VISIR USER MANUAL
Contents
1. 3 0 Calibration Units db ra ea kee se be A ew ee en A a NN 3 8 Data dcquisilon system 2 aa ze rd A ih 4 Observing with VISIR at the VLT Al Proposa prepaid Okee 228 ALLA EEE ee ee A 4 2 Telescope observing parameters 4 2 1 Instrument orientation on the sky 4 2 2 Chopping parameters 2 286 6 4424 u BA a ds asie EH 4 2 3 Nodding parameters za 2 04 same MN Las t s BD FON CL aOQUISILOA ee y e A A ee DE ea Ro E E Aw Guide stars a Ed AE ws Alo Brichtness Imitat ons ee 3 ns aa a anA a a EE a ALO ONCE eS ee re een ee ade Ge Ar Calnbrabion observations oa s auie ada ta MN de A Re 4 8 ARONA Problems AA as a e Dis 4 8 1 Decreased image quality ar 4 ac 34 hod ica BA a A do e 4 82 kLow levelsttipes ss mmm dr BS Gus ee Oh A Kb a ne 4S3 Bad tesiduals roer a a da A o ee dd 4 8 4 Residuals of sky emission lines a 4 4 24 iia a id AS SEID yc Sag ae ely oy at As oe Ge He A ES BS he A G 5 VISIR data O a fee acne ae ee ei Eee ee ee See nie Ds PpO as cries oe mn ne ah rede Be ke ee ec SS et e ea 9 3 VISIR spectrometer data a LEE he aa Fe hee eda OPS SEES be eee eS 6 VISIR templates description Dal Ze OED dt ea oe tae ee eet ee ee A ek a GS ee eee ade ee ee E 6 2 Observing with the imager 22 425 ei da seu ESE FEES S S34 TOE oe ws 6 3 Observing with the spectrometer a aooo m nn Ode Cal br sl on a2 at oS 2 ann a ee ee ee ee a ee a 7 C2. A checklist to help the preparation of OBs is available in 37 Acquisition observing and calibration templates are explained in 8 6 This manual reflects knowledge gathered during early phases of operations and is in some aspects to be considered to be preliminary Therefore we strongly recommend to consult http www eso org instruments visir for additional information and updates For support during proposal preparation and OB submission please contact ESO s User Support Department usd help eso org 2 Observing in the MIR from the ground 2 1 The Earth s atmosphere Our atmosphere absorbs the majority of the MIR radiation from astronomical sources The main absorbing molecules are H20 CH4 CO2 CO O2 O3 However the atmosphere is quite transparent in the two atmospheric windows the N and Q band They are centered around 10 and 20 um respectively The transmission in the N band is fairly good at a dry site and becomes particular transparent in the wavelength range 10 5 12 ym However the transmission of the Q band is rapidly decreasing with wavelength and can be viewed as the superposition of many sub bands having a typical spectral coverage of AA lum at an average transmission of 60 Observations in this band require low water vapor content in the atmosphere The atmospheric transmission in the N and Q bands is displayed on Fig 1 2 2 The spatial resolution The spatial resolution of an instrument is ultimately l
3. To avoid chopping inside the object it is recommended to use a chopping and nodding throw which is 1 5 times larger than the estimated MIR diameter of the object In the case of point sources the throw is usually set around 10 to ensure proper separation of the different beams The maximum chopping throw at the VLT is 30 and the minimum is 8 For good image quality and good background cancelation chopping and nodding throws below 15 are recommended see 4 8 1 Note that for chopping throws larger than the field of view the negative beams will not be seen on the detector and the integration times have to be adjusted accordingly The chopper position angle TEL CHOP POSANG is the angle of chopping counted East of North see Fig 16 This parameter can be set by the observer In order to keep the same distribution of beams on the detector for a different rotator angle TEL ROT OFFANGLE as in the default rotator position then TEL CHOP POSANG must be equal to TEL ROT OFFANGLE In particular this is the case in spectroscopy if the observer wishes to have the 3 beams along the slit As stated in Sect 3 5 the chopping frequency is not a parameter accessible to the observer it is fixed internally to ensure the best data quality Figure 16 Definition of chopping parameters from the telescope point of view If the position an gle PA is measured counter clockwise from North to East with PA between 0 and 360 then TEL CHOP POSA
4. PERPENDICU LAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Imager pixel scale Observation Category Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_img_obs_GenericChopNod tsf To be specified Range Default INS FILT1 NAME INS PFOV SEQ CATG SEQ JITTER WIDTH SEQ NOFF SEQ OFFSET COORDS SEQ OFFSET1 LIST SEQ OFFSET2 LIST SEQ TIME TEL CHOP POSANG TEL CHOP THROW SIC PAH ARII SIV_1 SIV SIV2 PAH2 PAH22 NEIL1 NEII NEII_2 Q1 Q2 Q3 NODE FAULT 0 075 0 127 0 127 PRE IMAGE SCIENCE SCI ENCE 0 10 0 1 100 NODEFAULT SKY DETECTOR NODE FAULT NODEFAULT NODEFAULT 180 3600 NODEFAULT 0 359 0 8 30 10 Imager Filter Imager pixel scale Observation Category Random Jitter Width arcsec Number of offset positions Offset coordinates List of offsets in RA or X List of offsets in DEC or Y Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_obs_LRAutoChopNod tsf To be specified Range Default INS GRAT1 WLEN SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW 8 5 8 8 9 8 11 4 12 2 12 4 NODE FAULT PARALLEL PERPENDICU LAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 Spectrometer Wavelength mi crons Relative Chop Nod Direction R
5. This limit is particularly constraining for VISIR spectroscopic observations All acquisition images are recorded and archived As part of the execution of the VISIR_spec_acq_MoveToSlit template an image used to measure the slit location is always taken and archived In service mode through slit images are also taken and archived so that the user can assess the correct centering of her his object The slit location image and the through slit images are automatic procedures that cannot be modified by a service mode observer Their execution time is included in the advertised execution time of the spectroscopic acquisition template e If the target coordinates are well known VISIR imaging modes allow to perform blind preset observations with the VISIR_img_acg Preset template In this case no acquisition images are taken The target will be located at the center of the detector with an accuracy limited by the accuracy of the guide star typically 1 RMS If both the target coordinates and the guide star ones are within the same astrometric systems the pointing accuracy is limited by the rel ative accuracy between the coordinates of the two objects In particular the pointing accuracy maybe affected by significant usually unknown proper motion of the guide star Note that the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers For a successful completion of an OB the observer has
6. a y sign e Re yR x 9 9 and along the dispersion fs v y x sign A Ra yR y y A 40 10 Finally the origin of the coordinate system is moved back from the fix point to 1 1 VISIR User Manual 26 fole y Ey E 11 Spectral extraction is similar to the TIMMI2 pipeline and described by Siebenmorgen et al 2004 AA 414 123 Wavelength calibration A first order wavelength calibration is given by the optical model of the instrument Its precision is about 10 pixels for the low and medium resolution mode and 15 pixels for the high resolution mode The wavelength calibration can be refined by using Fabry Perot Etalons plates or atmospheric lines In the VISIR FITS file chopper half cycle frames which are dominated by sky emission lines are stored 85 1 They can be used to fine tune the wavelength calibration to sub pixel precision by comparison with a model of the atmospheric lines This method is used by the pipeline More specifically the zero point of the wavelength calibration is obtained by cross correlating the observed sky spectrum with a HITRAN model of the sky emission lines The chopped frames cannot be used for calibration with atmospheric lines because the chopping pro cess results in a near perfect cancelation of sky lines Atmosphere absorption correction The atmosphere does not uniformly absorb the MIR radiation 3 2 1 At some wavelengths it is com ple
7. crossing should be avoided because of fast tracking speeds that do not allow proper background cancelation after nodding Questions related to the VISIR Phasel and Phase 2 observing preparation should be directed to the User Support Department usd help eso org 4 2 Telescope observing parameters 4 2 1 Instrument orientation on the sky By default the imager orientation is such that North is at the top and East is to the left For the spectrometer the default orientation is rotated by 90 respective to the imager so that the North is to the left and the East to the bottom with the slit orientation along the North South direction Since VISIR is mounted on a rotator at the Cassegrain focus of Melipal it is possible to change the default orientation of VISIR on the sky for example to obtain the spectra of two objects at once The parameter TEL ROT OFFANGLE defaulted to 0 is used for this purpose If PA represents the VISIR User Manual 18 required position angle on the sky east of north i e counted positively from north to east within the range O to 360 then TEL ROT OFFANGLE 360 PA 4 2 2 Chopping parameters The chopping technique as described in 32 is based on beam switching using the moving secondary mirror of the telescope It allows to alternatively observe a field then another field offset from the first by a chopping distance or throw called TEL CHOP THROW see Fig 16 This parameter can be set by the user
8. during the night 84 7 Flux and noise levels are extracted by multi aperture photometry using the curve of growth method the aperture used for all 4 beams in a given frame is the one for which the flux to noise ratio is the largest By combining all 4 beams the sensitivity in a given set up filter field of view is defined as the limiting flux of a point source detected with a S N of 10 in one hour of on source integration The growing calibration database allows a statistical analysis of the sensitivity with respect to instru mental and atmospherical conditions The values for each filter given in Table 2 refer to the median of more than 600 different observations during September and December 2004 A graphical compilation is presented in Fig 4 for the N band and Q band imaging filters Some of the best measurements approach theoretical expectations i e they are close to background limited performance BLIP Sensitivity estimates for the VISIR spectroscopy observing modes are obtained in a similar way However in this case chopping and nodding are executed in parallel Consequently only 3 beams are VISIR User Manual 4 Staring images Staring images n chopper position Chopped images subircaction subtraction Han B chopper position Sketch of mid infrared chopping and nodding technique of observation Chopped nodded image Final Image zoomed He2 16 blue compact galaxy Figure 3 Illustration of the choppi
9. offsets and allows the user to specify them in a list of relative offset positions In the most simple application only one offset position is specified This allows to record nodding pairs i e cycle of on off observations using a flexible offset position Additional jitter offsets can be specified More than one entry in the offset list results in a freely programmable pattern of nodding pairs Note that the integration time SEQ TIME specified refers to only one nodding pair The total observing time is given by the product of SEQ NOFFbySEQ TIME The offset positions are calculated as the cumulative sum of offsets i e are defined relative to the previous offset positions Note that the telescope always returns to the first reference position when specifying a list of offsets This mode can be exploited to perform mosaic or raster imaging The first reference position can then be considered as a sky observation while the offsets refer to object positions It is recommended to offset to positions that result in observations of overlapping fields which enhances the redundancy after image reconstruction Nodding Position B1 Nodding Position B2 Nodding Position B3 Preset Reference Position A Figure 20 Illustration of generating raster maps with VISIR_img_obs_GenericChopNod An illustration of generating an raster map can be found in Fig 20 The following parameters correspond to this setting SEQ NOFF 3 SEQ OFFSET
10. tt Ne H 7 att 4 amp wt 7 a we 9 6 2 a X i att NE DX E at Lasa i ve 1 5x10 ae 7 7 a D 1 35x100 2 5x10 AE gt ra x H r 4 e S 7 O Z E 10x10 E uo EEn o Ka TS L 7 al gt a a x B a x e a E a va be 6 a 5 y ORO ae E o 5 0x10 j va NY H Pal A Sen 2 oO E pe A D a En F eo 7 rid O L at J O at Cc 7 E O po O E AA AP AI A fen E E 0 2 4 6 8 0 0 0 2 0 4 0 6 0 8 1 0 integration time s integration time s Figure 11 Linearity curve of the detector in the large left and small right capacity modes The break in the response at 2 3 at 1 8 10 e of the large and at 1 9 10 e of the small capacity are indicated by full lines The top lines indicate the well capacities Figure 12 Bad pixel maps of the imager left and spectrometer right detectors The large grey rectangular areas correspond to electronically masked pixels in order to decrease detector striping VISIR User Manual 15 Figure 13 The DRS detector shows stripes and repeating ghosts for very bright sources left The ghosts are distributed every 16 columns For other sources striping is not apparent right Figure 14 Sequence of chop nod reduced spectra obtained in the Medium Resolution mode with a central wavelength 8 8um The TEL CHOP THROW 8 11 13 and 14 from left to right Note the presence of significant striping when the left beam hits some hot pixels
11. type standard star in the Low Resolution mode based on the same catalog as for imaging with an airmass difference no larger than 0 2 AM Such a calibration measurement will be performed at least once per night per instrument configuration More precisely the following settings of the VISIR_spec_cal_LRAutoChopNod template 6 will be used SEQ TIME 180 sec TEL CHOP POSANG 0 TEL CHOP THROW 8 SEQ CHOPNOD DIR PARALLEL The wavelength setting INS GRAT1 WLEN and INS SLIT1 WIDTH will be adjusted to the science obser vation Important note The observatory does not provide standard calibrations for VISIR medium and high resolution spec troscopy Thus for medium and high resolution mode the observer has to supply his own calibration by supplying a calibration OB to each science OB The observing time needed to execute this calibration is charged to the observer Ideally early type stars should be chosen For both imaing and spectroscopy day calibrations of VISIR are performed with an extended source that mimics a black body with adjustable flux by regulating its temperature For each instrument mode a corresponding flat field is recorded which consists of a series of images with different back ground levels Bad pixels gain maps and fringing patterns can in principle be derived from these flat fields However at the moment the scientific value of the application of these corrections is not established Day calibration
12. 1 LIST 30 10 10 SEQ OFFSET2 LIST 30 10 10 SEQ OFFSET COORDS SKY VISIR User Manual 29 Note that depending on choice of the integration time SEQ TIME several nodding cycles might result e g pattern like ABIBIAABIBI1A AB2B2A AB2B2A AB3B3A AB3B3A Pre imaging observations As of Period 76 the observatory supports a fast data release for VISIR pre imaging observations Pre imaging images must be obtained either with the VISIR_img_obs_AutoChopNod or VISIR_img_obs_GenericChopNod templates The SEQ CATG keyword must be set to PRE IMAGE In addition the name of the OB must start with the prefix PRE 6 3 Observing with the spectrometer Conceptually the same observing techniques applies for spectroscopy as well as for imaging The default slit orientation is in North South direction The length of the slit is selected by the keyword INS SLIT1 TYPE only for cross dispersed high resolution observations SHORT must be used otherwise LONG is the default setting A preferred observing strategy is called nodding on the slit where the chopping and nodding amplitudes are small SEQ CHOPNOD DIR PARALLEL Note that nodding on the slit requires to set the telescope rotator offset angle and the M2 chopping position angle to the same value which is in general different from 0 This is useful to acquire two targets simultaneously in the slit The keyword SEQ JITTER WIDTH allows to apply random offsets along the slit More complex
13. AA line order spect resol dispersion sensitivity um um theoretical pixels um Jy 100 1h HR 7 970 8 270 0 02420 H254 17B 32000 10544 3 HR 12 738 12 882 0 03571 Ne I 11A 17000 1145 0 9 HR 16 800 17 200 0 05156 H2 Sl 8B 14000 4950 lt 10 HRX 9 360 9 690 0 02325 H253 15A 25000 10974 av HRX 12 210 12 760 0 03864 H2 S2 11B 20000 6604 1 5 Table 6 VISIR high resolution long slit HR and cross dispersed HRX modes The second column gives the minimum and maximum allowed values for the central wavelength Ac in the given setting The wavelength range per setting in given in the 3rd column AA Offered slits have widths of 0 40 0 75 and 1 00 VISIR User Manual 12 Figure 8 Schematic drawing of the warm calibration unit on top of the VISIR vessel 3 7 Detectors The VISIR imager and spectrometer are each equipped with a DRS former Boeing 256 x 256 BIB detector The quantum efficiency of the detectors is greater than 50 and reaches 65 or more at 12 um Fig 9 The detector noise has to be compared with the photon noise of the background As shown in Fig 10 the measured noise in an observation consists of read out noise and fixed pattern noise which are both independent of the detector integration time DIT At the operating temperature of the detector 7K the dark current which is the signal obtained when the detector receives no photons is negligible compared to the background generated by
14. EL CHOP THROW along a position angle equal to e PA 90 360 TEL CHOP POSANG 90 if SEQ CHOPNOD DIR PERPENDICULAR e PA 180 180 TEL CHOP POSANG if SEQ CHOPNOD DIR PARALLEL The resulting distribution of images on a frame is illustrated in Fig 19 In imaging more flexibility on the nodding offsets are possible with the VISIR_img_obs_GenericChopNod template 4 3 Target acquisition Observing blocks must start with an acquisition template Pointing to a target can only be performed through an acquisition template Target coordinate name and proper motion are all set in the acquisition templates The execution of the acquisition templates presets the telescope to the target coordinate given by TEL TARG ALPHA and TEL TARG DELTA Offsets with respect to the target coordinates can be specified by TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA and allow for example to use a bright offset star for precise acquisition To guarantee proper centering within the slit when using an offset star the angular separation between the offset star and the target should not be larger than 60 Acquisition with an offset star has not been tested with the narrow 0 4 slit and should be avoided in P76 Note that the convention TEL TARG ALPHA TEL TARG OFFSETALPHA RA offsetstar TEL TARG DELTA TEL TARG OFFSETADELTA DEC offsetstar is used The target can be further offset to a particular position on the detecto
15. EST FLAT MSW FOCUS DETECTOR MECHANISM Figure 7 Schematic layout of the design of the VISIR spectrometer The long slits have a length of 32 5 and therefore cover the whole width of the detector The short slits only used in high resolution cross dispersed mode have a length of 4 1 The all reflective optical design of the spectrometer uses two TMA systems in double pass pass 1 collimator pass 2 camera A schematic layout of the VISIR spectrometer design is shown in Fig 7 The 3 mirror system of the low and medium resolution arm gives a 53mm diameter collimated beam the collimated beam diameter in the high resolution arm is 125mm Both subsystems image the spectrum onto the same detector selection between the two spectrometer arms is done by two pairs of folding flat mirrors In front of the actual spectrometer subsystems is a reflective re imager consisting of two off axis paraboloids and three folding flats The re imager provides a 16 mm diameter cold stop pupil in parallel light and transforms the incoming VLT Cassegrain beam of F 13 4 to an F 10 beam at the spectrometer entrance The spectrometer slit wheel is also equipped with a very wide slit 15 3 named OPEN in P2PP It gives the possibility to make imaging with the spectrometer detector and is used for object acquisition and centering on the detector The list of available filters for spectroscopic acquisition offered is given in Table 3 together with their me
16. EUROPEAN SOUTHERN OBSERVATOR Y Organisation Europ enne pour des Recherches Astronomiques dans 1 H misphere Austral Europaische Organisation f r astronomische Forschung in der s dlichen Hemisphare VERY LARGE TELESCOPE VISIR USER MANUAL Doc No VLT MAN ESO 14300 3514 Version 76 4 14 Jul 05 VISIR User Manual Issue Rev 04 09 04 10 12 04 01 02 05 06 07 05 14 07 05 14 07 05 Change Record Section Parag affected Reason Tnitiation Documents Remarks creation First release for science verification in P74 and OT proposals in P75 2 4 3 2 6 2 6 3 7 8 update for P75 Phase2 all update for P76 CfP all update for P76 Phase 2 4 8 1 Corrected Legend Fig 17 Cover pages Corrected typos v1 0 v1 1 v76 1 edited by R Siebenmorgen E Pantin M Sterzik v76 2 4 updated by A Smette Send comments to asmette eso org 11 VISIR User Manual Contents 1 Introduction 2 Observing in the MIR from the ground 21 e arts at mosphere see a Ls ti d en A Re ee are 22 Thespalialiresolution o 5 8 AE Sr DREI as la de 23 MIR Pace oro Ud a an u e ee Bee OME RA a as de 22 Chopping and nodding e 3 Lames Lis a 80er RT Eee 2 D IN re ee A ee ee ees cee ne 3 Instrument description and offered observing modes DL MAR ES III 3222 SO PECLTOMCLOS y ue SRE LAA RUN LAND AUS A ERR AA 39 com Midis a A A ds 32 I Ol O A II E Sh SA era N open kit es S 3 5 Low resolution offered central wavelengths
17. NG is 360 PA The positive beam is obtained when the M2 is at Chopping Position A and corresponds to the pointing position of the telescope as given in the FITS header The negative beam is obtained by moving the M2 so that it points to a position angle on the sky given by PA and a throw of TEL CHOP THROW from the telescope pointing position Chopping Position B If TEL CHOP POSANG TEL ROT OFFANGLE 360 PA the resulting image on the detector will appear as in one of the nodding position images illustrated in Fig 19 2In practice the telescope is actually given an offset equal to TEL CHOP THROW 2 along the angle given by PA Relative to its idle position and looking from the M1 to the sky the M2 oscillates along PA between two positions given by TEL CHOP THROW 2 This is completely transparent to the user VISIR User Manual 19 4 2 3 Nodding parameters The nodding technique allows to switch from one field to another by offsetting the telescope by several tens of arc seconds It allows to correct for optical path residuals that remain after chopping 8 2 The nodding period is 90s for exposure time equal or longer than 180s or is half this value for shorter exposure times This parameter can only be modified by the instrument operator In all the AutoChopNod templates the nodding offset is equal to TEL CHOP THROW and cannot be modified In order to reach Nodding Position B the telescope executes an offset of T
18. THROW 8 8 11 4 NODEFAULT PARALLEL PERPENDICU LAR PARALLEL 0 10 0 30 3600 NODEFAULT 0 359 0 8 30 8 Spectrometer Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_cal_HRAutoChopNod tsf To be specified Range Default INS FILT2 NAME INS GRAT1 WLEN SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW NEIL2 H2S_1 H254 NELLA 7 80 19 18 12 810 PARALLEL PERPENDICU LAR PARALLEL 0 10 0 30 3600 NODEFAULT 0 359 0 8 30 8 Spectrometer Filter Spectrometer Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR User Manual 36 VISIR spec_cal HRXAutoChopNod tsf To be specified Range Default INS GRAT1 WLEN 7 60 28 08 VODEFA ULT Spectrometer Wavelength mi crons SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL CHOP POSANG 0 359 0 Chopping Position Angle deg TEL CHOP THROW 8 30 8 Chopping Amplitude arcsec VISIR User Manual 37 9 Appendix Filter transmission curves The filter transmission has been measured using a Four
19. andom Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR User Manual 34 VISIR spec_obs MRAutoChopNod tsf To be specified Range Default 8 8 11 4 NODEFAULT INS GRAT1 WLEN SEQ CHOPNOD DIR PARALLEL PERPENDICU LAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW Spectrometer Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_obs_HRAutoChopNod tsf To be specified Range Default INS FILT2 NAME INS GRAT1 WLEN NEIL2 H2S_1 H2S 4 NEILA 7 80 19 18 12 810 SEQ CHOPNOD DIR PARALLEL PERPENDICU LAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW Spectrometer Filter Spectrometer Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_obs_HRX AutoChopNod tsf To be specified Range Default INS GRAT1 WLEN 7 60 28 08 NODEFAULT SEQ CHOPNOD DIR PARALLEL PERPENDICU LAR PARALLEL 0 10 0 180 3600 NODEFAULT 0 359 0 8 30 8 SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW Spectrome
20. asured bandpasses and approximate sensitivities for image acquisition 3 3 Slit widths Three different slit widths 0 4 0 75 and 1 are offered for all settings For over sized widths e g for the 1 slit with respect to the diffraction limit around 10um the spectral resolution of a VISIR User Manual 10 filter Ac half band width sensitivity um um mJy 100 1h NSW 8 85 1 35 40 NLW 121 1 9 40 ArII 8 94 0 11 200 Nell1 12 35 0 50 80 Nell2 12 81 0 10 50 Table 3 VISIR spectrometer filter characteristics The filters transmissions have been determined with a monochromator and the WCU The last column list the measured median sensitivities which were obtained using the curve of growth method on data obtained in parallel chopping nodding directions 3 beams point source spectrum is better than the one of the sky spectrum in addition the zero point of the wavelength calibration will be affected by an incorrect centering of the object within the slit 3 4 Resolution In the N band the low resolution and medium resolution modes provide spectral resolving power of 300 Table 4 and 3000 Table 5 respectively In high resolution long slit mode narrow wavelength ranges around the 8 02 H2_54 12 813 Ne II and 17 03 um H2_S1 line are offered With the 1 slit the measured spectral resolution is R 15000 Table 6 and a minimum flux in the line below 10716 W m arcsec can be achieved This value correspo
21. at the lower left of the detector For the location of the object along the slit pixel X 123 at row Y 128 this occured for TEL CHOP THROW between 10 and 13 approximatively The horizontal lines at the middle of the images are caused by the lack of detector response at 8 8m VISIR User Manual 16 image is stored as a plane in a data cube of a FITS file The number of chopping cycles corresponds to the time spent in one nodding position Ta This nodding period is typically Thoa 90s for science observations The chopper frequency DIT and also Thoq are predefined by the system The number of saved A B frames in one FITS file is Nro Till Lass 1 The number of nodding cycles is computed from the total integration time as given by the observer The total number of stacked images for each secondary position respectively chopper half cycle is NDIT This parameter is computed according to NDIT 2 DIT fenop NDITSKIP 2 and is given by the system It depends on DIT chopping frequency and NDITSKIP some read outs at the beginning of each chopper half cycle are rejected during stabilization of the secondary Typical stabilization times of the secondary are 25 ms The number of rejected exposures is given by NDITSKIP Similar during stabilization after each telescope movement respectively nodding position a number NCYSKIP of chopping cycles is ignored The timing organization of data is shown in Fig 15 The
22. ators unless provided by the observer are selected from the MIR spectro photometric standard star catalog of the VLT http www eso org instruments visir This catalog is based on the radiometric all sky network of absolutely calibrated stellar spectra by Cohen et al This list is supplemented by MIR standards used by TIMMI2 5 Cohen et al 1999 AJ 117 1864 Chttp www 1s eso org lasilla sciops 3p6 timmi html stand html VISIR User Manual 22 At present the standard star catalog contains 425 sources Zero point fluxes Jy have been calculated for the VISIR filter set by taking into account the measured transmission curves Fig 21 the detector efficiency Fig 9 and an atmosphere model Fig 1 A PSF can be derived from these photometric standard star observations However it is not guaran teed that the accuracy is sufficient for deconvolution purposes If the observer requires a specific PSF measurement s he has to provide the corresponding PSF OB Observations of photometric standards provided by the observatory are taken using the VISIR_img cal _AutoChopNod template 8 6 with the following settings SEQ TIME 180 sec for N and 360 sec for Q band TEL CHOP POSANG 0 TEL CHOP THROW 10 SEQ CHOPNOD DIR PERPENDICULAR Filter INS FILT1 NAME and pixel scale INS PFOV will be set according to the science observations In spectroscopy the observatory will provide spectro photometric observations of a telluric K
23. d the Q band between 16 5 and 24 5 um respectively In addition it offers a slit spectrometer with a range of spectral resolutions between 150 and 30000 The MIR provides invaluable information about the warm dust and gas phase of the Universe Micron sized particles such as silicates silicon carbide carbon coals aluminum oxides or polycyclic aromatic hydrocarbon PAH molecules are major contributors to the thermal MIR emission The gaseous phase emits through a large number of ionic and atomic lines Examples are Nell 12 8 um and the pure rotation lines of molecular hydrogen at 8 02 9 66 12 27 and 17 03 um Because of the very high background from the ambient atmosphere and telescope the sensitivity of ground based MIR instruments cannot compete with that of space born ones However ground based instruments mounted on large telescopes offer superior spatial resolution For example VISIR at the VLT provides diffraction limited images at 0 3 FWHM in the N band This is an order of magnitude better than what can be reached by the Spitzer Space Telescope SST The VISIR user manual is structured as follows Basic observing techniques of ground based MIR instruments are summarized in 3 2 y 3 provides a technical description of VISIR and its offered observing modes offered An overview on how to observe with VISIR at the VLT can be found in 8 4 A description of the structure of the imaging and spectroscopic data files is given in y 5
24. e O CO C lt L 0 1 N N 5 10 15 20 25 30 Wavelength um Figure 2 VLT diffraction limit full line versus seeing The Spitzer Space Telescope diffraction limits dashed are shown for comparison The Roddier dependence is shown for two optical seeings dashed dot The chopping technique cancels most of the background However the optical path is not exactly the same in both chopper positions Therefore a residual background remains It is varying at a time scale which is long compared to that of the sky This residual is suppressed by nodding where the telescope itself is moved off source and the same chopping observations as in the on source position is repeated An illustration of the chopping and nodding technique is shown on Fig 3 Depending on the choice of chopping and nodding amplitudes and directions up to 4 images of the source can be seen on the frame and used for scientific analysis Of course the free field of view on the chop nod images can be severely reduced depending on the particular chopping and nodding parameters chosen 2 5 Sensitivity Measurements of VISIR sensitivities are based on observations of mid infrared calibration standard stars Cohen et al 1991 AJ 117 1864 In imaging mode the stars are recorded in the small field 0 075 and intermediate field 0 127 by perpendicular chopping and nodding patterns with amplitudes of 10 Calibrators are frequently observed
25. e 18 5 Wavelenght um ikter 0 120 100 E 80 09 5 2 601 0 o 20 09 c 20 O J 18 5 19 0 19 5 20 0 Wavelenght um Filter SIV 120 100 DRE 09 5 gt 60 0 E AO 09 C 2 20 O f 10 1 10 3 10 5 LOT Wavelenght um 190 10 9 Transmissions in Transmissions in Filter QZ ee Wavelenght um 18 9 Filter Sic 120 100 SO 60 40 20 3 10 2 14 16 Wavelenght um Figure 18 continued 38 VISIR User Manual 39 Filter Nell_ref 1 Filter PAH ref c 1 2 c 22 O cure z v 0 8 v 0 8 G E O O os 0 6 N 0 4 N 0 4 O O E 0 2 E 0 2 O O 00 ZOO we 11 8 120 1 272 12 4 12 6 1038 1122 11 8 1222 12 8 Wavelenght um Wavelenght um Filter SIV ref 1 Filter SIV_ref2 a A c Q Q Be v 0 8 n E C o 5 076 O O D D N 0 4 N O O E 0 2 E O O Ze i 9 5 9 7 9 9 10 1 10 5 10 4 1026 10 5 1140 11 2 Wavelenght um Wavelenght um Figure 19 Transmission curves of VISIR imager filters manufactured by OCLI Overplotted dashed is the atmospheric transmission at low resolution Only relative transmissions have been determined their values are normalized so that their peak transmission is equal to 1
26. e is imaged on a cold stop mask to avoid straylight and excessive background emission The collimator mirror M1 is a concave aspherical mirror It is followed by a folding flat mirror M2 which eases the mechanical implementation e A set of three objectives mounted on a wheel Each objective is based on a three mirror anastigmatic TMA system Each of the TMA s is made of three conic mirrors The 0 075 small field SF and 0 127 intermediated field IF pixel scale are offered Table 1 These offered pixel fields of view pfov ensure a proper sampling of the images in the N and Q band pfov fov diffraction diffraction um pixels 0 127 32 5 32 5 94 1 88 0 075 19 2 x 19 2 159 3 18 Radius of first Airy ring at 7 7um Table 1 VISIR imager pixel scales offered The pixel size of the DRS 256x256 detector is 50 um The first airy ring at 7 7um corresponds to a radius of 0 24 on the sky The filter wheel is located just behind the cold stop pupil mask The list of filters offered is given in Table 2 The transmission curves of the filters measured at 35 K are plotted in the Appendix Note that ETC v3 0 5 does not properly account for the background behavior as a function of airmass Also it does not take into account the airmass dependence of the seeing VISIR User Manual T IA oe 1 I IT bafe tel T I E A D OT AT AT TI N NA 1 PT OT TT TO TT En theoretical limit sensitiv
27. ected on the sky is straight There is an additional linear distortion in both dispersion and cross dispersion direction of the detector The following algorithm is supported by the pipeline for low and medium resolution mode Let us define the detector pixels in dispersion direction by x and in cross dispersion direction by y respectively a The skew angle along x with and along y with W b The maximum curvature along x with A and along y with e is defined positive in clockwise direction and Y counter clockwise A is positive by increasing x and by decreasing y respectively Measured values of the distortion parameters are in the low and medium resolution mode Y 1 6 and Y 0 7 The curvatures in the low resolution mode are e 1 04 pixel A 0 08 pixel and for the medium resolution mode are 0 26 pixel A 0 08 pixel The center of the lower left of the detector is at 1 1 Therefore the fix point which is the detector center is at 128 5 128 5 for the n 256 pixel array of the DRS The fix point is moved to 1 1 by n 1 n 1 Al y 7 E 6 and the skew is corrected along the cross dispersion falz y u y tan v y 7 and along the dispersion direction falx y x y z tan 8 The curvature is a segment of a circle with radius R in x direction given by n 2y 2R and in y direction by n 2y A 2RA A It is corrected along the cross dispersion falz y
28. es for J2000 0 Entries for equinox and proper motions are not yet taken into account VISIR User Manual 21 4 4 Guide stars Guide stars are mandatory for active optics and field stabilization Any VLT programme should make sure that a guide star USNO catalog with a V 11 13 mag is available within a field of 8 around the object If TEL AG GUIDESTAR is CATALOGUE a guide stars from the guide star catalog will be automat ically selected by the TCS If TEL AG GUIDESTAR is SETUPFILE the observer has to provide the coordinates of the GS The coordinates of the guide star also fix the reference point for the World Coordinate System coordinates that appear in the FITS header of the files In both cases the telescope operator acknowledges the guide star Depending on the weather con ditions or if the star appears double in the guide probe the telescope operator may have to select another guide star Therefore If the observer has selected a guide star for astrometric purposes for example to insure the repeatability of the pointings between different OBs a clear note should be given in the README file for service mode observations or be specifically mentioned to the night time astronomer in visitor mode As stated above the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers 4 5 Brightness limitations There are currently no brightness limitations wi
29. g_acq MoveToPixel tsf To be specified Range Da INS FILTI NAME INS PFOV SEQ CHOPNOD DIR SEQ TIME TEL AG GUIDESTAR TEL CHOP POSANG TEL CHOP THROW TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPHA TEL TARG ADDVELDELTA TEL TARG ALPHA TEL TARG DELTA TEL TARG EQUINOX TEL TARG OFFSETALPHA TEL TARG OFFSETDELTA SIC PAH ARII SIV_1 SIV SIV2 PAH2 PAH22 NEIL1 NEII NEIL2 Q1 Q2 Q3 NODE FAULT 0 075 0 127 0 127 PARALLEL PERPENDICU LAR PARALLEL 30 3600 NODEFAULT CATALOGUE SETUPFILE NONE CATALOGUE 0 359 0 8 30 8 ra dec 0 359 0 0 0 0 0 0 ra dec 2000 0 0 0 0 0 Imager Filter Imager pixel scale Relative Chop Nod Direction Total integration time sec Get Guide Star from Chopping Position Angle deg Chopping Amplitude arcsec Guide star RA Guide star DEC Rotator on Sky PA on Sky RA additional tracking velocity DEC additional tracking velocity RA blind offset DEC blind offset VISIR_img_acq_Preset tsf To be specified Range Da TEL AG GUIDESTAR TEL GS1 ALPHA TEL GS1 DELTA TEL ROT OFFANGLE TEL TARG ADDVELALPHA TEL TARG ADDVELDELTA TEL TARG ALPHA TEL TARG DELTA TEL TARG EQUINOX CATALOGUE SETUPFILE NONE CATALOGUE ra dec 0 359 0 0 0 0 0 0 ra dec 2000 0 Get Guide Star from Guide star RA Guide star DEC Rotator on Sky PA on Sky RA additional tracking velocity DEC add
30. hecklist for Phase 2 preparation ill O NN Ek en Om 0 Ne 10 10 12 13 17 17 17 17 18 19 19 21 21 21 21 22 22 23 23 23 24 24 24 24 25 27 27 21 29 29 30 VISIR User Manual 8 Appendix VISIR template parameters 81 Acqu s on ee 3 oe 2 8 Soe ka a ee eee pi DD IODSELVALION cess ee en ee ee ee e AE OS ad de See ee eR 83 AIDE AIO x y ERA A E a as ble oe ere i as 9 Appendix Filter transmission curves IV 31 31 33 39 37 VISIR User Manual List of acronyms BIB BLIP BOB DIT ETC FWHM ICS IR IRACE MIR OB P2PP PAE pfov PSF S N UT VISIR TCS TMA WCU Blocked impurity band Background limited performance Broker of observation blocks Detector integration time Exposure time calculator Full width at half maximum Instrument control software Infrared Infrared array control electronics Mid infrared Observing block Phase 2 proposal preparation Preliminary acceptance in Europe pixel field of view Point spread function Signal to noise ratio Unit telescope VLT imager and spectrometer for the mid infrared Telescope control system Three mirrors anastigmatic Warm calibration unit VISIR User Manual 1 1 Introduction The VLT spectrometer and imager for the mid infrared VISIR built by CEA DAPNIA SAP and NFRA ASTRON provides diffraction limited imaging at high sensitivity in two mid infrared MIR atmospheric windows the N band between 8 to 13 um an
31. ier Transform Spectrometer at a temperature of 35 K for filters manufactured by the company READING Their absolute transmission curves are displayed in Fig 21 The other filters manufactured by OCLI have been measured using the WCU and wavelength scans with the monochromator Note that for these filters the transmission curves are normalized to 1 see Fig 19 Filter Arlll Filter Nell_ref2 120 1 20 100 amp 100 80 80 Y 09 O 6 gt 60 en 60 ace po cy 40 E AG WY WY E G o 20 220 O O 3 80 8 90 XOU 9 10 a2 1236 12 8 13 0 Rene 15 4 Wavelenght um Wavelenght um Filter Nell Filter PAH1_ARlllref 1 120 1 20 Q 100 be 100 80 80 09 Cp O 6 80 S 60 2 ae E 40 20 J 09 WY C C E 20 Z 20 O O gt 12 4 12 6 1226 a 10 2 a 3 0 8 5 gt 20 90 Wavelenght um Wavelenght um Fler Paz Filter QO 120 t20 100 100 80 80 WY WY 5 S 60 S 60 2 D E 40 ce AO N WY C gt 20 Z 20 O EN O la TUS 159 11 6 122 12 8 1340 1022 16 8 1722 17 8 Wavelenght um Wavelenght um Figure 21 Transmission curves of VISIR imager filters manufactured by READING Overplotted dashed is the atmospheric transmission at low resolution The absolute transmission values are given expressed in percent VISIR User Manual Filter 07 120 100 0 09 5 gt 360 0 gt A0 09 c 30 O _ u 15 20 16 5 pac
32. imited either by the diffraction of the telescope or the atmospheric seeing The diffraction limit as measured by the diameter of the first Airy ring increases with wavelength as 1 22 A D where A is the observing wavelength and D the diameter of the telescope mirror see solid line in Fig 2 The wavelength dependence of the seeing can be derived by studying the spatial coherence radius of the atmosphere in the telescope beam and is to first order approximated by the Roddier formula where the seeing is x A7 see dot dashed lines in Fig 2 VISIR User Manual 2 MIR atmosphere transmission at Paranal Transmission 5 10 15 20 Wavelength um 5 30 Figure 1 MIR atmospheric transmission at Paranal computed with HITRAN for an altitude of 2600 m and 1 5 mm of precipitable water vapor at zenith The US standard model atmosphere is used However initial results from VISIR data indicate that this formula overestimates the measured MIR seeing at Paranal by 20 50 as the size of a UT mirror is comparable to the turbulence outer scale As a result VISIR data are already diffraction limited for optical seeing below 0 6 2 3 MIR background The atmosphere does not only absorb MIR photons coming from astrophysical targets but also emits a strong background with the spectral shape of a black body at about 253K Kirchhoff s law The telescope gives an additional MIR background The VLT telescopes emits at 283 K with a preliminar
33. imposed is the theoretical photon noise BLIP performances are approached for higher fluxes and larger DIT respectively memory effects Stabilization is ensured by introducing dead times where necessary It is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q These artifacts are less important in spectroscopy due to the lower light levels but clearly visible on objects brighter than 2 of the background However a TEL CHOP THROW between 9 to 13 shoud be avoided in particular for objects bright enough to be seen in individual DITS as one of the beams will hit some particularly hot pixels in the lower left of the spectrometer detector see Fig 14 3 8 Data acquisition system Both VISIR detectors are controlled by the ESO standard IRACE acquisition system In imaging the read out rate of the detector is high Up to 200 frames per s are read for a minimum detector integration time of DIT 5ms Such a frame rate is too high to store all exposures One VISIR image is of size 256x256 each pixel is coded with 4 bytes long integer Thus one read out has a size of 262 kB During each chopping cycle the elementary exposures are added in real time and only the result is stored on disk At a chopping frequency of fchop 0 25 Hz every Tenop 48 one VISIR VISIR User Manual 14 GE SST o S m nn Zn 6 5 T 7 e 31 90x10 e Bx 0 PA att Le A tt D p a gett
34. ities are the reference for classification of VISIR service mode observations and the basis to assess the feasibility of an observing programme In particular classifications of service mode OBs will be based on sensitivity measurements made at zenith Calibrations will be provided following the guidelines given in 34 7 For up to date information please consult http www eso org instruments visir The use the VISIR exposure time calculator ETC lo cated at http www eso org observing etc is recommended to estimate the on source integra tion time 3 Instrument description and offered observing modes For P76 VISIR offers two spatial scales in imaging and several spectral resolution modes in slit spectroscopy The imager and spectrograph are two sub instruments They have independent light paths optics and detectors The cryogenic optical bench is enclosed in a vacuum vessel The vessel is a cylinder 1 2m long and 1 5m in diameter Standard Gifford McMahon closed cycle coolers are used to maintain the required temperature 33 K for most of the structure and optics and lt 15K for the parts near the detector The detectors are cooled down to 7K 3 1 Imager The imager is based on an all reflective design The optical design is shown in Fig 6 It consists of two parts e A collimator which provides an 18mm diameter cold stop pupil in parallel light As generally designed for IR instruments the pupil of the telescop
35. itional tracking velocity VISIR User Manual 32 VISIR_spec_acq_MoveToslit tsf To be specified Range Default INS FILT2 NAME NSW NLW ARII NEIL1 Acquisition Filter NEIL2 NODEFAULT INS SLIT1 TYPE LONG SHORT LONG Spectrometer Slit Type long or short INS SLIT1 WIDTH 0 40 0 75 1 00 NODEFAULT ic O Slit Width arcsec SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 NODEFAULT Total integration time sec TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE TEL CHOP POSANG 0 359 0 Chopping Position Angle deg TEL CHOP THROW 8 30 8 Chopping Amplitude arcsec TEL GS1 ALPHA ra Guide star RA TEL GS1 DELTA dec Guide star DEC TEL ROT OFFANGLE 0 359 0 0 Rotator on Sky PA on Sky TEL TARG ADDVELALPHA 0 0 RA additional tracking velocity TEL TARG ADDVELDELTA 0 0 DEC additional tracking velocity TEL TARG ALPHA ra TEL TARG DELTA dec TEL TARG EQUINOX 2000 0 TEL TARG OFFSETALPHA 0 0 RA blind offset TEL TARG OFFSETDELTA 0 0 DEC blind offset VISIR User Manual 8 2 Observation 33 VISIR_img_obs_AutoChopNod tsf To be specified Range Default Imager Filter INS FILT1 NAME INS PFOV SEQ CATG SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW SIC PAH ARII SIV_1 SIV SIV2 PAH2 PAH22 NEILI NEII NEII 2 Q1 Q2 Q3 NODE FAULT 0 075 0 127 0 127 PRE IMAGE SCIENCE SCI ENCE PARALLEL
36. ity mJy 100 1h 8 9 10 11 12 13 wavelength um 100007 model M median p 1000 sensitivity mJy 100 1h HAIR 12 70 12 75 12 80 12 85 12 90 wavelength um Figure 5 Sensitivity as a function of wavelength for low top and high bottom resolution mode Four offered settings of the N band low resolution are stitched together Atmospheric molecular absorption e g at 9 55 11 8 and 12 5 um is evident Note the detector feature at 8 8 ym Dots indicate individual observations full lines represent median and the dashed line the best sensitivities In the 12 81 um region several settings of the high resolution mode are shown Theoretical model curves correspond to BLIP VISIR User Manual 8 entrance window diaphragm focal plane i IN TMA optics filter AN Figure 6 The optical path of the imager in the intermediate field 0 127 pixel is shown from the entrance window down to the detector filter Ac half band width maximum sensitivity um um transmission mJy 100 1h 7 theory median BLIP SF IF PAH1 8 59 0 42 77 1 6 5 8 ArTII 8 99 0 14 72 4 1 6 70 SIV_1 9 821 0 18 12 4 0 30 60 SIV 10 49 0 16 70 4 5 8 13 SIV 2 10 77 0 19 70 4 6 9 20 PAH2 11 25 0 59 75 2 3 6 9 SiC 11 85 2 34 75 1 2 7 18 PAH22 11 88 0 37 58 4 1 7 l5 Nell1 12 27 0 18 51 6 9 12 20 Ne
37. ll 12 81 0 21 64 6 1 12 18 Nell_2 13 04 0 22 68 6 3 15 22 Q1 17 65 0 83 59 11 1 50 120 Q2 18 72 0 88 49 13 6 50 80 Q3 19 50 0 40 50 41 7 100 160 Table 2 VISIR imager filter characteristics following the manufacturer specifications except for the central wavelengths noted with which were re determined with a monochromator and the WCU because they deviate from specifications The last 3 columns give respectively the theoretical expec tations under BLIP and excellent weather conditions and the measured median sensitivities for the Small and Intermediate Field obtained in various weather conditions The measured sensitivities were obtained using the curve of growth method on data obtained in perpendicular chopping nodding directions 4 beams VISIR User Manual 9 3 2 Spectrometer VISIR offers slit spectroscopy at three spectral resolutions with a pixel scale of 0 127 This is obtained by means of two arms one with low order gratings for the low and medium spectral resolution the other with large echelle gratings providing high spectral resolution VLT FOCAL DIAPHRAGM WHEEL i IMAGER OLDSTOP ILTER WHEEL TALON WHEEL LMR De GRATING UNIT kan r Er 4 GRATING SCANNERS DUO ECHELLE 1 RETURN FLAT GRATING UNIT Ny ESOLUTION SELECTION MECHANISM LMR MLW COLLIMATOR R CAMERA N COLLIMATOR en IN CAMERA a w wu a E renner lt EE oo OoOO V Sl Z wW PUPIL gt IMAGING LENS T
38. monochromatic point source with adjustable wavelength or an extended black body source with adjustable temperature A selection mirror allows to switch from the telescope to the simulator beam It can be used for calibration and tests also during daytime Fig 8 shows the unit on top of the enclosure VISIR User Manual 11 range Ac grating spect resol dispersion um um order measured 1 slit pixels um 7 7 9 3 8 5 2 300 390 160 01 8 0 9 6 8 8 2 300 390 160 02 9 0 10 6 9 8 2 305 360 160 05 10 34 12 46 11 4 1 185 220 119 94 11 14 13 26 12 2 1 215 250 119 96 11 34 13 46 12 4 1 215 250 119 96 Table 4 VISIR low resolution offered settings The first column gives the wavelength range of a spectrum for the central wavelength Ace listed in the 2nd column The measured sensitivities are 50mJy at 100 1h in the clean regions of the spectrum cf Fig 5 for a slit width of 1 Offered slits have widths of 0 40 0 75 and 1 00 The spectral resolution of the 8 5um and 12 2um settings has not been independently measured values for the 8 8um and 12 4um settings are reported instead A range Ne grating spect resol dispersion um um order measured 1 slit pixels um 8 706 8 893 8 8 2 3500 1367 43 11 274 11 526 11 4 1 1800 1011 2 Table 5 VISIR medium resolution setting The measured sensitivities are 200 mJy at 100 1h Offered slits have widths of 0 40 0 75 and 1 00 mode Ac
39. nds to an approximate sensitivity limit around 1 Jy in the continuum A high resolution cross dispersed mode with a 4 short slit is available for wavelength settings around 9 66 H2_S3 and 12 27 um H2_S2 Please consult http www eso org instruments visir for the latest update of the list of offered modes and slits 3 5 Low resolution offered central wavelengths In addition to the 4 central wavelengths 8 8 9 8 11 4 and 12 4 um announced in Phase 1 two additional ones are offered for Phase 2 at 8 5 and 12 2 um The main reason justifying these two new settings is of cosmetic nature As described below the detector has a number of bad pixels In particular some of them mainly located at the bottom left of the detector appear to cause particularly severe striping in the chopped images if their illumination is slightly different in the two chopping positions This situation can be produced either if one of the beams of a bright object falls on these pixels or because of a residual scanner jitter of the grating units The 8 5um setting moves the blue wing of the ozone band off the wavelength range covered by the detector at the expense of having a larger part of the detector covering the red wing of the water vapour band The 12 2um setting avoids the CO2 band 3 6 Calibration units A warm calibration unit WCU is located on top of the VISIR vacuum enclosure The WCU is also called star simulator It simulates either a
40. ng and nodding technique on observations of the blue compact galaxy He2 10 The galaxy only appears after chopping and nodding courtesy VISIR commissioning team June 2004 VISIR User Manual 5 DEERE RAS OEA IRENE ei rer A median small field 1 00 E y median intermed field E a E VW Y O 2 E a 4 gt E u y y el 10 ii a 4 a Yo a4 7 das A y 7 r io ie 1 4 a LE ea TAUR en i D A Y F e y ARIII SIV PAH2 PAH2_2 NEII 4 PAH1 SIV_1 SIV_2 SIC NEIl 1 NEII_2 pas yo e sh the A A AA ERAN Fear Ar DS e ITA ERE O AE j ATA di BER 8 9 1 11 12 13 wavelength um SE E sl A median small field median Intermed field i y S 100 4 E 2 i 6 y d k A a E A l gt 2 0 E O 2 10F k Q1 Q2 Q3 1 170 175 180 185 19 0 19 5 20 0 wavelength um Figure 4 Sensitivities for the VISIR imager for the N top and Q band bottom Small and in termediate field observations are displaced for clarity Background noise limits are indicated for the individual filter bandpasses VISIR User Manual 6 obtained with the central one containing twice as much flux as the two other ones Table 4 to 6 list typical sensitivities measured in low medium and high resolution modes away from strong sky emission lines for the wavelength ranges offered in P76 Figure 5 shows the dependence of the sensitivity on wavelength The median sensitiv
41. ng to the two chopper positions may therefore not be equal If this difference is larger than a few tens of ADUs structures in the gain maps will appear as low level stripes Such stripes tend to smooth out on long integrations 4 8 3 Bad residuals The chopping and nodding technique does not always lead to a satisfactory removal of all the structures seen in individual images Bad residuals have been found to occur in the following situations e in observations carried out close to zenith and to a lesser extent close to the meridian in general the likely cause is the fast rotation of the field relative to the telescope structure e in variable atmospheric conditions In addition it seems that imaging of extended objects are also more likely to be affected by low level bad residuals similar to fringes in some aspects whose orientation on the images changes at the same angular velocity as the rotator The origin of these structures is not understood 4 8 4 Residuals of sky emission lines In spectroscopy the scanners of the grating units may still show a small residual motion at the beginning of an exposure or mainly for the HR or HRX modes show some jitter after a nodding offset The first few frames at a given wavelength setting may therefore show stronger than expected residuals at the wavelength of the sky emission lines more exactly of the wings of sky emission lines For the HR and HRX modes the residuals of the scanner ji
42. of bad pixels lt 2 Fig 12 The imager detector also suffers from striping and appearances of ghosts The relatively wide rectangular area in the lower right corner South West corner for PA 0 deg of the imager detector or some other rectangular areas are masked out to avoid such disturbances Fig 13 For bright objects the DRS detector shows VISIR User Manual 13 0 4 F QZ Detective quantum efficiency nG Oe ee NE E NE ne ne EEE O 10 Es 20 2 SO Wavelength um Figure 9 Detector quantum efficiency at 12K provided by DRS solid line The same curve dashed but scaled by 0 72 reflects a lower limit of the quantum efficiency The scaling was derived from laboratory measurements Note the sharp absorption feature at 8 8 um that will appear in raw spectroscopic data 6000 En Zr E ee 1500 a t Large Cap tel sus 0 5 me Small Cap det_sub OV T 5 1 K x a ey Se 1022 Jake R se L 6G 1 013 _ 4000 j Ja WOOO R O ar eo f nn D O 0 n O B Z Z 2000 f 500 Data Data gt Fit of the data Fit of the data L Photon noise Photon noise O 1 1 1 f f j fi f O Ll L Ll L Ll Ll 1 f Ll Ll Ll 0 5 0x10 1 0x10 1 5107 0 5 0109 1 0x10 1 5x10 Signal e Signal e Figure 10 Noise as a function of the incoming flux in the large left and small right capacity mode Super
43. onding science templates but allow to monitor the sensitivity and image quality by observing calibration standard stars 7 VISIR User Manual 30 Checklist for Phase 2 preparation Acquisition Are the coordinates accurate in the equinox J2000 0 reference frame For high proper motion objects are they valid for the epoch of the observations For solar system objects are they in the topocentric ICRF or FK5 J2000 0 reference frame at the epoch of the observations Acquisition Is relative good astrometric accuracy required if yes a guide star should be provided whose distance relative to the target is accurately known Pre imaging If OBs are part of the pre imaging run of your programme the name of the OB must start by PRE and the SEQ CATG keyword must be set to PRE IMAGE Calibrations For calibration OBs use the appropriate VISIR img cal AutoChopNod or VISIR_spc_cal_LR MR HR HRXAutoChopNod templates Position angle If the observations must be carried out at a position angle different from 0 check 34 2 1 and 34 2 2 In particular it is useful to clearly indicates in the README file if TEL CHOP POSANG is not equal to TEL ROT OFFANGLE to warn the instrument operator about the non standard configuration In spectroscopy TEL CHOP POSANG must be equal to TEL ROT OFFANGLE in order to have the 3 beams along the slit VISIR User Manual 8 Appendix VISIR template parameters 8 1 Acquisition 31 VISIR _im
44. org observing p2pp P2PP too1 html Acquisitions observations and cali brations are coded via observing templates One or more templates build up an observing block OB They contain all the information necessary for the execution of a complete observing sequence An overview of the available VISIR templates and their parameters is given in 86 of this manual e For each science template the user has to provide a finding chart so that the target can be acquired In addition to the general instruction on how to create these finding charts see http www eso org observing p2pp ServiceMode html the following VISIR requirements apply All finding charts have to be made using existing infrared K band or longer wavelength images Typically 2MASS or DENIS K band images are acceptable although higher spatial resolution may be preferable If the wavelength at which the finding chart has been taken is different from that of the science observation e g a K band finding chart for a 10um spectroscopic template the user has to describe clearly how to identify the target at the observing wavelength in the README section of the programme description Adequate examples of such comments are x The target will be the brightest source in the field of view at 10um x At 10um there will be two bright sources in our field of view The science target is the southernmost of these two Note that observations close to zenith during meridian
45. pendicular direction Note that while the telescope offset is in positive East direction the resulting image on the detector will move to the West This technique is recommended for point or relatively small extended lt 5 sources Fig 3 spar Nodding Position A Nodding Position B Nodding Position A Nodding Position B Figure 19 Schematic drawing of the content of a frame obtained with TEL CHOP POSANG O and SEQ CHOPNOD DIR PARALLEL top and SEQ CHOPNOD DIR PERPENDICULAR bottom In the indi vidual nodding positions the positive beams correspond to the chopper position A and the negative beams to the chopper position B Note that the default pointing position of the telescope corresponds to the center of the detector Within the accuracy of the telescope pointing this location matches the nodding position A chopper position A if SEQ CHOPNOD DIR PARALLEL The keywords SEQ JITTER WIDTH allows chopping and nodding with random offsets so that a jitter pattern is performed This technique allows to reconstruct bad pixels For SEQ JITTER WIDTH Ono jitter is performed and the resulting image depends on the setting of SEQ CHOPNOD DIR The chopping period is set by the system and the nodding period is fixed to 90s The number of nodding cycles Neyel_nod 18 computed according to the total observation time 3 8 VISIR User Manual 28 VISIR_ img _obs GenericChopNod This imaging template enhances the flexibility of nodding
46. r or in the slit by manual intervention by the operator There are two acquisition templates for imaging VISIR_img_acq Preset VISIR_img_acq MoveToPixel and one for spectroscopy VISIR_spec_acq_MoveToSlit The observing parameters are described in 38 1 The effect of all acquisition templates is first to point the telescope so that the center of the detector match the target coordinates entered by the user within the accuracy of the VLT pointing see below For VISIR_spec_acq MoveToSlit the first acquisition images are obtained with the OPEN 15 37 slit Then e Both VISIR_ img _acq MoveToPixel and VISIR_spec_acq_MoveToSlit requires interaction with the instrument operator or night support astronomer in order to center the target at the appro priate location on the detector Without further indication given by the observer the default locations are the center of the detector for VISIR_img_acq MoveToPixel and SEQ CHOPNOD DIR PAR ALLEL 3 This convention is identical to the UVES one but differs from example from the ISAAC or NACO one VISIR User Manual 20 in the top left quadrant of the detector at a distance equal to TEL CHOP THROW 72 from the center of the detector in both X and Y for VISIR img acq MoveToPixel and SEQ CHOPNOD DIR PERPENDICULAR at the center of the chosen slit for VISIR_spec_acq_MoveToSlit In service mode acquisition with these templates are limited to objects brighter than 0 2 Jy
47. s are supplied to the user on an experimental basis 4 8 Known problems In addition to effects caused by the cosmetic quality of the detectors mentioned above 83 7 the following problems may affect the quality of the observations 4 8 1 Decreased image quality The image quality can be severely degraded in observations obtained with a large gt 15 chopper throw as can be seen in Fig 17 The origin of this problem has been localized and all efforts will be made to implement a solution as soon as possible VISIR User Manual 23 oe x VISIR 2005 01 29703 48 24 517_tp1_0000 fits Figure 17 Image of a star obtained in the PAHI filter in the Small Field 0 075 pixel and with TEL CHOP THROW 25 SEQ CHOPNOD DIR PARALLEL and TEL CHOP POSANG 90 Left the core of the star image appears double with two peaks separated by 0 2 Right the wings of the image reveals 2 additional components on both sides of the core separated by 1 8 and containing 4 of the total flux Note also the electronic ghosts that appear as white features immediately above and below the core and which only affect bright sources 4 8 2 Low level stripes The background level of individual DIT images fluctuates not only with the varying sky background but also with the detector temperature The latter follows the 1Hz period of the closed cycle cryo cooler The mean background level in two consecutive half cycle frames correspondi
48. source geometries might require larger amplitudes and or SEQ CHOPNOD DIR PERPENDICULAR in order to avoid self cancellation Low and medium resolution Templates for low and medium resolution spectroscopy are VISIR_spec_obs_LRAutoChopNod and VISIR_spec_obs_MRAutoChopNod respectively Observing pa rameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR 8 6 2 High resolution long slit mode Template for high resolution spectroscopy is VISIR_spc_obs HRAutoChopNod Three order sorting filter at 8 02 12 81 and 17 03um INS FILT2 NAME H2_S4 Ne 11 H2 S1 are available Other observing parameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR 6 2 High resolution cross dispersed mode VISIR_spc_obs_HRXAutoChopNod is functionally similar to VISIR_spc_obs_HRAutoChopNod but uses a grism for cross dispersion and order separation Two central wavelength settings 9 66 and 12 27 um are currently available Note that the effective length of the spectrograph slit is limited to 4 Total integration time SEQ TIME the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR are specified as usual 8 6 2 6 4 Calibration Specific templates exist for the observations of photometric and spectro photometric standard stars They offer the same functionality as the corresp
49. tely transparent at others partly or completely opaque Differential absorption is often corrected by dividing the extracted spectrum by a reference spectrum This procedure may cause numerical instabilities at wavelengths close to strong sky lines that might amplify the noise Photometry Spectro photometric calibration of low and medium resolution spectra can be achieved with the MIR standard star list provided by the Observatory see 4 7 For high resolution spectroscopy only calibrators known with high precision such as A stars or asteroids should be considered However even early stars are known to have some hydrogen absorption lines in the N and Q band VISIR User Manual 27 6 VISIR templates description 6 1 Acquisition Each OB needs to start with an acquisition template they are described in 8 4 3 6 2 Observing with the imager VISIR_img_obs_AutoChopNod This template permits observing a source in imaging configuration with various sub settings The observer must specify filter pixel scale chopper throw which is in the range of 8 to 30 The keyword SEQ CHOPNOD DIR is set to PARALLEL or PERPENDICULAR which results in images as shown in Fig 19 PARALLEL considers an equal nodding and chopping amplitude which are both in parallel direction It is recommended for faint extended sources for which the spatial resolution is not so crucial PERPENDICULAR considers an equal nodding and chopping amplitude however in per
50. ter Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR User Manual 8 3 Calibration INS FILT1 NAME INS PFOV SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW VISIR_img_cal_AutoChopNod tsf To be specified Range Default Imager Filter SIC PAH ARII SIV_1 SIV SIV2 PAH2 PAH22 NEILI NEII NEII 2 Q1 Q2 Q3 NODE FAULT 0 075 0 127 0 127 PARALLEL PERPENDICU LAR PERPENDICULAR 0 10 0 30 3600 NODEFAULT 0 359 0 8 30 8 99 Imager pixel scale Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_cal_LRAutoChopNod tsf Range Default To be specified Parameter INS GRAT1 WLEN SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP THROW 8 5 8 8 9 8 11 4 12 2 12 4 NODE FAULT PARALLEL PERPENDICU LAR PARALLEL 0 10 0 30 3600 NODEFAULT 0 359 0 8 30 8 Spectrometer Wavelength mi crons Relative Chop Nod Direction Random Jitter Width arcsec Total integration time sec Chopping Position Angle deg Chopping Amplitude arcsec VISIR_spec_cal MRAutoChopNod tsf To be specified Range Deja INS GRAT1 WLEN SEQ CHOPNOD DIR SEQ JITTER WIDTH SEQ TIME TEL CHOP POSANG TEL CHOP
51. th VISIR However it is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q to avoid detector artifacts 8 3 7 4 6 Overheads The VLT telescope overhead for one OB which includes active optics setting selection of guide star field stabilization is 6 min VISIR instrument configurations can be changed in a short time For example a complete change of instrument settings takes less than 2 minutes The total time for an image acquisition of a bright sources gt 1Jy takes 5min for one fine acquisition iteration or in blind preset 2min Spectro scopic acquisitions take longer and are strongly dependent on the source brightness an overhead of 15min is accounted for sources gt 1 Jy while 30min are required for sources between 0 2 and 1 Jy respectively Instrument overheads due to chopping and nodding duty cycle losses have been measured to be 25 of the observing time for the imager and 50 for the spectrometer respectively The total observing time requested by the observer must include telescope and instrument overheads 4 7 Calibration observations MIR observations depend strongly on the ambient conditions such as humidity temperature or air mass In service mode science observations are interlace by calibration observations on a timescale of 3h Observations of photometric standards will be provided by the observatory within a time interval of three hours w r t the science observations Calibr
52. the photons emitted by the telescope and the atmosphere The dark current is removed by the observation technique chopping or nodding It is at least 6 times lower than the photon noise for the spectrometer and negligible for the imager The detectors have a switchable pixel well capacity The large capacity is used for broad band imaging and the small capacity for narrow band imaging and spectroscopy Detector saturation due to the enormous MIR background is avoided by a storage capacity of 1 9 10 e in small and 1 8 107 e7 in large capacity modes respectively For background limited noise performance BLIP the optimal operational range of the detector is half of the dynamic range for the large capacity and between 1 2 and 1 5 for the small capacity The detector is linear over 2 3 of its dynamic range Fig 11 and its working point is set in the middle of the dynamic range During commissioning it was found that for about half of the array the gain does not differ by more than 2 peak to peak By comparison with other limitations flat field corrections which are difficult to implement in the MIR are not considered important The detector integration time DIT is a few milli seconds in broad band imaging and may increase to 2s in high resolution spectroscopy The DIT is determined by the instrument software according to the filter and pfov It is not a parameter to be chosen by the observer The DRS detectors contain a fair fraction
53. to ensure that correct target coordinates are provided for the equinox J2000 0 ideally at the epoch of the observations The following cases require special care e imaging in the small field in some conditions an error of less than 10 on the coordinates can bring the target outside of the field e spectroscopic acquisition in some conditions an error of less than 7 5 on the coordinates can bring the target outside of the wide slit used Errors of such scale are common in the following situations e high proper motion stars in particular if the epoch of the VISIR observations is significantly different from the epoch for which the coordinates were determined e point like sources within extended objects such as an AGN a number of catalogues do not provide accurate coordinates of the nucleus Coordinates given by 2MASS are more reliable e coordinates obtained with low spatial resolution instrument such as MSX etc For solar system objects the J2000 0 equinox topocentric ICRF or FK5 coordinates at the epoch of the observations are required as the Telescope Control System takes into account precession nutation annual aberration and refraction On the contrary the topocentric apparent coordinates at the observatory often used in other observatories should not be used Additional velocity parameters corresponding to a cos and u must be given in s In particular note that P2PP only accepts coordinat
54. total on source integration time is leonie 4 Neyanod Nesei ch p NDIT DIT 3 The total rejected time is tskip 4 Noyel_chop DIT NDITSKIP Neyel_noa NDIT NCYSKIP 4 and the total observing time is ttot source E tskip 5 Typical duty cycles tsource ttot are about 70 nereko eyo chop Ncyskp N Cycl_ chop mg gt An e Bn Si Bn An T_nod re DIFSRIP NDIT NDITSKIP ee bs DIT T_chop Figure 15 Data timing in VISIR Ac and Bc refer to the two chopper positions An and Bn refer to the two nodding telescope positions Note the AnBnBnAn cycle sequence for the nodding to save observing time VISIR User Manual 17 4 Observing with VISIR at the VLT 4 1 Proposal preparation Tools are available to prepare the observations either during phase 1 call for proposals or during phase 2 creation of observing blocks by the observer e The exposure time calculator ETC available at http www eso org observing etc may be used to estimate the integration time needed to obtain the required S N for a given instrument setting because of the numerous sky absorption lines and the detector feature see Fig 5 it is recommended to display the S N as a function of wavelength when using the spectrograph ETC e As for all VLT instruments astronomers with granted VISIR telescope time prepare their obser vations using the phase 2 proposal preparation tool P2PP described at http www eso
55. tter tend to cancel out on long integrations and lead to a very slight decrease of the spectral resolution VISIR User Manual 24 4 8 5 Fringes The DRS detector shows fringes which are generated in the detector substrate One example of such fringes is shown in Fig 18 for the medium resolution mode The fringes are stable and are not appar ent in chopped images but the spectra are modulated Division of the extracted spectra by standard star spectra simultaneously removes most of the fringes and corrects for telluric features u A QFLPbP a adl Figure 18 VISIR spectrum in staring medium resolution mode showing the detector fringing white The detector absorption feature at 8 8 um is visible as black horizontal bar cf Fig 9 Dark vertical stripes are caused by the non uniform gain of the different electronic amplifiers These features are largely removed by chopping 5 VISIR data 5 1 Data format One FITS file is saved for each telescope nodding position This file is a data cube and contains for each chopping cycle 1 half cycle frames of the on source position A of the chopper 2 the average of the current and all previous A B chopped frames In addition the last plane of the cube contains the average of all chopped frames For the default value of the rotator angle 00 images are oriented North up and East left Spec troscopic data are aligned horizontally in the spatial and verticall
56. y emissivity estimate of lt 15 The VISIR instrument is cooled to avoid internal background contami nation The detectors are at 7K and the interior of the cryostat at 33K The background radiation at 10um is typically my 5 mag arcsec 3700 Jy arcsec and at 20um mo 7 3 mag arcsec 8300 Jy arcsec Consequently the number of photons reaching the detector is huge often more than 10 photons s Therefore the exposure time of an individual integration the Detector Integration TIme DIT is short of the order of a few tens of milli seconds in imaging mode 2 4 Chopping and nodding The basic idea to suppress the MIR background is to perform differential observations using the chopping nodding technique In the chopping technique two observations are performed One set of exposures on source include the background and the astronomical source second set of off source exposures measures the pure background The on and off source observations have to be alternated at a rate faster than the rate of the background fluctuations In practice it is achieved by moving the secondary mirror of the telescope For VISIR at Paranal a chopping frequency of 0 25 Hz has been found to be adequate for N band imaging observations while 0 5 Hz are adopted for Q band imaging Spectroscopic observations are performed with lower chopper frequencies at 0 1 Hz VISIR User Manual 3 10 0 Y D nn 2 C z Er O N
57. y in the dispersion direction cf Fig 18 For the LR and MR modes the short wavelength appear at the top of the frames For the HR and HRX modes the short wavelength is at the top of the frame if the side B of the dual grating is used and at the bottom of the frame of the side A is used 5 2 Pipeline The VISIR pipeline has been developed by ESO DMD and uses the ESO CPL library The main observation templates are supported by the pipeline reductions Raw images of imaging and spectro scopic observations are recombined Spectra are extracted and calibrated in wavelength 8 5 3 for all VISIR User Manual 25 spectroscopic modes in low medium and high resolution Sensitivity estimates based on standard star observations are provided both in imaging and spectroscopy 8 4 7 Public release of the VISIR pipeline is foreseen for the beginning of P76 The pipeline currently supports the following templates e VISIR img _obs_ AutoChopNod e VISIR_spec_obs_LRAutoChopNod e VISIR spec_obs MRAutoChopNod e VISIR spec_obs HRAutoChopNod e VISIR spec_obs HRXAutoChopNod In mosaic or raster mode VISIR_img_obs_GenericChopNod only raw frames are delivered e g mapping reconstruction algorithms are not supported 5 3 VISIR spectrometer data Optical distortion correction Spectra are deformed by optical distortion and slit curvatures The VISIR spectrograph uses curved slits to cancel the distortion of the pre slit optics Thus the slit proj
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