bright and objects are usually sparse you can calculate a good sky model from just 5 dither pointings If you feel this is not good enough decrease the exposure time from 15 to 10 seconds and increase ncyc 5 3 2 Example 2 You want a total integration time of 5000s in J band The sky varies much slower than in Ks and is also less bright Thus you can expose 60s which would require the use of the autoguider for optimal results 60s does not divide 5000s so increase it to e g 36 CHAPTER 5 IMAGING Table 5 1 Typical maximum integration times used for imaging Filter z J Ks Exp time s 120 60 30 20 62 5s or accept a slightly smaller total integration time Hence with 62 5s you must take 80 exposures This calls for the 8 point dither script which you could repeat 10 times ncyc 10 and nruns 1 or ncyc 5 and nruns 2 Expect a loss of 5 max 10 of exposures
72. scopy are available in polarimetry mode as well The only restriction for spectro polarimetry is that a particular slit of 0 75 width has to be used 8 CHAPTER 1 LIRIS AN OVERVIEW Table 1 5 LIRIS slits and grisms Mask name slit width 10p65 0 65 10p75 0 75 10 75 0 75 larger wavelength coverage see Sect 6 3 11 10 15 5 0 Grism name resolution spectral range scale lr zj 960 z 0 887 1 531um 6 1 pix lr hk 945 K 1 388 2 419um 9 7 pix 2500 mr h 2500 H 12 2007 mr j 2500 J 12 2007 mrz 2500 z 12 2007 LIRIS displays four polarisation angles of 0 45 90 and 135 degrees simultaneously see also Fig 5 1 For this purpose the field of view is truncated to 4 x1 1 2 4 Coronography The diameter of the coronographic mask is 1 2 It can be used with any imaging filter The coronographic mask is located in the slit wheel and can be replaced by a MOS mask if demand for multi object spectroscopy is high Chapter 2 Before your time application observing run LIRIS is a very easy to use instrument Therefore only a minimum of preparations is required before your observing run In order to use your time at the telescope in an optimal way e make sure you know when your targets are visible ING offers very nice tool for that purpose at http www ing iac es ds staralt index php e prepare finder charts in advance e decide if and which standard stars you wan
73. t and write down not only their names but also their coordinates this sounds self evident but unfortunately unprepared observers are still not uncom mon in these days Thus please don t leave this to the very last moment 2 1 Exposure time calculator Use the exposure time calculator at http www iac es project LIRIS to estimate the total amount of observing time you need for your targets However be aware that the ambient conditions atmospheric temperature dust level solar activity can change the IR sky background brightness by more than magnitude Thus the ETC estimates can always only be a rule of thumb and you may want to sacrifice or gain the one or other target However this decision can only be made at night when you know the conditions 2 2 Observing strategy 2 2 1 Blank sky fields In order to get proper sky background model you need to dither the exposures For point like sources or sources much smaller than the dither pattern you can stay on the 9 10 CHAPTER 2 BEFORE YOUR TIME APPLICATION OBSERVING RUN target field However if your target is extended it is mandatory that you go to a blank sky field very close to the target to obtain a sky estimate In that case only 50 of your observing time is available for the actual science target There is way you can obtain the sky background from images of an extended target if the dither pattern is not at least 2 5 as big as the target itself If
74. ter a skycat version exists for each image which can be used for overplotting To subtract the remaining sky residuals enter 1 5 and 256 for DT DMIN and SIZE Choose Model sky for the method or any of the other options The manual will describe which ones are useful for which case After sky subtraction images OFCUC sub fits exist in the SCIENCE directory E 7 Coaddition Lastly the coadded image is created You do not need to provide any parameters Just execute the task This will create a SCIENCE coadd DEFA folder the coadded image therein E 7 COADDITION 71 will be called coadd fits If the astrometric solution has failed i e you see double sources or big warps switch to a different solution method than the one used and or obtain a denser less dense object reference catalogue Run the astrometric solver again and then the coaddition You do not need to re run the sky subtraction If you do not want to overwrite the old coaddition enter an arbitrary 4 character long string in the JD field If you enter e g BBBB the new coaddition directory will be called BBBB Unless you have chosen Shift only for the astrometry the coadded image has a full astrometric calibration No absolute photometric calibration is done
75. troduction into LIRIS imaging only takes about one hour for observers with no previous observing experience at the WHT For spectroscopy or com plicated imaging programmes that require a detailed discussion of the observing strategy the introduction can last 2 hours The SA will usually stay with the observer during the first night of the run until around 23 00 unless problems or other issues require a longer stay The WHT telescope operator TO will be present all night The SA and the duty engineer can be contacted on their mobile phones written on the white board in the control room 3 2 The WHT LIRIS observing system The observing system will be ready and waiting for you to start your observing run It consists of some basic commands and several windows which are explained in detail below 3 2 1 Basic observing commands Most operations with LIRIS are handled from the command line in a console showing SYS gt prompt From therein you launch exposures scripts change filters or switch e g between imaging and spectroscopy Most tasks are explained further below in the corre sponding sections Here we give an overview of the most basic commands To change into imaging mode and put a certain filter see Table 1 4 limage lt filter name gt To change into spectroscopy mode and put a certain slit see Table 1 5 lspec lt grism name gt slit name gt 13 14 CHAPTER 3 GETTING STARTED AT THE TELESCOPE To take an exposure

The LIRIS@WHT cookbook - Isaac Newton Group of Telescopes

Contents

Download Pdf Manuals

Related Search

Related Contents