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Very Large Telescope Paranal Science Operations VISIR User Manual

1. Figure 8 Schematic layout of the design of the VISIR spectrometer The long slits have a length of 32 5 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 spec trometer design is shown in Fig 8 The 3 mirror system of the low and medium resolution arm gives a 53 mm diameter collimated beam the collimated beam diameter in the high resolution arm is 125 mm 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 incom ing 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 is given in Table 4 together with their measured band passes and approximate sensitivities for image acquisition
2. wavelength um 1000 T 8005 E F 3 6007 q L E L 4007 Ei aja L 4 1 L 200 4 F L Nn L a 0 L j 1 1 i 1 1 i i i i f 1 L LEO 8 0 8 5 Wavelength um Figure 21 Measured sensitivity as a function of wavelength for low resolution mode measured on the old DRS detector but also valid for Period 95 Top 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 um Dots indicate individual observations full lines represent median and the dashed line the best sensitivities Theoretical model curve correspond to BLIP is shown with a blue line Bottom Bluer setting centered at 8 1jm 2000 TI TO OT TO SERES a E 1500F 3 E 1000 E L J AS ka Si a 500 al n ll J go care ai 0 airis Ly EE EEES TNA E
3. 12 6 12 8 13 0 13 2 Wavelenght um Filter PAH 1_ARIllref1 fy yla Al af TY H A _ i E y 8 0 8 5 9 0 9 5 Wavelenght um Filter QO 13 8 16 2 16 8 Wavelenght um 142 Figure 19 Transmission curves of VISIR imager filters manufactured by READING Over plotted dashed is the atmospheric transmission at low resolution The absolute transmission values are given expressed in percent Tronsmissions in Tronsmissions in Transmissions in Filter Q1 Filter Q2 120 100 q 80F Sob the A D F d h 2 Md E aop iH E hi E OM Wh E 1955 165 1725 18 5 19 5 16 5 17 5 18 5 19 5 20 5 Wavelenght um Wavelenght um Filter Q3 Filter SiC 120 e 100F ve Wu P KA TA 80 d ke ui d i E 40F l E 20 F 7 18 5 19 0 19 9 20 0 20 5 8 10 12 14 16 Wavelenght um Wavelenght um Filter SIV 120 100 80 60 40 20 O a a Sh pa 10 1 10 3 10 5 10 7 10 9 Wavelenght um Figure 18 continued Filter Nell rer Filter PAH ref erie lt 121 CS o DB 1 0 9 1 0F CAPITA E E My v 0 8 o D r d 4 Cc E S So K 06 0 6t 7 ES KO IR 10 oO N 0 4 N O4F 7 o Kai t E 0 2 E 0 21 J O O E ont EE 0 01 i i 11 8 12 0 12 2 12 4 12 6 10 8 11 2 11 8 12 2 12 8 Wavelenght um Wavelenght um Filter SIV_ref1 Fil
4. 0 19 90 20 00 20 10 20 20 Wavelength um Figure 24 Measured sensitivity as a function of wavelength for different settings in the medium resolution mode NOT Offered in Period 95 obtained in very good weather conditions II for 19 9um lt 4A lt 20 3um 8000 6000 F 7 4000 F 7 2000 7 Sensitivity mJy 100 in 1h 9 00 9 05 9 10 SL Wavelength um 8000 6000 F 4000 2000 Sensitivity mJy 100 in 1h EE 10 48 10 50 10 52 Wavelength um 10 54 Figure 25 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 1 4000 3000 2000 1000 Sensitivity mJy 100 in 1h 0 propa q pet eal e o rl po ea e lio o e e PO peo pearl qe jo pe 11 540 11 545 11 550 11 555 11 560 11 565 Wavelength um 4000 E a E 3000F 7 b o E J E 20007 4 he E gt E J L A 1000 Z F E d E E 7 n E J 0 E A Aaa e pr A AO e e A J 11 745 11 750 11 755 11 760 11 765 11 770 11 775 Wavelength um Figure 26 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 ID 4000 3000 2000 1000 S
5. The cosmetic quality of the AQUARIUS detector is very good However this and eventual other problems e g decreased image quality bad residuals stripes etc still need to be investigated We emphasize that due to the central outward readout of the Imager and Spectrometer detectors the science targets need to avoid the central 20 pixels horizontal stripe around Y 512 6 VISIR data 6 1 Data format One FITS file is saved for each telescope nodding position This file is a data multi extension fits file and contains for each chopping cycle 1 general header 2 each half cycle frame of the on source position A of the chopper 3 the average of the current and all previous A B chopped frames In addition the last extension of the file contains the average of all chopped frames In Burst mode a number of FITS files is saved Each file is a data cube containing up to 1500 ele mentary frames The total number of files is such that it sums up the total integration time requested For the default value of the rotator angle 0 images are oriented North up and East left Spec troscopic data are aligned horizontally in the spatial and vertically in the dispersion direction cf Fig 12 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 6 2
6. 0 pepe to a aja Poe EE la A A UA Aa lA 18 65 18 70 18 75 18 80 18 85 18 90 18 95 Wavelength um Figure 30 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 VD OA A Rs EU EEES SE 1 5x 104 1 0x104 T 5 0x1083 F Sensitivity mJy 100 in 1h 21 280 21 290 21 300 21 310 Wavelength um Figure 31 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 VID
7. 4 beams The sensitivity values are still valid for Period 95 proposal preparation Filter Ac halt band sensitivity um width 100 1h mJy um SF B 8 7 8 92 0 97 B 9 7 9 82 0 84 9 B 10 7 10 65 1 37 5 B 11 7 11 52 0 85 5 B 12 4 12 47 0 99 8 J7 9 7 76 0 55 14 J 8 9 8 70 0 73 3 J 9 8 9 59 0 94 7 J12 2 11 96 0 52 8 Table 3 VISIR imager filter characteristics determined with a monochromator and the WCU The last 2 columns give the measured median sensitivities for the Small Field obtained in good weather conditions The measured sensitivities were obtained using the curve of growth method on data obtained in perpendicular chopping nodding directions 4 beams The sensitivity values are still valid for Period 95 proposal preparation 4 2 Spectrometer VISIR offers slit spectroscopy at three spectral resolutions with a pixel scale of 07076 This is ob tained 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 PLANE WHEEL q Y MO A BIC IMAGER TRE IMAGER y COLDSTOP Y ILTER WHEEL P E EA WHEEL HR LIT WHEEL DUO ECHELLE GRATING UNIT GRATING UNIT 4 GRATING SCANNERS 1 RETURN FLAT RESOLUTION SELECTION MECHANISM R COLLIMATOR CAMERA
8. PARALLEL top and SEQ CHOPNOD DIR PERPENDICULAR bottom In the individual 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 SEO 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 O no 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 90 s The number of nodding cycles Noye1_nod 18 computed according to the total observation time Sect 4 5 VISIR_img_obs_GenericChopNod This imaging template enhances the flexibility of nodding offsets and allows the user to specify them in a list of relative offset positions In the most simple application only one offset posi tion 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 off set list results in a freely programmable pattern of nodding pairs Note that the integration time SEO TIME specified refer
9. and intermediate field 0 076 not offered in Period 95 by perpendicular chopping and nodding patterns with amplitudes of 10 Calibrators are frequently observed during the night Sect 5 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 T T Amedan small field 1 00 d i median intermed field e 6 1 o A E A De e os Toy E A e Vi Y ih 2 10 Eos jo 4 e E y Si A 4 4 m 4 E d Puw ST ES a i Ki ARIII SIV PAH2 PAH2_2 NEII 4 PAH1 Siv_1 siv_2 SIC NEIL1 NEIL 3 LA L saab ss a 8 9 10 11 12 13 wavelength um oot median small field median intermed field o lt K y z amp 100 R 4 ES E A Y 3 tb A D E A i gt RK E c HAY mn 10 Q1 Q2 Q3 170 175 180 185 190 195 200 wavelength um Figure 6 Sensitivities for the VISIR imager for the N top and Q band bottom measured on the old DRS detector but also valid for Period 95 Small and intermediate IF is not offered in Period 95 field observations are displaced for clarity Background noise limits are indicated for the individual filte
10. 4 2 1 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 Filter Ae halt band sensitivity um width um 100 1h mJy Nell 1 12 35 0 50 80 Nell 2 12 81 0 10 50 Table 4 VISIR spectrometer acquisition filter characteristics The filters transmissions have been determined with a monochromator and the WCU The last column list the measured median sensi tivities which were obtained using the curve of growth method on data obtained in parallel chop ping nodding directions 3 beams The sensitivity values are still valid for Period 95 proposal preparation 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 4 2 2 Resolution In the N band the low resolution and medium resolution modes provide spectral resolving power of 300 and 3000 Table 5 respectively In high resolution long slit mode narrow wavelength ranges around the 8 02 H2 S4 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 an emission line below 107 W m arcsec can be achieved This value corresponds to an approximate sen
11. 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 nuta tion 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 6 and 0 must be given in s 3In particular note that P2PP only accepts coordinates for J2000 0 5 4 Guide stars Guide stars are mandatory for active optics and field stabilization Any VLT program should make sure that a guide star UCAC3 with a V 10 5 14 0 mag is available within 7 5 around the object Sensitivity in the mid IR for a ground based observatory is strongly limited by the sky brightness In addition the VISIR field is small compared to other VLT instruments Therefore images of a field can often appear empty in short to medium length exposures However objects may become visible in longer ones Combining different exposures taken on different nights may be tricky if a proper astrometric alignment is not carried out Since the overall astrometric accuracy of an image is actually limited by the accuracy on the coordinates of the guide star it is strongly recommended that all OBs of a same field use the same guide star in particular for faint objects In addition objects within optically dark mole
12. Pointing position East Figure 15 Definition of chopping parameters from the telescope point of view If the position angle PA is measured counter clockwise from North to East with PA between O and 360 then TEL CHOP POSANG 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 idle position 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 5 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 Sect B The nodding period is a parameter that can only be modified by the instrument operator For expo sures shorter than 180s SEQ TIME lt 180s as possible in acquisition images the nodding time is set to half the requested exposure time For exposures longer than 180s the nodding time is set to 90s In particular exposure time given in the template will be internally changed by the software to be the closest to a multiple
13. TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec 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 img acq Preset tsf To be specified Parameter Range Default Label TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE 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 veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec 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 spec acg MoveToSlit tsf To be specified Parameter Range Default Label INS FILT2 NAME NEI 1 NEM 2 NODE Acquisition Filter FAULT INS SLIT1 TYPE LONG SHORT LONG Spectrometer Slit Type long or short INS SLIT1 WIDTH 0 40 0 75 1 00 NODE Ee Slit Width arc FAULT sec SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE
14. 512 pixels all 64 outputs from the two areas being read in parallel This readout scheme also allows for 16 outputs rather than 64 to simplify system design for low background applications With this multiplexer configuration it is possible to read the detector out at 150 Hz 7 milli seconds frame rates each output operational at 3 MHz pixel rates 4 4 2 Detector Readout Note for Period 95 Detector windowing is forbidden until further notice Detector readout modes and all other parameters of the intrinsic read such as the DIT cannot be user con trolled 4 4 3 Detector Thermal Oscillations The pre intervention DRS detector suffered thermal osculations they are common for mid IR detec tors and occur at a frequency of 1 Hz Oscillations usually appear because the Closed Cycle Coolers used to cool the detectors and the optical parts do not provide a constant temperature in their cooling cycle These thermal oscillations can result in additional detector noise estimated to be 250 DN which translates to an additional noise of approximately 50 e rms added in quadrature to the read noise The solution to this problem was to mount a large block of lead approximately 0 6 kg in weight serving as thermal capacitor to smooth the thermal oscillation by a factor of at least fifty Thermal oscillations induced noise has not been observed after the upgrade and we keep this section for historic reasons and for the benefit of VISIR s archival d
15. 7 5 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 SC Amplitude arcsec VISIR_spec_obs_HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME NEI 2 H2S_1 H2S 4 Spectrometer Filter NEII 2 INS GRAT1 WLEN 7 80 19 18 12 810 Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 Ce Amplitude arcsec VISIR_spec_obs_HRXAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 60 28 08 VODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec SEQ DIT ALG
16. As for Exposure time calculation the astronomical community is invited to use the well established DRS detector sensitivities both for spectroscopy and imaging as provided in Tables 2 3 6 and 7 Indeed the new AQUARIUS detector has proven to show better performance however a final characterization of the achieved sensitivities is to await the commissioning and science verification The cosmetic quality of the AQUARIUS detector is proven to be excellent The regions of masked pixels Fig 12 and stripes present on the old DRS detector are not there anymore 1 2 Low Resolution Spectroscopy The second major improvement is that concerning the N band 8 13um Low Resolution Spec troscopy Before the upgrade this was achieved by means of a grating and has the disadvantage of requiring 4 independent exposures in order to cover the 8 13 5um range The introduction of the low resolution prism R 300 for a 073 slit will allow to achieve the same wavelength coverage in a single exposure and reach improved sensitivities 1 3 Precipitable Water Vapor The amount of Precipitable Water Vapor PWV present in the Earth s atmosphere can heavily im pact on Mid infrared observations However the effect of PWV is strongly dependent on wave length Whereas a PWV column of 3 mm or larger is generally acceptable for observations in the N band the sensitivity of observations in the Q band depends strongly on the PWV contents and typically can
17. Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 ate Amplitude arcsec 10 Appendix Filter transmission curves The filter transmission has been measured using a Fourier Transform Spectrometer at a temperature of 35 K for filters manufactured by the company READING Their absolute transmission curves are displayed in Fig 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 Transmission curves for the intermediate band imaging filters are shown in Fig 2 Transmissions in 2 Transmissions in Tronsmissions in 120 100 80 60 40 20 Filter Arlll O 8 80 8 90 9 00 9 10 9 20 120 100 80 Wavelenght um Filter Nell AS NS 60 40 20 OL i PA PE E 12 4 12 6 12 8 13 0 13 2 Wavelenght um Filter Pan2 120 SS RS 100 80 60 40 20 LORS TS EE ENKE 10 9 11 6 12 2 Wavelenght um 12 8 Transmissions in Transmissions in Transmissions in 120 100 80 d eck 40 20H 0 Filt er Nell_ref
18. Ss SEES a REP RES BER 17 2 Observing with the imager as KRESS PER ag 7 3 Observing with the spectrometer 2 6225 a PR e ows oe eee Bee Bae bee bee eee oS Shou Sd oe Bae se 8 Checklist Sl Phase Ult Las tata dd E ove he a Sew ME e a oth Sls oh s 8 2 Phase assa SS ad e ee ee EE TA EE 9 Appendix VISIR template parameters 9 1 CIRO css ap ok ke O AAA a SAT AP o AR A vs ye See Ee ER de a Se BA a BO GS GS a SECOS 10 Appendix Filter transmission curves 11 Appendix Observed sensitivities in various spectroscopic settings 30 30 31 31 33 33 33 35 36 36 36 37 38 40 42 45 49 List of acronyms BIB BLIP BOB DIT ETC FWHM ICS IR IRACE MIR OB P2PP PAE pfov PSF S N SAM 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 Sparse Aperture Masking Unit telescope VLT imager and spectrometer for the mid infrared Telescope control system Three mirrors anastigmatic Warm calibration unit 1 VISIR Upgrade Project VISIR has been undergoing an upgrade starting from May 2012 P89 Pending successful recom missioning during P
19. TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 Se 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 veloc ity sec TEL TARG ADDVELDELTA 0 0 DEC additional tracking ve locity sec 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 9 1 Observation VISIR_img_obs_AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME PAH1 ARII SIV 1 SIV Imager Filter SIV2 PAH2 PAH22 NEI 1 NEO NEO_2 B8 7 B9 7 B10 7 B11 7 B12 4 J7 9 J8 9 J9 8 J12 2 Q1 Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 045 Imager pixel scale SEQ CATG PRE IMAGE SCIENCE Observation Category SCIENCE SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 Ge Amplitude arcsec VISIR_img_obs_GenericChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME PAH ARII SIV_
20. 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 5 5 Brightness limitations There are currently no brightness limitations with VISIR However it is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q to avoid detector artifacts Sect 4 4 5 6 Overheads The VLT 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 1 Jy takes 5 min for one fine acquisition iteration or in blind preset 2 min S
21. quadrant of the detector at a distance equal to TEL CHOP THROW 2 from the center of the detector in both X and Y for VISIR spec acq MoveToSlit at 3 South from the center of the slit In service mode acquisition with the VISTR_spec_acq_MoveToSlit template is limited to objects brighter than 0 2 Jy All acquisition images are recorded and archived Note that except if specifically requested in the README file photometric standard stars are not necessarily observed in the same filter as the acquisition filters 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 obtained using the filter set by the INS FILT2 NAME parameter 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 Only the exposure time of the through slit images can 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_acq_Preset template In this case no acquisition images are taken By default if TEL TARG ALPHA and TEL TARG DELTA contain the accurate coordinates of the target the target will be locate
22. see Sect 5 7 For high resolution spectroscopy only calibrators known with high precision such as A stars or asteroids should be considered However even early A stars are known to have some hydrogen absorption lines in the N and Q band 7 VISIR templates description 7 1 Acquisition Each OB needs to start with an acquisition template they are described in Sect 7 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 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 perpendicular 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 EI pra Nodding Position A Nodding Position B Nodding Position A Nodding Position B Figure 17 Schematic drawing of the content of a frame obtained with TEL ROT OFFANGLE TEL CHOP POSANG and SEQ CHOPNOD DIR
23. strongly affected by the detector feature at this wavelength The users can evaluate the impact of the PWV value on their program with the new advanced SKYCALC Sky Model Calculator this is a line by line radiative transfer model available un derhttps mm eso org observing etc bin gen form INS MODE swspectr INS The tool calculates the telluric interference with planned observations as a function of the PWV and some other parameters As for all VLT instruments astronomers with granted VISIR telescope time prepare their ob servations using the phase 2 proposal preparation tool P2PP described at Acquisitions observa tions and calibrations 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 com plete observing sequence An overview of the available VISIR templates and their parameters 1s given in Sect 7 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 sci observing phase2 SMGuidelines 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 wavelengt
24. 0 104550 Nill 8A 14600 10133 8 HRX 18 680 18 960 0 164225 SIN 7B 11150 6450 4 HRX 21 295 0 104900 NalV 7A 14300 10097 9 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 4 in the given setting The wavelength range per setting in given in the 3rd column AA R is the theoretical spectral resolution Offered slits have widths of 0 40 0 75 and 1 00 Note that the range 12 210 12 760 also covers HD 0 0 R 9 while the Nell emission line can be observed up to z 0 038 The dispersion is given in the 7th column and has been estimated for the new AQUARIUS detector pixel size The sensitivity values are still valid for Period 95 proposal preparation mm 08 32 outputs vAOut 1 32 Column Column Amplifiers l 512 rows Row 1A Row 1B Bi 512 rows Row 5128 A Sie outputs vBOut 1 32 Row 512A ST Y T li om O 64 outputs mode 2 sides Figure 10 Left panel shows the AQUARIUS multiplexing readout scheme Right panel shows the detector mounted in its socket 4 4 Detectors The VISIR imager and spectrometer are currently equipped with two new AQUARIUS 1k x 1k detectors with pixel size of 30 um The optical design of VISIR was based on a hypothetical detector with 512 x 512 pixels and 50 um pixel size The actual AQUARIUS 1024 x 1024 array is 20 larger a
25. 1 SIV Imager Filter SIV2 PAH2 PAH22 NEIL 1 NEII NEI 2 B8 7 B9 7 B10 7 B11 7 B12 4 J7 9 J8 9 J9 8 J12 2 Q1 Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 045 Imager pixel scale SEQ CATG PRE IMAGE SCIENCE Observation Category SCIENCE SEQ NOFF 1 100 NODEFAULT Number of offset positions SEQ OFFSET COORDS SKY DETECTOR NODE Offset coordinates FAULT SEQ OFFSET1 LIST NODEFAULT List of offsets in RA or X SEQ OFFSET2 LIST NODEFAULT List of offsets in DEC or Y SEQ TIME 180 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 30 10 e Amplitude arcsec VISIR_spec_obs_LRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 8 1 8 5 8 8 9 8 11 4 12 2 12 4 Spectrometer Wavelength NODEFAULT microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 GE Amplitude arcsec VISIR_spec_obs_MRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN
26. Angle de TEL CHOP THROW 8 10 10 GE Amplitude arcsec VISIR spec cal MRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 5 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 SE Amplitude arcsec VISIR_spec_cal_HRAutoChopNod tsf To be specified Parameter Range Default Label INS FILT2 NAME NEI 2 H2S_1 H2S 4 Spectrometer Filter NEII 2 INS GRAT1 WLEN 7 80 19 18 12 810 Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 NODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 Ce Amplitude arcsec VISIR spec cal HRX AutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 7 60 28 08 NODEFAULT Spectrometer Wavelength microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod
27. At the operating temperature of the detector 6 7 K the dark current which is the signal obtained when the detector receives no photons is negligible compared to the background generated by the photons emitted by the telescope and the atmosphere The dark current is removed by the observation technique chopping or nodding The detectors have a switchable pixel well capacity For reasons of excessive noise the large capacity is not offered Detector saturation due to the enormous MIR background is avoided by an operation with the small wells with a storage capacity of 0 6x10 e 60000 0 5 det 0 4 50000 oe Low Gain 0 3 y 32881 4x 5763 o 2 40000 ee 3 EN i 30000 o e 3 wo 1 5 y 40815x 4146 8 So 0 10000 2 30000 400 50000 2 20000 r 5 Signal Level D 02 t amp t t E 10000 0 3 e o gt 0 4 e 0 9 0 0 5 2 25 70 5 Ekposu re tite s Figure 11 Left panel displays the measured AQUARIUS detector linearity for both high and low gain read modes Right panel displays the non linearity as a function of the signal level as computed for the high gain setup in large capacity modes respectively The detector integration time DIT is a few milli seconds in broad band imaging and may increase to 2 s in high resolution spectroscopy The DIT is determined by the instrument software using a user selectable algorithm The options are 1 AUTO in this case the sky background
28. ES O EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans H misph re Austral Europ ische Organisation f r astronomische Forschung in der s dlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei Miinchen Very Large Telescope Paranal Science Operations VISIR User Manual Doc No VLT MAN ESO 14300 3514 Issue 95 Date 27 08 2014 V D Ivanov amp the VISIR IOT Prepared A Date Signature C Dumas Approved A A Ee Date Signature A Kaufer LESA A e ei ewe ke dd een This page was intentionally left blank Change Record Issue Date Section Parag affected Reason Initiation Documents Remarks Rev 1 0 04 09 04 creation First release for science verification in P74 and OT proposals in P75 1 1 10 12 04 2 4 3 2 6 2 6 3 7 8 update for P75 Phase2 v76 1 01 02 05 all update for P76 CfP v76 2 06 07 05 all update for P76 Phase 2 v76 3 14 07 05 4 8 1 Corrected Legend Fig 17 v76 4 14 07 05 Cover pages Corrected typos v77 1 04 09 05 3 5 7 4 1 4 3 4 8 1 7 8 1 10 update for P77 CfP v77 2 05 09 05 4 6 match imager overhead of CfP v77 3 20 12 05 1 2 3 4 3 6 4 2 4 4 4 7 7 8 update for P77 Phase2 v78 1 27 02 06 3 6 3 10 update for P78 CfP v78 19 06 06 cover 2 2 3 2 4 3 1 P78 release v79 30 11 06 4 8 P79 release v80 28 02 07 3 1 4 2 1 5 1 6 2 8 2 P80 release burst mode includ
29. O AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle deg TEL CHOP THROW 8 10 10 GE Amplitude arcsec 9 2 Calibration VISIR_img_cal_AutoChopNod tsf To be specified Parameter Range Default Label INS FILT1 NAME SIC PAH1 ARII SIV_1 Imager Filter SIV SIV 2 PAH2 PAH2_2 NEIL 1 NEII NEI 2 Q1 Q2 Q3 NODEFAULT INS PFOV 0 045 0 076 0 045 Imager pixel scale SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PERPENDICULAR SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 30 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle deg TEL CHOP THROW 8 10 10 con Amplitude arcsec VISIR_spec_cal_LRAutoChopNod tsf To be specified Parameter Range Default Label INS GRAT1 WLEN 8 1 8 5 8 8 9 8 11 4 12 2 12 4 Spectrometer Wavelength NODEFAULT microns SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ JITTER WIDTH 0 10 0 Random Jitter Width arcsec SEQ TIME 180 3600 VODEFAULT Total integration time sec SEQ DIT ALGO AUTO GOOD WEATHER Total integration time sec BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position
30. Pipeline A pipeline for the reduction of VISIR data has been developed by ESO The main observation tem plates are supported by the pipeline reductions Raw images of imaging and spectroscopic observa tions are recombined Spectra are extracted and calibrated in wavelength Sect 6 3 for all spectro scopic modes in low medium and high resolution Sensitivity estimates based on standard star observations are provided both in imaging and spectroscopy Sect 5 7 The public release of the VISIR pipeline is accessible at 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 6 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 projected on the sky is straight There is an additional linear distortion in both dispersion and cross dispersion direction of the detector The distortions have not been estimated yet for the new AQUARIUS detector and will be reported after the full commissioning of the new detectors Wavelength calibration A first order wavelengt
31. S 2 eee HS ewe E ER RIIIE Instrument description and offered observing modes 4 1 Imager 4 2 1 Slit widths 4 2 3 Low resolution offered central wavelengths 4 2 4 Medium resolution offered central wavelengths 4 2 5 High resolution offered central wavelengths 4 3 Calibration units 4 4 Detectors 4 4 1 Detector Architecture 44 2 Detector Read Ale AAA A A 4 4 3 Detector Thermal Oscillations 444 Detector Dark Current 4 4 5 Detector Linearity eses sis irradia 4 5 Data acquisition System ssa 44 40444 o AA A Observing with VISIR at the VLT 5 1 Proposal preparation 5 2 Telescope observing parameters o e ee 5 2 1 Instrument orientation on the sky o e 5 2 2 Chopping parameters oe SES A RAE OMG See he A RO a Ch SE E E E ap sa ee E E e ee 5 3 1 Introduction 5 3 2 Description 11 11 12 12 12 12 13 15 15 16 16 16 16 19 SE IT STA uy See ETE IA A DE A e E KC BEE A RARA RA A he ce eee On ae ee a 5 6 Overheads 5 7 Calibration observations ae Ga GOA we rs EENEG 5 9 Known problems ssa ssa E E E wa She ee Be RT E 6 VISIR data 6 1 Dataformat 0 0 00 0 adig eioi ee ee A lt i segs ee ee i e SS Se SN Ooo Sle ed ee de os 6 3 VISIR spectrometer Dala 4224424 Pt ast ae e ern Ses ern CN 7 VISIR templates description all ACQUSNOM eg bse pus RES
32. Wavelength um Figure 28 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 IV The region on the top panel encompasses the observed wavelength of Nell up to z 0 038 2 0x104 JT E E 1 5x104 m a i L a i 1 0x104 E L J Ki A E 5 0x10 4 n C 7 n 0 E hy o oy EE f pe qu EA EL A jo a 16 900 16 910 16 920 16 930 6 940 16 950 Wavelength um 10000 7 T 8B000F a H d f 6000 a gt L g L 4000 gt te 2 2 2000 3 n 0 F 1 1 l 1 1 1 1 l f fi Ll l fi fi 1 1 1 1 1 i 17 80 17 85 17 90 17 95 18 00 Wavelength um Figure 29 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 V LOGO A 8000 F 6000 F 7 4000 F 2000 F J Sensitivity mJy 100 in th 0 E E O ap A SRE SAE E S E fe E E ELE E EA E E LE 18 210 18 220 18 230 18 240 Wavelength um 6000 ER a AA 5000 4000 3000 2000 Sensitivity mJy 100 in 1h 1000
33. a A asi atera ERA AA 7 50 7 60 7 70 7 80 7 90 8 00 Wavelength um 1000 7 T 800 g i 3 600 E gt E 1 400 SES gt S 200 4 dA 0 E 1 fi 1 li 1 i fi i 1 Ll 1 1 1 1 fi 1 Ll Ll 1 Ll i L 80 82 8 86 88 90 92 Wavelength um Figure 22 Measured sensitivity as a function of wavelength for different settings in the medium resolution mode NOT Offered in Period 95 obtained in very good weather conditions I for 7 5um lt A lt 94um 1000 7 8800F i q 2 600 gt E 400 S E TE a 200 o Nn d 0 L L fi f f L f L f 1 f f 1 f 1 10 5 11 0 11 5 12 0 12 5 Wavelength um 2000 TP MATA I E i amp 1500F 2 E 1000 E E 500 Nn 4 C 5 UN 4 0 Kr AN SM LSS ep 1 178 18 0 182 184 186 188 19 0 Wavelength um Figure 23 Measured sensitivity as a function of wavelength for different settings in the medium resolution mode NOT Offered in Period 95 obtained in very good weather conditions ID for 10 lum Je 12 5um and 17 7um lt lt 19 1um 4000 E TT yy 3000 2000 1000 Sensitivity mJy 100 in 1h
34. a chopping distance or throw called TEL CHOP THROW see Fig This parameter can be set by the user 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 cancellation chopping and nodding throws below 15 are recommended but for Period 95 the maximum throw is limited to 10 arcsec for reasons related to the increased chopping frequency 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 PA is the angle of chopping counted East of North see Fig 15 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 see Fig IC 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
35. and spectro photometric standard stars They offer the same functionality as the corresponding science templates but allow to monitor the sensitivity and image quality by observing calibration standard stars Their use is recommended to be properly recognized by the VISIR pipeline S Checklist This section provides a number of advice regarding the preparation of the proposal 8 1 Phase 1 It is very important that the time justification Box 9 of the proposal contains enough information so that its feasibility can be correctly assessed The following points must be respected 1 the expected S N for each object and modes must be given 2 in particular for extended sources does the reported S N refer to an area of 1 arcsec as given by the imaging ETC to an extent of 1 arcsec in the spatial direction as given by the spectroscopy ETC or to the whole spatial extent of the object 3 in spectroscopy does the S N refer to one pixel in the dispersion direction as given by the ETC or to one resolution element 4 in case of large throw does the S N take into account the fact that some beams would fall outside the detector 5 does the overhead calculation include the time required for each preset given that OBs should in general not be longer than 1 hour 8 2 1s there a guide star brighter in the interval V 10 5 14 0 mag within a radius of 7 5 arcmin around the object The PWV constraint under which the observation
36. ata users 4 4 4 Detector Dark Current Mid IR detectors operate at a temperature range that is set by the Closed Cycle Coolers typically 6 9 K Lower operating temperatures allow to minimize the leakage and most importantly the dark current Laboratory experiments showed that the dark current ranged between 2200 and 0 56 e pixel s at 10 0 and 5 6 K respectively The upgrade operational goals aimed at reaching a 1 0 e pixel s and this was achieved at an operating temperature of 9 K The dark current if the AQUARIUS detectors is negligible for all practical purposes and no darks are required for a science grade data reduction 4 4 5 Detector Linearity The AQUARIUS detector linearity was derived for both the high gain and the low gain configura tions The left panel of Fig 11 displays the measured signal level as a function of exposure time Typically these data are taken in a non destructive read mode such that many hundreds of frames are taken between the signal detector starvation level up to its saturation level A linear fit was applied to the data points between 15 000 40 000 DN and the differences between the fit and the data points is plotted for the high gain setup right panel of Fig 11 Over this signal range the detector shows an excellent linearity of the order of 0 5 For this particular detector the gain and therefore the detector saturation level and read noise can be changed by a factor of approximately eight
37. be done under PWV columns between 1 and 3 millimeters Operations wise a prior knowledge of the PWV content will seriously impact on efficient service and visitor mode VISIR Paranal PWV across the year 175 quartile PWV mm B50 quartile a25 quartile Day of year Figure 1 Average PWV distribution over Paranal across the year observations As part of the VISIR upgrade project and starting December 2011 real time PWV monitor is now available on Paranal The commissioning of the PWV monitor shows that it meets all specifications e PWV range 0 5 9 mm validated e PWV precision ca 30 um e PWV accuracy ca 0 1 mm e High time resolution sec e All sky pointing 2D capability e Autonomous operation The median PWV over Paranal is 2 1 mm with strong seasonal variations see Fig 1 The fraction of time in which the PWV contents over Paranal is lower than 1 mm is about 10 The PWV value will be used as user defined constraint parameter from Period 90 October 2012 April 2013 on wards The users can evaluate the impact of the PWV value on their program with the new advanced SKY CALC Sky Model Calculator this is a line by line radiative transfer model available under https www eso org observing etc bin gen form INS MODE swspectr INS NAME SKYCALC The tool calculates the telluric interference with planned observations as a function of the PWV and some other parameters http www eso org tecarc
38. by taking into account the measured transmission curves Fig 19 the detector efficiency and an atmosphere model Fig 2 Continuous observations over 3 hours of the same standard star indicates that photometric stability better than 3 can be achieved with VISIR at the VLT In order to test 1f a photometric precision of the same order can be obtained a reduced set of standard stars has been built consisting of the Cohen et al stars which obey the following criteria e visibility from Paranal e no variability detected by Hipparcos non variables Var 0 in the Hipparcos catalogue e absolute flux calibration errors as reported by Cohen et al lt 20 e all spectral types reported in SIMBAD no more than 1 sub class different from that used by Cohen et al 4Cohen et al 1999 AJ 117 1864 e not visual binaries as reported by SIMBAD This catalogue of 81 stars is also made available at http www eso org instruments visir From this catalogue a further selection to provide a reduced list of 12 stars has been carried out see also http www eso org instruments visir These stars are distributed as uniformly as possible in Right Ascension with spectral types as similar as possible In addition their flux in the N band of the order of 10Jy is bright enough to be observable in the Q band without reaching non linearity levels in the N band even in non ideal background conditions At least one star in this reduced catalogue wil
39. c_cal_LR MR HR HRXAutoChopNod templates Position angle If the observations must be carried out at a position angle different from 0 check Sect 5 2 1Jand Sect 5 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 particular in spectroscopy TEL CHOP POSANG must be equal to TEL ROT OFFANGLE in order to have the 3 beams along the slit 9 Appendix VISIR template parameters VISIR_img_acq_MoveToPixel tsf To be specified Parameter Range Default Label INS FILT1 NAME K BAND PAHI ARII Imager Filter SIV_1 SIV SIV_2 PAH2 PAH2 2 NEI 1 NEI NEI 2 B8 7 B9 7 B10 7 B11 7 B12 4 J7 9 J8 9 J9 8 J12 2 Q1 Q2 Q3 NODE FAULT INS PFOV 0 045 0 076 0 045 Imager pixel scale SEQ CHOPNOD DIR PARALLEL PERPENDICU Relative Chop Nod Direction LAR PARALLEL SEQ TIME 30 3600 VODEFAULT Total integration time sec TEL AG GUIDESTAR CATALOGUE SETUPFILE Get Guide Star from NONE CATALOGUE SEQ DIT ALGO AUTO GOOD WEATHER DIT determination algorithm BRIGHT SOURCE NODE FAULT TEL CHOP POSANG 0 359 0 Chopping Position Angle de TEL CHOP THROW 8 10 10 Ge 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 veloc ity sec
40. cross dispersed mode with a 4 1 long slit A or Range AM Grating Resolution Dispersion Sensitivity um um Order measured 1 slit px um 100 1h mJy 7 5 8 0 0 488 2 3500 2192 1000 8 0 9 3 0 470 2 3500 2267 200 10 2 13 0 0 525 2 3500 2417 1733 200 17 1 19 0 0 925 1 1800 1158 1200 20 12 0 900 1 1800 720 1200 Table 5 VISIR medium resolution settings The first column gives the minimum and maximum allowed values for the central wavelength 4 lt in the given setting The wavelength range per setting in given in the 2nd column AA The spectral resolution measured with a 1 slit is given in the 3rd column The dispersion is given in the 4th column and has been estimated for the new AQUARIUS detector pixel size Typical offered sensitivities are given in the last column Examples of dependence of sensitivity with wavelength are shown in Figures 22 to 24 Offered slits have widths of 0 40 0 75 and 1 00 Note for Period 95 this mode is not offered Offered modes and sensitivities are given in Table 6 The ETC offers the possibility to take into account the earth motion to predict the observed wavelength of a given line depending on the fore seen date and time of observations In particular this feature allows to determine the dates when the emission line under study would appear at the same wavelength as a sky line 4 3 Calibration units A warm calibratio
41. ctured as follows Basic observing techniques of ground based MIR instruments are summarized in Sect Sect 4 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 Sect 5 A description of the structure of the imaging and spectroscopic data files is given in Sect 6l A checklist to help the preparation of OBs is available in Sect 8 Acquisition observing and calibration templates are explained in Sect 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 3 Observing in the MIR from the ground 3 1 The Earth s atmosphere Our atmosphere absorbs the majority of the MIR radiation from astronomical sources The main absorbing molecules are HO CH4 CO2 CO O2 O3 However the atmosphere is quite transparent in two atmospheric windows the N and Q bands They are centered around 10 and 20 um respec tively The transmission in the N band is fairly good at a dry site and becomes particular transparent in the wavelength range 10 5 12 um 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 spec tral coverage of AA lum at an average transmission of 60 Observatio
42. cular clouds may have few or no suitable guide stars at least in the catalogues currently used by the Telescope Control System the UCAC3 Alternatively some bright nebulae may saturate the region of the digital sky surveys used by the telescope operator to select guide stars Considerable amount of telescope time will be saved if such cases are identified before an OB is started Providing the coordinates of a guide star in the acquisition template of an OB is therefore strongly recommended in a number of situations e observations of faint objects hardly or not visible even after a long exposure in particular if this exposure has to be combined with other ones e observations of objects within optically dark molecular clouds where few suitable guide stars are expected e observations of objects within bright nebulae larger than the field of view accessible by the guide probe that appeared saturated in the digital sky surveys example Orion e observations for which astrometric accuracy is important In all these cases the use of the guidecam tool see http www eso org instruments visir for VISIR is strongly recommended and the coordinates of a suitable guide star should be inserted in the acquisition templates If TEL AG GUIDESTAR is CATALOGUE a guide star 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
43. d at the center of the detector including if the observing templates use SEQ CHOPNOD DIR PERPENDICULAR In this case in order to avoid to lose the chopnod images it is advisable either to change the parameters TEL TARG ALPHA and TEL TARG DELTA so that they are off set by half the TEL CHOP THROW values to south and west for TEL ROT OFFANGLE TEL CHOP POSANG 0 oruse the parameters TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA as above the convention final coordinates RA DEC of the center of the field plus offsets equal initial coordinates is used which translates into RA TEL TARG OFFSETALPHA TEL TARG ALPHA DEC TEL TARG OFFSETDELTA TEL TARG DELTA Therefore if TEL ROT OFFANGLE TEL CHOP POSANG 0 TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA should be positive in order to reproduce the scheme shown in Fig A typical value for these parameters is TEL CHOP THROW 2 where TEL CHOP THROW is the chop throw used in the subsequent templates If both the target and guide star coordinates are within the same astrometric systems the point ing accuracy is limited by the relative accuracy between the coordinates of the two objects In particular the pointing accuracy maybe affected by significant usually unknown proper mo tion of the guide star Note that the observatory does not guarantee the accuracy of the world coordinate systems WCS keywords in the FITS headers Acquisition wi
44. 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 cali bration is charged to the observer Ideally early type stars should be chosen In particular for high resolution spectroscopy asteroids provide mostly featureless spectra on the VISIR spectral range For service mode observations all Calibrator Observations should be Concatenated to their sci ence OB In addition the DIT determination Algorithm should be set to AUTO or BRIGHT SOURCE for the first occurrence of each filter or wavelength setting and it should be set to PRE VIOUS in all subsequent observing template For both imaging 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 most instrument modes a corresponding flat field is recorded which consists of a series of images with different background levels Exceptions are all imaging obtained with the spectroscopy detector for spec troscopy acquisition 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 These day calibrations are supplied to the
45. ed v8l 31 08 07 3 1 4 2 1 4 3 2 P81 release new filters included v87 22 09 10 4 3 2 8 2 8 3 P87 release exclusion of K band in science imaging templates 3 2 5 P87 release update of HR allowed 2 First line of Table 7 modified 8 2 P87 release non availability of jitter with IMG GenericChopNod v88 22 02 11 4 1 P88 release upper limit of 5 filters in a single service mode OB 4 4 P88 release UCAC3 substituting USNO for guide stars selections v89 31 08 11 all P89 release removing most references to the old DRS detector and reporting the first properties of AQUARIUS detector v90 26 02 12 1 Update new schedule of VISIR upgrade v90 17 08 12 all New updates after the commissioning 1 v95 27 08 14 all P9S release updates after 07 2014 tests v1 0 v1 1 v76 1 edited by R Siebenmorgen E v76 2 4 v77 1 3 updated by A Smette v78 80 updated by L Vanzi v87 90 updated by Y Momany v95 and on updated by Y Momany V D Ivanov et al Pantin M Sterzik Contents 1 3 4 5 VISIR Upgrade Project 1 1 Detector Upgrade 1 2 Low Resolution Spectroscopy eee eee eee 1 3 Precipitable Water VADOR sos Dear Pe ER A a er e 2 Introduction Observing in the MIR from the ground SCH The Earth s atmospherej scans EPE RS AA ERO ESOS SE EE EE EE EC Buda o ee ee TA DE 3 3 MIR backoround parar A 3 4 Chopping and nodd ing is dr rr E
46. eld SF and 07076 intermediate field IF not offered in Period 95 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 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 Starting Period 81 and in addition to the ones listed in Table 2 another set of filters is offered Their characteristics are summarized in Table B Imaging Mode Pixel size Field of view Note Small Field 07045 4070 x 4070 offered in P95 Table 1 The offered pixel scales of the IMAGER detector and the corresponding usable field of view The pixel size of the AQUARIUS 1kx1k detector is 30 um entrance window diaphragm focal plane TMA optics filter Figure 7 The optical path of the imager is shown from the entrance window down to the detector Normally the burst read out is offered for the imager in visitor mode only but it is not offered in Period 95 The burst read out allows the user to save every single DIT frame of the exposure In this way it is possible to follow rapidly evolving events or to improve the spatial resolution by taking short enough exposures to freeze the atmospheric turbulence This mode can be used only for objects bright enough to provide a S N high enough in a single elemen
47. ensitivity mJy 100 in 1h E A E PE 0 i f fi li f 1 fi f i f fi 1 Ll 12 44 12 46 12 48 12 50 Wavelength um 10000 EST TT TOO 7 L model J L median 7 L 7 6 j 7 o gt L E D 2 1000 4 8 N L L K L ee is ENE a aL Ki L J A E A 1 La JV Ku N L i y OI th l pt el e e I E 12 70 12 75 12 80 12 85 12 90 wavelength um Figure 27 Measured sensitivity as a function of wavelength for high resolution mode measured on the old DRS detector but also valid for Period 95 III For the region on the bottom panel the observed sensitivities were obtained on various nights and are compared with a theoretical model curve corresponding to BLIP blue line 6000 5000 4000 3000 2000 an ES 1000 Sensitivity mJy 100 in 1h E 0 1280 12 90 1300 13 10 13 20 13 30 Wavelength um 2 0x104 MMT TNT TRETA 1 5x 104 1 0x 104 5 0x 103 Sensitivity mJy 100 in 1h 0 E A O PR E ES E O love FE RU E 16 375 16 380 16 385 16 390 16 395 16 400 16 405
48. eriod 94 the instrument is offered again in Period 95 for the first time after the upgrade Only service mode is available and a limited number of instrument modes small scale imaging with pixel size of 0 045 arcsec long slit low resolution spectroscopy and long slit and cross dispersion high resolution spectroscopy The pixel size for the spectroscopy is 0 076 arcsec Burst mode Sparse Aperture Masking SAM mode and medium resolution spectroscopy are not available The maximum chop throw is limited to 10 arcsec At the time of finalizing this manual the post upgrade VISIR capabilities are not yet fully character ized The instrument overheads remain the same and the users should assume the same performance as for the pre upgrade VISIR The astronomical community is encouraged to monitor the latest VISIR news reported on http www eso org sci facilities paranal instruments visir news html http www eso org sci facilities paranal instruments visir upgradeproject html 1 1 Detector Upgrade The major part of the upgrade project concerns the replacement of the current detector DRS 256 x 256 pixel array with the Raytheon AQUARIUS 1024 x 1024 pixel array This hardware upgrade is expected to improve the VISIR performance in terms of field coverage and sensitivity The new AQUARIUS detector will be offered in two pixel scales 07045 and 07076 SF and IF respectively The projected and usable field of view is 4070x4070 and 6270 x 6270
49. for spectroscopy VISIR spec acq MoveToSlit and VISIR spec acq ImgMoveToSlit The latter one allows to 2This convention is identical to the UVES one but differs from example from the ISAAC or NACO one perform spectroscopic acquisition with the imager detector in intermediate field only and therefore offers the possibility to acquire fainter objects in a larger variety of filters The spectroscopic ac quisition must always use the SF 0 045 px scale The observing parameters are described in Sect 9 The effect of all acquisition templates is first to point the telescope so that the coordinates at the center of the detector match e the target coordinates if no blind offset is used e the offset star coordinates otherwise within the accuracy of the VLT pointing see below For For VISIR spec acq MoveToSlit the first acquisition images are obtained with the OPEN 15 377 slit Then e The VISIR_img_acq_MoveToPixel and VISIR_spec_acq_MoveToSlit require interac tion with the instrument operator or night support astronomer in order to center the target at the appropriate location on the detector Without further indication given by the observer the default locations are for VISIR_img_acq_MoveToPixel and SEQ CHOPNOD DIR PARALLEL 3 North from the center of the detector to avoid the central outward readout of the detector for VISIR img acq MoveToPixel and SEQ CHOPNOD DIR PERPENDICULAR in the top left
50. g cycles is ignored The timing organization of data is shown in Fig The total on source integration time is tsource 4 Neyel_nod Neyel_chop NDIT DIT 3 The total rejected time is tskip 4 Noel nod DIT NDITSKIP Nec chop NDIT NCYSKIP 4 and the total observing time is tiot source skip 5 Typical duty cycles tsource ftot are about 70 wee H ocd chop nevskip Neel chop t Sek Bn Bn An e 7 gt g T_nod NDITSKIP NDIT NDITSKIP ape a pm Ac Bc gt Ac Bc T_chop Figure 13 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 5 Observing with VISIR at the VLT 5 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 see Fig 2I and following it is recommended to display the S N as a function of wavelength when using the spectrograph ETC This advice is particularly relevant for spectroscopic settings with wavelengths centered at 8 8um as they will be
51. h Documents VLT 14300 mid_ir_imager_spectrometer 14330 VISIR_ Upgrade SoW_for 20_RS_campaign_5504 pdf 2 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 and the Q band between 16 5 and 24 5 um In addition it offers a slit spectrometer with a range of spectral resolutions between 250 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 073 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 stru
52. h 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 e It is mandatory to check that a guide star in the range V 10 5 14 0 mag within a field of 7 5 arcmin radius around the science target is available This can be done using the guidecam tool see http www eso org instruments visir doc See Sect 5 4 for details Note that observations close to zenith during meridian crossing should be avoided because of fast tracking speeds that do not allow proper background cancellation after nodding A final recommendation concerning service mode observations is that no more than 5 filters are grouped together in a single Observing Blocks This is rather necessary for a proper calibration of each single filter Moreover it is also recommended that N and Q band filters are not grouped together as the Q band sensitivities can be quite lower from that in the N band Questions related to the VISIR Phase 1 and Phase 2 observing preparation should be directed to the User Support Departmen
53. h 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 Sect 6 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 process results in a near perfect cancellation of sky lines Atmosphere absorption correction The atmosphere does not uniformly absorb the MIR radiation Sect 3 1 At some wavelengths it is completely 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
54. l be observed every night VISIR is in use Note that this list could be modified without previous notice A PSF can be derived from these photometric standard star observations However 1t is not guar anteed that its S N 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 Sect 7 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 or M 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 respect to the science target Such a calibration mea surement will be performed at least once per night per instrument configuration More precisely the following settings of the VISIR spec cal LRAutoChopNod template Sect 7 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 observation Important note The observatory
55. m Figure 3 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 T T 3 T T Pa e 0 6 E L v On vos a AC A bg D OI CH be 204 KR o e E 4 k 4 E AS o Ed 0 2 F ei m z ES L 4 0 6 F do d Pa d Z e ge L I Pia e a E 04 r A Diet D op af P a4 F d on A ua 7 E H pS D kee pa Bat 7 7 ab mA GA Bel er o 0 0 2 Y E soak o a e e E A L l l AR A 1 A A A 1 1 J 0 5 1 15 2 FWHM VIS Figure 4 Measures of the VISIR image quality versus optical seeing obtained during 2005 The dashed lines indicates the prediction of Roddier s formula 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 A 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 The chopping technique cancels most of the background However the optical path is not exactly the same in both chopper positions The
56. 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 VISIR img obs BurstAutoChopNod The observations in burst mode are analogous to the observations with the template VISIR img obs AutoChopNod but single elementary frames are saved These can be single DIT frames or the average of a number NDIT of DIT frames The minimum total integration time is 2 minutes and the total integration time must be a multiple of this value NDIT can assume values from 1 to 10 7 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 oth erwise 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 M
57. n unit WCU is located on top of the VISIR vacuum enclosure The WCU is also called star simulator It simulates either a 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 9 shows the unit on top of the enclosure ama 3 Monochromator Figure 9 Schematic drawing of the warm calibration unit on top of the VISIR vessel Mode A or Range AM Spectral Or Reso Dispersion Sensitivity um um features der lution px um 100 1h Jy HR 7 800 8 100 0 024200 H2_S4 17B 32000 17573 3 HR 12 738 12 882 0 035710 Ne I 11A 17000 11908 0 9 HR 16 800 17 200 0 051560 H2 S1 8B 14000 8250 lt 10 HRX 8 970 9 140 0 056750 Ar 16A 27100 18757 4 HRX 9 360 9 690 0 058125 H2 S3 ISA 25000 18290 5 HRX 10 480 10 540 0 079000 SIV Coll 12B 24000 13407 4 HRX 11 540 11 570 0 080250 HD 0 0 R 10 12B 23400 13333 2 HRX 11 762 0 081500 CHV 12A 19100 13067 3 HRX 12 210 12 760 0 096600 H2_S2 11B 20000 11007 1 5 HRX 12 814 13 364 0 088750 Nell 11A 17500 11917 2 HRX 16 390 0 088775 Coll 9A 17300 12100 12 HRX 16 925 0 131000 Col 8B 14100 8067 8 HRX 17 790 17 980 0 117675 PII Fell 8A 13140 9000 4 HRX 18 246
58. nd hence cannot be illuminated completely Availing the two pixel scales of 07045 and 07076 SF and IF respectively IF is not offered in Period 95 the finally offered field of view e g the illuminated parts of the detector are 400x400 and 6270 x 6270 respectively The AQUARIUS array was developed at Raytheon Vision Systems at Santa Barbara USA The development was funded by ESO to upgrade the VISIR instrument at VLT for next generation VLTI instruments and for the future mid IR candidate instrument METIS at the E ELT For a detailed presentation of the AQUARIUS detector we refer the reader to Ives et al Proc SPIE 8453 38 in the following we will highlight on the major properties of the detector To properly operate in the mid IR window the AQUARIUS detector is designed to deliver low ther mal background high quantum efficiency and high sensitivity With respect to conventional ones these goals have been achieved by the introduction of a new class of photo conductors called the Impurity Band Conduction IBC Raytheon designation Their Si As IBC design achieved higher sensitivities by decreasing the thickness of the photo conductor and increasing the doping of the Si As diodes 4 4 1 Detector Architecture Figure 10 displays the architecture of the AQUARIUS detector It is split into two perpendicular areas each made of 512 rows and 1024 columns Each area has 32 outputs such that each output is configured to read out 32 x
59. ns in this band require low water vapor content in the atmosphere The atmospheric transmission in the N and Q bands is displayed on Fig 3 2 The spatial resolution The spatial resolution of an instrument is ultimately limited 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 MIR atmosphere transmission at Paranal 1 2 1 0 0 8 0 6 Transmission i 5 10 15 20 5 30 Wavelength um Figure 2 MIR atmospheric transmission at Paranal computed with HITRAN for an altitude of 2600m and 1 5mm of precipitable water vapor at zenith This is a coarse overview For plan ning of observations ESO provides a sky model tool athttps www eso org observing etc bin gen form INS MODE swspectr INS NAME SKYCALC The US standard model atmosphere is used of the telescope mirror see solid line in Fig 3 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 179 see dot dashed lines in Fig B 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 turb
60. of 90s 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 TEL 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 i7 In imaging more flexibility on the nodding offsets are possible with the VISIR img obs GenericChopNod template 5 3 Target acquisition 5 3 1 Introduction Observing blocks OB must start with an acquisition template Pointing to a target can only be performed through an acquisition template The target coordinates name and proper motion are all set in the acquisition templates The execution of the acquisition templates presets the telescope to the target coordinates given by TEL TARG ALPHA and TEL TARG DELTA Offsets with respect to the target coordinates can be spec ified by TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA and allow for example to use a bright reference star for precise acquisition see Fig 16 To guarantee proper centering within the slit when using a reference star the angular separation between the reference star and the target should not be larger than 60 Acquisition with a reference star has not been tested with the narrow 0 4
61. ore complex 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 Ob serving parameters are total integration time SEQ TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR Sect 7 2 High resolution long slit mode Template for high resolution spectroscopy is VISIR spc obs HRAutoChopNod Three order sort ing filter at 8 02 12 81 and 17 03um INS FILT2 NAME H2 S4 Ne 11 H2_S1 are avail able See Table 6 for the corresponding list of offered central wavelengths Other observing param eters are total integration time GEO TIME central wavelength INS GRAT1 WLEN the slit width INS SLIT1 WIDTH and SEQ CHOPNOD DIR Sect 7 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 See Table 6 for a list of offered wave lengths 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 Sect 7 2 7 4 Calibration Specific templates exist for the observations of photometric
62. pectro scopic acquisitions take longer and are strongly dependent on the source brightness an overhead of 15 min is accounted for sources gt 1 Jy while 30 min 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 50 of the observing time The total observing time requested by the observer must include telescope and instrument overheads The post upgrade overheads have not been fully characterized Users are encouraged to consult the VISIR web page for updates http www eso org sci facilities paranal instruments 5 7 Calibration observations MIR observations depend strongly on the ambient conditions such as humidity temperature or air mass In service mode science observations are interlaced 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 Calibrators unless provided by the observer are selected from the MIR spectro photometric stan dard star catalog of the VLT http www eso org sci facilities paranal instruments visir tools html This catalog is a sub set of the radiometric all sky network of absolutely cal ibrated stellar spectra by Cohen et alf At present the standard star catalog contains 425 sources Zero point fluxes Jy have been calcu lated for the VISIR filter set
63. r The use of the VISIR exposure time calculator ETC located at http www eso org observing etc is recommended to estimate the on source integration time 4 Instrument description and offered observing modes 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 15 K for the parts near the detector The detectors are cooled down to 9 K 4 1 Imager The imager is based on an all reflective design The optical design is shown in Fig 7 It consists of two parts e A collimator which provides an 18 mm diameter cold stop pupil in parallel light As generally designed for IR instruments the pupil of the telescope is imaged on a cold stop mask to avoid stray light 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 07045 small fi
64. r band passes The growing calibration database allows a statistical analysis of the sensitivity with respect to instru mental and atmospheric 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 compi lation is presented in Fig 6 for the N 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 obtained with the central one containing twice as much flux as the two other ones Table 5 to 6 list typical sensitivities measured in low medium and high resolution modes away from strong sky emission lines for the offered wavelength ranges Figures to in the Appendix Sect 11 shows the dependence of sensitivity on wavelength The median sensitivities are the reference for classification of VISIR service mode observations and the basis to assess the feasibility of an observing programme In particular classification of service mode OBs will be based on sensitivity measurements made at zenith Calibrations will be provided following the guidelines given in Sect 5 7 For up to date information please consultihttp www eso org instruments visi
65. refore 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 Staring images Staring images A chopper position A chopper position ERREI BET BE Chopped images te Nodding A beam Nodding B beam a dek do subixaction B chopper position B chopper position Sketch of mid infrared chopping and nodding ae registration technique of observation Chopped nodded image Final Image zoomed He2 10 blue compact galaxy Figure 5 Illustration of the chopping and nodding technique on observations of the blue compact galaxy He2 10 The galaxy only appears after chopping and nodding courtesy VISIR commission ing team June 2004 An illustration of the chopping and nodding technique is shown on Fig 5 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 3 5 Sensitivity Measurements of VISIR sensitivities are based on observations of mid infrared calibration standard stars Cohen et al 1999 AJ 117 1864 In imaging mode the stars are recorded in the small field 0 045
66. rs and these are the intermediate results in a given nodding position and lastly 111 a single intermediate result image for that given nodding position During each chopping cycle the elementary exposures are added in real time and stored into disk In particular at a chopping frequency of fcnop 0 25 Hz every Tenop 4 s two VISIR Half Cycle images are stored as frames in the fits extension file The number of chopping cycles within one nodding position is defined by the time spent integrating in that nodding position Taoa This nodding period is typically Tu 90 s for science observations The chopper frequency DIT and also To are predefined by the system The number of saved A B frames in one FITS file is Neycl_chop Thoa T chop 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 Typ ical 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 posi tion a number NCYSKIP of choppin
67. s need to be executed needs to be specified as a comment in the Target List of the proposal Phase 2 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 If the VISIR_img_acq_Preset is used and the following templates have SEQ CHOPNOD DIR PERPENDICULAR the target will appear at the center of the detector by default with the risk of losing 3 beams that would appear outside of the field Either slightly change the co ordinates of TEL TARG ALPHA and TEL TARG DELTA or use TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA See Sect Acquisition It is strongly recommended that a same guide star be selected and inserted in the acquisition template for all OBs of a same field in particular if e relatively good astrometric accuracy is required e the object is faint or diffuse and unlikely to be visible on short exposures e the object appears in the field of a bright nebula that saturates the digitized sky survey DSS used by the telescope and instrument operator The guidecam tool see http www eso org instruments visir doc can help in se lecting appropriate guide stars Calibrations For calibration OBs use the appropriate VISIR img cal AutoChopNod or VISIR_sp
68. s to only one nodding pair The total observing time is given by the prod uct of SEQ NOFF x SEQ TIME The offset positions are calculated as the cumulative sum of offsets 1 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 18 Illustration of generating raster maps with VISIR_img_obs_GenericChopNod An illustration of generating an raster map can be found in Fig The following parameters correspond to this setting SEQ NOFF 3 SEQ OFFSET1 LIST 30 10 10 SEQ OFFSET2 LIST 30 10 10 SEQ OFFSET COORDS SKY Note that depending on choice of the integration time SEQ TIME several nodding cycles might re sult e g pattern like ABIBIAAB1B1A AB2B2AAB2B2A AB3B3AAB3B3A Currently images obtained with the VISIR_img_obs_GenericChopNod are not reduced by the ESO VISIR pipeline Pre imaging observations The observatory supports a fast data release for VISIR pre imaging ob servations Pre imaging images
69. sitiv ity limit around 1 Jy in the continuum A high resolution cross dispersed mode with a 4 1 short slit is available for a number of wavelength settings Table 6 Please consult http www eso org instruments visir for the latest update of the list of of fered modes and slits 4 2 3 Low resolution offered central wavelengths Following the VISIR upgrade project and throughout Period 95 the N band 8 13um Low Resolu tion Spectroscopy will require only one exposure to cover the 8 13um range This is achieved by replacing the old gratings with a new R 300 for a 073 slit prism The sensitivities measured on the old DRS detector still apply for Period 95 proposal preparation These are 50 mJy at 100 1h in the clean regions of the spectrum Offered slits have widths of 0 4 0 75 and 1 4 2 4 Medium resolution offered central wavelengths Note for Period 95 this mode is not offered In Medium Resolution mode the central wavelength A can be freely chosen within the wavelength ranges listed in Table Note that the exposure time calculator ETC cannot currently provide estimates of S N for lt 7 6um Table 5 provides offered sensitivities 4 2 5 High resolution offered central wavelengths The VISIR spectrometer offers a high resolution long slit mode for 3 pass bands centered in the wavelengths of the H2_S4 Nell and H2_S1 lines A wider range of wavelengths is accessible with the high resolution
70. slit and should be avoided Note that the coordinates of the target TEL TARG ALPHA TEL TARG DELTA and the offsets to the reference star TEL TARG OFFSETALPHA TEL TARG OFFSETADELTA must be indicated in the acquisition template Thus the following conventior TEL TARG ALPHA TEL TARG OFFSETALPHA RA offsetstar TEL TARG DELTA TEL TARG OFFSETADELTA DEC offsetstar will be used and the telescope is preset to the reference star Once the reference star is properly centered TEL TARG OFFSETALPHA is subtracted back and the telescope is moved to the target WI Is E E Figure 16 Setting the correct values of the TEL TARG OFFSETALPHA and TEL TARG OFFSETDELTA for a blind offset Here the object A is a bright star used to center the target the faint object B at the center of the field The telescope will first point at the object A The instrument operator centers 1t properly Once done the telescope is offset so that object B is now properly centered and the observation templates can be executed Following the convention described in the text and since the target object B is at the east of the offset star TEL TARG OFFSETALPHA is negative on the other hand the target is at the south of the offset star so TEL TARG OFFSETDELTA is positive 5 3 2 Description There are two acquisition templates for imaging VISIR_img_acq_Preset and VISIR_img_acq_MoveToPixel Two acquisition templates are also available
71. t usd help eso org 5 2 Telescope observing parameters 5 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 Figure summarizes the situation 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 A and B at once The parameter TEL ROT OFFANGLE defaulted to 0 is used for this purpose If PA represents the required position angle of object B relative to A measured on the sky east of north e counted positively from north to east within the range 0 to 360 then setting TEL ROT OFFANGLE 360 PA allows one to have both A and B objects on the slit pix 0 076 pix 0 045 62 40 IMG IF IMG SF IMG SPC Figure 14 Field orientation and scale for the imaging and spectroscopic modes of VISIR 5 2 2 Chopping parameters Note for Period 95 the maximum permitted chopping throw is 10 arcsec The chopping technique as described in Sect 3 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
72. tary exposure Typical flux limits are of the order of 5 Jy in N and 10 Jy in Q Some compromise between sensitivity and time resolution can be reached by averaging a number of elementary frames Further and more up to date information can be found at http www eso org instruments visir inst Filter Ae half max sensitivity 100 1h Note um band trans mJy width mission theory median um BLIP SF PAHI 8 59 0 42 77 1 6 5 All 8 99 0 14 12 4 1 6 SIV_1 9 82 0 18 12 4 0 30 SIV 10 49 0 16 70 4 5 8 SIV_2 10 77 0 19 70 4 6 9 PAH2 11 25 0 59 75 2 3 6 PAH2_2 11 88 0 37 58 4 1 7 Nell 1 12 27 0 18 51 6 9 12 use for spectr acquisition Nell 12 81 0 21 64 6 1 12 Nell_2 13 04 0 22 68 6 3 15 use for spectr acquisition Ql 17 65 0 83 59 11 1 50 Q2 18 72 0 88 49 13 6 50 Q3 19 50 0 40 50 41 7 100 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 ex pectations under BLIP and excellent weather conditions and the measured median sensitivities for the Small 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
73. tector artifacts are less important in spectroscopy Before the upgrade the users were adviced to avoid a TEL CHOP THROW between 9 0 and 1370 to avoid artifacts on the old spectroscopic detector this limitation is no longer relevant Figure 12 Upper An example of the good cosmetics on the new Aquarius detector used for ac quisition of a Cohen standard in the 1 O slit Note the central division of the detector due to the intrinsic central outward readout and how the target was placed slightly above Lower A sequence of chop nod reduced spectra obtained in the Low Resolution mode covering the entire N band with a single exposure The TEL CHOP THROW was set to 107 4 5 Data acquisition system Note for Period 95 Detector windowing is forbidden As part of the upgrade project the VISIR new AQUARIUS detectors will be controlled by the new NGC acquisition system In imaging the read out rate of the detector is high Up to 200 frames per second are read for a minimum detector integration time of DIT 5 ms Such a frame rate is too high to store all exposures One VISIR image is of size 1024 x 1024 each pixel is coded with 4 bytes long integer Thus one read out has a size of 262 kB The current version of the NGC by default provides output files in the format of fits extensions These are structured in the following way 1 a general long header 11 a number of sub images Half Cycle which have their own short heade
74. ter SIV_ref2 c 1 2 AT gis ae ie c 1 2 7 7 O O Kb en ET E 5 0 8 E C o 2 7 opt O ES WI oO L D N 0 45 4 o o S E ozr 7 o o Z Sege 9 5 9 7 9 9 10 1 10 3 10 4 10 6 10 8 110 CL Wavelenght um Figure 19 Transmission curves of VISIR imager filters dashed is the atmospheric transmission at low resolution Wavelenght um manufactured by OCLI Over plotted Only relative transmissions have been determined their values are normalized so that their peak transmission is equal to 1 Transmissions in Transmissions in Transmissions in 120 100 120 100 120 100 Filter B 8 7 TTT 7 8 9 Wavelenght um 10 Filter J 7 9 Ima y Popo 6 7 8 9 Wavelenght um Filter J 9 8 8 9 10 Wavelenght um 11 Transmissions in Transmissions in Transmissions in 120 100 120 100 80 60 40 20 E 120 100 E 80 60 E 40 20 E Filter B 9 7 d j ie d 8 9 10 Wavelenght um 11 Filter J 8 9 Wavelenght um Filter J 12 2 11 Wavelenght um 12 13 Figure 20 Transmission curves of intermediate band VISIR imager filters Over plotted dashed is the atmospheric transmission at low resolution 11 Appendix Observed sensitivities in various spectroscopic set tings median best ever theoretical limit sensitivity mJy 100 1h
75. th the VISIR_img_acq_MoveToPixel or VISIR_spec_acq_ImgMoveToSlit tem plates can make use of the K Band filter for which a preliminary conservative limiting magnitude is 12 in 60s on source integration for a S N 10 However we strongly recommend to limit the acquisition filters to the N band filters as observations in the K band filter employ the detector at starvation levels Lastly we emphasize that the K band filter is not offered for science purposes For a successful completion of an OB the observer has 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
76. ulence outer scale As a result VISIR data are already diffraction limited for optical seeing below 0 6 The results of measures obtained in 2005 are shown in Fig 3 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 253 K Kirchhoff s law The telescope gives an additional MIR background The VLT telescopes emits at 283 K with a preliminary emissivity estimate of lt 15 in N The VISIR instrument is cooled to avoid internal background contamination The detectors are at 9K and the cold optical bench at 33 K The background radiation at 10um is typically my 5 mag arcsec 3700 Jy arcsec and at 20um mg 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 3 4 Chopping and nodding Note for Period 95 the maximum permitted chopping throw is 10 arcsec 10 0 er BEE a eee SPITZER diffraction O 7 5 L ee C DESE 2 r f i 40 a s VLT VISIR diffraction _ O E 1 visible seeing S a O e EE 3 A ca gt 0 5 visible seeing 7 7 lt 0 1 i 1 1 1 L f 1 1 1 L RK 1 1 1 L 1 1 1 1 L 1 1 1 1 3 10 Es 20 25 30 Wavelength u
77. user on an experimental basis and may be discontinued with no previous notice 5 8 OBs Classification The sky transparency constraints used at Paranal are photometric PHO clear CER thin THN and thick THK They mostly refer to the optical band and their translation to the IR domain and specially to the MIR is not obvious The following scheme is applied for VISIR OBs requiring PHO conditions will be executed and classified as A Fully within constraints if the sensitivity in the corresponding band is equal or better then the nominal median value and if the conversion factor is constant within 10 Refer to the web page http www eso org instruments visir inst to know the values of nominal sensitivities in each mode OBs requiring CLR THN and THK conditions will be executed and classified as A when the sensitivity is respectively within 20 30 and 50 the nominal values Classification for VISIR observations conducted in service mode is also based on the PWV con straint OBs executed with the requested PWV constraint will be classified A Fully within con straints those executed within 10 of the requested PWV value will be classified B Almost within constraints and OBs executed under PWV conditions greater than 10 of the requested PWV value will be classified C out of constraints Observations qualified as C will be repeated The time required to do so will not be charged to the observer s program 5 9 Known problems
78. value is estimated by examining the background level in exposures obtained with the lowest available DIT Consequently the DIT value is increased until the background level is conservatively within the linearity operating regime of the detec tor and the highest good DIT is selected 2 GOOD WEATHER in this case one assumes that weather conditions in terms of PWV and transparency are nominally good and selects a single tabulated DIT value typically of the order of 8 milli seconds that will ensure the operation of the detector at a reasonable 25 of the potential well 3 BRIGHT SOURCE this option is useful in case the science target is a significantly bright source and one is interested in selecting the lowest possible DIT value to ensure the non saturation of the detector The new AQUARIUS detector cosmetic testing shows that it does not show the high fraction of bad pixels seen in the old DRS detector Moreover the AQUARIUS detector can be considered free from stripping effects and ghosts Furthermore due to the new spectroscopic imaging detectors readout structure from the center outwards it is now recommended that the scientific standard stars targets are not placed in the very center of the detectors but slightly offset by few arcseconds either above or below the central division of the detectors For Period 95 it is advised to observe only sources fainter than 500 Jy in N and 2500 Jy in Q Due to the low flux levels eventual de

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