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FORS 1+2 User Manual

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1. 30000 T T T T d T T T T Oft Dot APHOH A o0 Qo N SE 858 EE 9 15 858 SELLE 38860080809 8 88 98 98 8 5 e 20000 4 L J 2 L J o L J n L 4 E 10000 ke 0 0 500 1000 1500 2000 pixels Figure D 13 Calibration spectrum taken with the SR collimator and grism GRIS_600z 23 FORS2 6000 T T T T T T T T T T T T T T T on oy e o o 5 5 ae E L Ss 88 5 x tT wo 2 T L Q L o s n 4000 D n 4 4 bal 9 2000 5 kel L 4 Ke T L 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 500 1000 1500 2000 pixels Figure D 14 2nd order calibration spectrum taken with the SR collimator and grism GRIS_600z 23 FORS2 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 30000 T T T T T T T T T T T T T T T T iD iD oo e Q m pt 40 x KKK D E Ki on aw oou uv NO KE oov Q 7 eno 0 0 o Q 2 e 20000 gt e r 4 D o 4 n Be 4
2. FILT_485_37 100 T 1 TT TT 1 ITT 1 ITT kel H SS 7 gor F ZS 60 0 A 1 E 40 sot gt o H LEA Lu 400 450 500 550 600 wavelength nm FILT 691 55 FILT_815_13 100 m 100 T 4 Ka 55 60 4 60 gt 4 a L 4 D gt J 40 40 4 2 n L 4 20 H 20 5 o H Arrild o H Li 600 650 700 750 800 700 750 800 850 900 wavelength nm wavelength nm FILT_834_48 z SPECIAL 100 T T 1 Lob dT 1 TT TT 1 100 TT TT 1 TT 1 ITT 1 80 KE ux 4 80 4 Ve 60 60r 0 F 1 F J 40 40 L 4 2 B 4 20 E 20 H S Li LL 700 750 800 850 900 800 850 900 950 1000 wavelength nm wavelength nm Figure B 3 FORS intermediate band filter transmission curves Appendix Efficiency Curves for the FORS Grisms C 1 FORSI and FORS2 Grisms This appendix contains the efficiency curves of all standard grisms available for FORS1 and FORS2 and the approximate wavelength range for a slit which is located in the field centre Tables of the measured efficiency values will be available on WEB pages Figure C 1 Efficie
3. 2 E Um E 8 x 2 8 8 E 3 E 5 8 3 20000 10000 0 E 0 500 1000 1500 2000 pixels Figure D 3 Calibration spectrum taken with the SR collimator and grism GRIS 1200g4 96 FORS 1 30000 T T T T I T T T I T T T T T T T T Q de AN 98 PRO P E Si Oy q e KEEN E 355 55 552 BON 253088 6 gs 20000 10000 Lal Ga 0 1 1 1 1 0 500 1000 1500 2000 pixels Figure D 4 Calibration spectrum taken with the SR collimator and grism GRIS_1028z 29 FORS2 57 arc lines GRIS_600B 12 arc lines GRIS_600B 22 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 30000 T T T T on e N oo x KE 0 Qe Qu ka e o N N M N Dei ei e ee e e N Dei NN 20000 10000 0 500 1000 1500 2000 pixels Figure D 5 Calibration spectrum taken with the SR collimator and grism GRIS_600B 12 FORS1 30000 T T T T T T T T T T T T T T T T
4. 10000 0 500 1000 1500 2000 pixels Figure D 15 Calibration spectrum taken with the SR collimator and grism GRIS 300V 10 FORS1 30000 T T T T T Lh T T T E T T T T b MO ola oui Or 2 o LNA oO 0 H O aot och S NNN 0 Be Q or FAM o9 P Q tt oo OG Ala odo Q Cx se X0 wou ROG E oo Q L 4 80000 CH _ ue gt L 4 O L 4 L 20000 a 9 0 EN L bs LUUD VICA ES 0 500 1000 1500 2000 pixels Figure D 16 Calibration spectrum taken with the SR collimator and grism GRIS 300V 20 FORS2 64 arc lines GRIS_300I 11 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 30000 T T T T uu 8 45 588 28 A TE B
5. 30000 1 1 1 1 1 1 1 e e e e Q ST I009 6149 5 2000 1500 1000 500 pixels Figure D 10 Calibration spectrum taken with the SR collimator and grism 6001 15 FORS1 61 VLT MAN ESO 13100 1543 FORS1 2 User Manual Issue 2 8 OS voce 268816 2876588 v62998 85 7998 997698 960 1698 vvloc8 9 S6v8 99 278 12 8078 262468 EE O0 8 2597928 IgSsIIg 69 018 6479108 8T 8764 LAZA 085662 299164 79 5094 4 98872 068674 662 22 4VeveL VEOELIL r 70 2 714 0602 576969 276269 704159 1 30000 20000 10000 62 1009 1459 2000 1500 1000 500 pixels Figure D 11 Calibration spectrum taken with the SR collimator and grism 5 6001 25 FORS2 T LOS ET LY DG APCE 182200 95 9707 02 920 OL V96E 048888 JL 6000 4000 2000 F C2 1009 SIND pug 2000 1500 1000 500 pixels Figure D 12 2nd order calibration spectrum taken with the SR collimator and grism GRIS 6001 25 FORS2 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543
6. E XE X1 I1 28 or Qoo wu 1 BO 4 565 8 5 828 T e e 00 20000 10000 0 i ENSE li J i 0 500 1000 1500 2000 pixels Figure D 6 Calibration spectrum taken with the SR collimator and grism GRIS 600B 22 FORS2 59 VLT MAN ESO 13100 1543 FORS1 2 User Manual Issue 2 8 ARCA v G969 4v 6269 v041429 968699 8872659 6579059 DG BCE Se c0v9 66 C869 tv vtt9 62 vO 9 L 09 9929 8272129 696919 r90 vI9 91 9609 PE v209 866209 6992464 CR 65 29 65288 672989 690649 656945 08 5809 0425106 26126 066649 30000 20000 10000 v6 A009 51 9 2000 1500 1000 500 pixels Figure D 7 Calibration spectrum taken with the SR collimator and grism GRIS 600V 94 FORS1 6 2424 Fal oped 7074912 05 5904 6602 279969 436269 024149 568659 8872659 659099 Se 20v9 66 2859 V 7659 D FOCH 0 9959 822129 686919 906 19 9179609 PE V409 866209 6552 65 ER PPOO 2952 85 672988 690645 656945 92 0978 30000 20000 10000 009 SIND Sail 2000
7. 68 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 HgCd ECH 110B ESO ech hgcd fits Objectname 21 20 00 000 90 00 00 00 LINEAR astro Rtd Jun 09 2000 at 07 40 23 Figure D 23 Calibration spectrum of the Echelle grism configuration EGRIS 110B XGRIS 600B taken with the SR collimator and HgCd lamps Line identification list see Table D 3 3610 5000 3650 1440 3654 8401 3663 2739 4046 5569 4077 8313 4358 3428 4678 1602 4799 9199 5085 8242 5460 7422 5769 5981 5790 6558 a b f g h i j k 1 Table D 3 Line identification list for Echelle grism configuration EGRIS 110B XGRIS 600B He lines see Figure D 22 Appendix FORS Image Orientation E 1 MOS Orientation The orientation of the FORS image in MOS mode is given below for rotator position 0 deg Note that the sky directions in this schematics change for different rotator angles while the orientation on the CCD remains unchanged The orientation of the images on the CCD is given in parenthesis Also given are the locations of the CCD readout ports MOS Orientation and CCD Read out Ports Rotator angle 8 position angle on sky 0 TOP MOS 1 north pixel 2000 2000 D MOS A __ __ MOS B east west LEFT CRIGHT pixel 1 1 MOS 19 south BOTTOM 69 70 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 131
8. 252 ERO n Ree ers 9 2 3 6 Image Motion due to 9 2 4 Spectroscopy cuum es mud dehet S ha bets umi Mis 10 2 4 1 Grisms and Order Sorting 22 10 2 4 2 Relative Astrometric Accuracy Requirements for 10 24 3 Instrument Flexures sg E eves OE hele GPE Eod 11 24 4 Longslit Spectroscopy LSS mode 11 24 5 Multi Object Spectroscopy with Movable Slitlets MOS 13 2 4 6 Wide Slit Spectro Photometry SPECPHOT mode 13 2 4 Multi Object Spectroscopy with masks on FORS2 MXU mode 13 2 4 8 Echelle Spectroscopy with 2 14 24 9 Slitless Spectroscopy gt o e s ss ok Re m m onm 14 2 5 Polarimetry with FORSI ous RM EUR RH 16 2 5 1 Imaging 1 16 2 5 2 Spectropolarimetry PMOS mode 17 2 5 3 Performance of the Polarimetric Modes of FORSI 17 2 6 High Time Resolution 18 256 Al HOVERVIEW hoes Ree An e ek ea tech ux AUS eu deb gerd ch 18 2 6 2 High Time Resolution M
9. Issue 2 8 GRIS_600B 300 350 400 450 500 550 600 650 wavelength nm GRIS_600R 14 500 550 600 650 700 750 800 wavelength response X response VLT MAN ESO 13100 1543 100 80 60 40 20 100 80 60 40 20 5 600 400 450 500 550 600 650 700 750 wavelength nm GRIS 600I 650 700 750 800 850 900 950 wavelength nm 51 Figure C 2 Efficiency curves of the medium resolution grisms The vertical lines mark the approximate limits of the spectral range with the slit in the center of the field dotted lines FORSI solid lines FORS2 52 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 GRIS 1400V 100 T 7 Mom L P d 2 60 9 7 NJ X 40 7 Uu kf L t 20 ol peg 450 500 550 600 wavelength nm GRIS 1200R GRIS 1028z 100 T 100 A T m KS 1 gee eX UAR 80 5 80 Ss LUNAM M NP 60 60 Lk Ka Uu n m n al amp 40 gt 8 401 4 ul Uu L 4 L 4 D 20 r 20 o 550 600 650 700 750 750 800 850 900 950 wavelength nm wavelength nm GRIS_600RI 19 GRIS_600z 23 ee 100 em been
10. Responsive Quantum Efficiency Standard Resolution Signal to Noise To Be Confirmed To Be Defined Telescope Control System 43 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 UV Ultraviolet VIMOS Visible Multi Object Spectrograph VLT Very Large Telescope WCS World Coordinate System Angstrom Electron Centimeter h Hour kpx KiloPixel min Minute mm Millimeter nm Nanometer px Pixel Second um Micrometer Appendix FORS Filter Characteristics B 1 Broadband Filters Table 1 lists all presently see issue date of this document available FORS1 and FORS2 broadband filters The transmission curves are given thereafter Note the transmission curve of filter USPECIAL 73 is not yet available Tables of the measured transmission values will be available via the ESO web pages http www eso org instruments fors filters html Filter FORS1 FORS2 FORS 1 2 FORS 1 2 FORS1 FORS2 FORS 1 2 FORS 1 2 FORS 1 2 FORS 1 2 FORS2 U_BESS 33 1 U_SPECIAL 73 B_BESS 34 74 2 V_BESS 35 75 R_BESS 36 R_SPECIAL 76 1 55 37 77 u_GUNN 38 v_GUNN 39 g GUNN440 3 r GUNN441 z GUNN 42 78 GG375 30 80 4 GG435 31 81 4 0G590 72 32 4 FILT_465_250 82 4 edge filter n a edge filter n a edge filter n a 465 Table B 1 Characteristics of the FORS1 2 broadband filters is the central wavelength in nm 1 red leak 7 x 10 2 red leak lt 4 x 1074 3 this fi
11. Southern 22 Organ i des Recherches Astro onomique s dans l H misph re Austral Europ i ics afir astronomische Forschung in der s dlichen Hemisph re Paranal Observatory Very Large Telescope Focal Reducer Low Dispersion Spectrograph FORS1 2 User Manual VLT MAN ESO 13100 1543 Issue 2 8 Date June 30 2004 VLT Paranal Observatory Telephone 56 55 435000 Fax 56 55 435001 ii FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Change Record Issue Rev sections affected Reason Remarks July 9 1998 all Draft delivered by VIC Feb 11 1999 first release March 25 1999 all LADC setting recommended August 5 1999 all document re arranged page and section numbering changed Sept 1 1999 all more information on FORS2 Feb 1 2000 all revision for SM P65 amp proposals P66 July 10 2000 all revision for SM P66 amp proposals P67 Sept 17 2000 all revision after MXU commissioning and split up of manuals FORSI 2 User s FORS1 2 FIMS FORS1 2 Templates Dec 27 2000 all revision for SM p67 and proposals P68 June 27 2001 all all sections restructured SM P68 amp CFP P69 Jan 5 2002 all MIT CCD mosaic detectors May 22 2002 all updates for SM P70 Dec 24 2002 2 3 chapters 2 and 3 re sorted small changes elsewhere July 12 2003 2 V BESS offset ECU pre image policy and small changes in the other chapters Jan 5 2004 2 appendix HIT mode POL figure FORS2 vigne
12. 1500 1000 500 pixels Figure D 8 Calibration spectrum taken with the SR collimator and grism GRIS_600R 14 FORS 1 VLT MAN ESO 13100 1543 FORS1 2 User Manual Issue 2 8 60 59 2 8 1e 8078 262468 EE 0068 29 928 TESTIB L 69 5018 8T 8v64 08 S 9z 29 194 v9 094 288872 06 8 v4 986 8 4 071854 SKI v6 412 YO ZvV14 r9esgez TY 2602 575969 46269 072129 Cap gece 88 6699 6979069 098 9 6272079 662869 577559 627059 059929 8272129 696919 9179609 ve v 09 86 6209 29 9268 8 vv6S 295 86 6 2586 6970646 L 696946 rs4e9vs 30000 20000 61 14 009 SIND seul 10000 2000 1500 1000 pixels Figure DO Calibration spectrum taken with the SR collimator and grism GRIS_600RI 19 FORS2 2876588 6 2 998 8 7598 9 7598 921658 9656 8 So vere 128078 2672268 ee ooeg 297928 9118 69 5018 627108 91 9008 81 8v64 r F SE9 085654 29 16 r PR 5054 amp 06 8 v4 86 8 4 22 62222 217624 v6 414 PO VIA oz s902 6602 79969 196269
13. 4 6 Calibrating Polarimetric Measurements lee 38 4 6 1 Circular polarimetry 2222 2 5 2 22 22 2 22224 2 2 42458 4 gt 38 4 6 2 LinearPolanmetry uem BUR eA Ree qs Re d E RS 39 AT Pipeline Reduction 4 sik Rom Rec Re NIE REGE RUM RU RE 40 47 1 Supported modes is sa piu d BR de 9 de ee Ro ee Red me 40 4 7 2 Quality Control Pipeline Service Mode 40 4 7 3 Paranal Science Operation Pipeline IMG LSS mode 41 Abbreviations and Acronyms 43 FORS Filter Characteristics 45 B Filters Re e EUR E Pas Ia Rupe RM E eue 45 B 2 Interference Filters 4 07 eame el eeu ree ade RH pope exe s 48 Efficiency Curves for the FORS Grisms 50 C1 FORSI and FORS2 50 Wavelength Calibration Spectra for the FORS Standard Grisms 54 FORS Image Orientation 69 Bl MOS Orientation AN Be aes ted Bob Eve xu eer Rue bw 69 E2 LSS Orientation sx aes eae eb ae ames 70 World Coordinate System Information 71 Field vignetting with the FORS2 CCD 73 List of Figures 2 1 Schematic view of the FORS instruments lA 4 2 2 Light paths for the standard and high resolution collimators of FORSI andFORS2 4 2 3 Spectral format of the FORS2 blueEchellegrism 15 2 4 Strip Mask for
14. Pu 3 Ba Kee jn d i a bo L d i J E E o 60b 4 60 Ba 7 P 7 amp 40 S 8 40 n p n 4 L D 20 20 EEN 500 550 600 650 700 750 800 850 700 750 800 850 900 950 100010501100 wavelength nm wavelength nm Figure C 3 Efficiency curves of the medium resolution volume phased holographic grisms The vertical lines mark the approximate limits of the spectral range with the slit in the center of the field dotted lines FORSI solid lines FORS2 FORS1 2 User Manual Issue 2 8 VL I MAN ESO 13100 1543 53 EGRIS 110 98 XGRIS_600B 92 1 EORR Rep He o S 8 E SL S P Zo 10 0 RS a ol 1 1 1 ET gl 1 400 600 800 1000 400 500 600 700 800 wavelength nm wavelength nm Figure 4 Efficiency curves of the orders 4 12 of Echelle grism EGRIS_110B 98 and the cross disperser grism XGRIS 600B The curves of the Echelle orders have to be folded with the efficiency of the cross disperser Appendix D Wavelength Calibration Spectra for the FORS Standard Grisms This Appendix gives a wavelength table for the calibration lamps used FORS1 and FORS2 together with the arc line spectra taken with the FORS grisms and the SR collimator The measurements were done with MOS slits located in the center of the field of view and a slit width of 1 0
15. between 0 5 and 5 arcseconds The absolute photometric accuracy will be poor since it is not possible to do a differential photometric measurements with a 2nd star on a slit Equivalent widths of lines and for a wide slit also the colors should be less compromised by the image motion Same as for the imaging mode The slits are on the extreme left side of the field of view offset by about 3 arcminutes The slits are little squares The grism XGRIS 300I can be used with order separation filter OG590 or without In the later case there would be some 2nd order overlap typically at the red end of the 1st order where the CCD response would be reduced The 2nd order overlap would start at gt 6600 but would become important at wavelengths above about gt 8000 depending on the color of the target The following slit masks will be available mask name slit width _0_5 900015 0 5 HITS_0_7 900016 0 7 HITS_1_0 900017 1 0 _1 3 900018 1 3 HITS 1 6 900019 1 6 HITS_2_0 900020 2 0 HITS_5_0 900021 5 0 The respective cross disperser grisms are either identical copies to the standard FORS2 grisms 600 3001 or con verted former standard grisms 600R The wavelength range of the cross disperser grims are however slightly different from the standard grisms This is primarily caused by the asymmetric mount of the FORS2 MIT CCD mosaic which is off centered by 33 FORS2 cross disperser grisms for the HITS mode Gris
16. 13100 1543 File View Graphics Data Servers FORS2 fsmosaic 1007 1886 0 Y 2316 0 301 00 24 07 536 72 08 51 43 2000 E image select object scroll image measure WCS Control select region Figure G 2 In case of the high resolution collimator the corners of the field of view are vignetted by the camera lenses Index calibration plan 31 overview table 32 unsupported modes 31 calibration units 22 new calibration units 22 night time calibrations 22 parasitic light 22 calibrations 31 CCD 19 contamination 21 conversion faction 19 dark current 20 defects 20 exposure shutter 21 fringes 20 linearity 20 readout modes 19 readout modes standard modes 19 readout noise 19 readout time 20 22 data reduction 31 pre imaging data 33 ECH mode 14 exposure shutter 21 filters 43 broad band filters 7 43 combinations 5 exchangeable components 5 interference filters 7 46 fims manual 1 FORS instrument components 3 instrument overview 3 observing modes 3 WEBpage 1 FORS upgrades 23 new calibration units 22 replaced components 23 grisms 12 48 high time resolution modes 18 HIT mode 18 imaging 7 broad band filters 7 43 filters 43 75 flat fields 33 instrument flexures 8 interference filters 7 46 occulting masks 8 order separation filters 7 scale and field distortion 32 user provided filters 8 world co
17. 59 D 9 Calibration spectrum SR 5 600 1 60 D 10 Calibration spectrum SR 5 6001 60 D 11 Calibration spectrum SR 5 6001 61 D 12 2nd order Calibration spectrum SR 1 6001 61 D 13 Calibration spectrum SR 5 6002 62 D 14 Calibration spectrum SR 5 6002 62 D 15 Calibration spectrum SR GRIS_300V 63 D 16 Calibration spectrum SR GRIS_300V 63 D 17 Calibration spectrum SR 5 3001 64 D 18 Calibration spectrum SR 5 3001 64 D 19 Calibration spectrum SR 5 2001 65 D 20 Calibration spectrum SR 5 1501 65 D 21 Calibration spectrum SR 5 1501 66 D 22 Calibration spectrum EGRIS_110B XGRIS_600B using He 67 D 23 Calibration spectrum EGRIS_110B XGRIS_600B using HgCd lamps 68 G 1 Vignetting of FORS2 CCD by MOS standard resolution mode 73 G 2 Vignetting of the FORS2 CCD in high resolution 74 List of Tables 2 1 2 2 2 3 2 4 2 5 2 6 2 8 2
18. 9 2 10 2 11 2 12 2 13 3 1 4 1 4 2 4 3 4 4 4 5 4 6 1 2 DI D 2 D 3 FORSI standard configuration of opto mechanical components 5 FORS2 standard configuration of opto mechanical components 6 Optical properties OF FORS be up Ru us eese Rm 7 Exchangeable filter set for both FORS instruments 2 8 Characteristics ofthe FORS grisms 12 The FORS longslits eee ees Ga quee edes raa Se 13 The FORS2 Echelle grisms e ek er jl ses Ede oso es d 14 Retarder plate angles for circular and linear 17 Detector set ups 5 22 Detector readout noise and conversion factors 24 Basic characteristics of 24 Approximate CCD 25 Retired instrument 26 Operational overheads with FORS on the VLT The through slit exposure is typically executed twice It is important to include the overhead times while preparing proposals and service mode observations Packages eoe Ee ea v BEER EE alle ds 30 FORS Calibration Plan TasKs e 34 Large sc
19. SITE CCD FORS2 RON 7 e ADUI RON e ADU 6 2540 x 128 001 5 2 0 1 1 85 0 03 6 90 0 09 1 60 0 01 5 5 0 1 2 00 0 02 6 70 0 09 144 0 02 5 3 0 1 1 90 0 02 6 96 0 16 1 41 0 03 5 50 1 83 0 05 7 21 0 08 3 51 0 04 6 0 2 62 0 03 8 07 0 06 4 36 0 03 9 0 2 81 0 05 7 69 0 06 3 89 0 03 8 0 2 68 0 04 7 78 0 16 4 09 0 06 7 0 2 61 0 03 master low 200kHz slave low 200kHz master high 100kHz slave high 100kHz Table 2 10 Detector readout noise and conversion factors Port A of FORSI is the lower left B the lower right C the upper left and D the upper right quadrant FORSI J FORS2 SITE FORS2 MIT photosensitive pixels HxV 2048x2048 2048x2048 2 4096 2048 pixel size um 24 24 15 dark current at 120C e px h 8 15 25 3 linearity up to full well RMS lt 0 5 cosmic ray rate events min cm 4 0 2 4 0 3 Table 2 11 Basic characteristics of the FORS CCDs H horizontal V vertical lines in spectroscopic modes fringe amplitudes were found to be of the order of 5 in the worst cases Jitter and nodding on the slit It will be mandatory to use offset techniques jitter images nodding on the slit to subtract the sky background at wavelengths greater 800nm for spectroscopy and due to the detector cosmetics at any wavelength in imaging mode The fringes are quite stable but depend on the spectrum of the night sky which will be variable To subtract a scal
20. We are very greatfull for the input from the members of the FORS instrument operation team from the team of the Paranal observatory and last but not least for the feedback from the users 2 Instrument Characteristics 2 Overview Instrument Concept FORS is the visual and near UV FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope VLT of the European Southern Observatory ESO It is designed as an all dioptric instrument for the wavelength range from 330 nm to 1100 nm and provides an image scale of 0 2 pixel or 0 1 pixel with the 2048x2046 pixels CCD detectors pixel size of 24x24 um of FORSI Since April 2002 FORS2 is equipped with a mosaic of two 2kx4k MIT CCDs pixel size of 15x15 um with a pixel scale of 07125 pixel 070625 pixel Two versions of FORS have been built and installed on the Cassegrain foci and have been moved to different telescopes in the last years The main instrument components shown in Figure 2 1 are The Top Section with the focal plane equipment including the multi object spectroscopy MOS unit with 19 movable slits longslits the polarimetry mask FORS1 only the MXU mask exchange unit FORS2 only and the two calibration units The Collimator Section with the two collimators and the electronic cabinets The Filter Camera Section with the retarder plate mosaics FORS1 only the wheel for the Wollaston prism and optional optical analyzers filters and or grisms the gri
21. are given in A Users Guide for the Flexible Image Transport System FITS version 3 1 NASA Definition of the Flexible Image Transport System FITS NOST 100 1 2 and the Data Interface Control Document GEN SPE ESO 19400 0794 71 72 FORS1 2 User Manual Issue 2 8 CTYPE1 RA TAN tangential projection type CRVAL1 12 345678 x coord of reference pixel CRPIX1 512 0 x coord of reference pixel CTYPE2 DEC TAN tangential projection type CRVAL2 12 34567 y coord of reference pixel CRPIX2 525 5 y coord of reference pixel CD1 1 3 185E 5 partial derivative CD1 2 5 616 5 partial derivative CD2 1 5 616E 5 partial derivative CD2 2 3 185E 5 partial derivative EQUINOX 2000 0 equinox VLT MAN ESO 13100 1543 RA in deg PIXEL DEC in deg Pixel Appendix Field vignetting with FORS2 fsmosaic 1007 X 10460 Equinox 000 Min nj 51079 Bitpix 16 Low 5000 High 10000 Auto Set Cut Levels Scale fios 212 Sj 2t k BOTTOM 19 img E image select object scroll image measure WCS Control select region Figure G 1 The field of view of FORS2 with MIT CCDs is restricted by the MOS unit in the focal plane of the unit telescope to about 6 8 arc minutes for the standard resolution collimator 73 74 FORS1 2 User Manual Issue 2 8 VLT MAN ESO
22. element 55 56 FORS1 2 User Manual Issue 2 8 30000 T T T T T VLT MAN ESO 13100 1543 4713 20 4921 93 5015 70 5047 70 5085 80 20000 10000 arc lines GRIS_1400V 18 1000 pixels 5341 10 5400 56 1500 5769 59 5790 69 5852 49 2000 Figure D 1 Calibration spectrum taken with the SR collimator and grism GRIS_1400V 18 FORS2 30000 T T T I T T T T d T T T I T T T d o e OD ooo o Ki 010 Oe GIP SOM QW os oh ON nO TO lt T Oz E NO oo bh i NN H 2 od O OM th te OH oo NN re NN OG eee e L 20000 L M 4 12 L 4 n L 4 10000 24 0 p er A BEE eee d 0 500 1000 1500 2000 pixels Figure D 2 Calibration spectrum taken with the SR collimator and grism GRIS 1200R 93 FORS2 51 arc lines GRIS_1028z 29 arc lines GRIS 12002 96 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 30000 T T T T T T T T e e om 2
23. known to perform bad in terms of it s instrument response We are planning to order a volume phased holographic grism to be used on FORSI which should provide at least a factor of 3 more response then the Echelle mode of FORS2 or the respective 2nd order observations which also show relatively low performance The respective wavelength range would be 3730 to 4970 with the central wavelength at 4340 a dispersion of 0 61 J pxl and a spectral resolution of 1430 for a slit width of 1 arc second 2 11 Retired Instrument Components Table 2 13 lists the instrument components which are no longer offered for FORS and the time period during which they were used Removed Component Used in Availability Reason for Replacement Removal H_Alpha 59 FORS1 2 1 4 99 30 9 00 ghosts H_Alpha 83 GRIS_600z 16 FORS 1 1 4 00 3 1 3 01 low response GRIS_600z 26 GRIS_600R 24 FORS2 1 4 00 3 1 3 02 low response GRIS_600RI 19 XGRIS_600V 90 FORS2 no red Echelle none XGRIS_300I 91 FORS2 no red Echelle none 2kx2k Site CCD FORS2 1 4 00 31 3 02 red optimization MIT mosaic GRIS 600z4 26 FORS1 2 1 4 00 31 9 02 low response none Table 2 13 Retired instrument components 3 Observing with FORS observations with FORS are done via observing blocks OBs OBs contain of the target information and a small number of users selected observing templates depending on the observing mode The users will fill out the parameter fields ke
24. obtain master calibration data These are optimized for e g low noise level and they are quality checked Hence they give the optimum calibration data to the best present knowledge Master calibration data are of the following types e master BIAS bias level read out noise e master SCREEN FLAT IMG high spatial frequency flat 51 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 41 master SKY FLAT IMG high and low spatial frequency flat taken in twilight master NIGHT FLAT IMG as previous obtained from night science exposures photometric zero points from standard star observations bad pixel tables master SCREEN FLAT LSS high spatial frequency flat slit function WAVE DISPERSION LSS wavelength calibration Usually master SKY FLAT IMG are used for flattening IMG data This removes all multiplicative artifacts in the image different gain values in the four CCD ports pixel to pixel gain variations instrument and CCD efficiency Since the illumination during dusk dawn is however different from night conditions a large scale gradient of a few percent may remain which can be easily removed by e g fitting a polynomial A better large scale illumination correction can be obtained from night flats which are pipeline processed from jittered science images Since these usually have a lower signal to noise ratio than lamp flats it is preferable to use them for large scale correction only Photomet
25. or other LSS longslits shift the wavelength range on the CCD accordingly 1 The start wavelength of the 2nd order overlap is given in parenthesis 2 This order separation filter OG550 is cemented to the grism itself 3 Low performance is expected since the grisms are not optimized for 2nd order observations 4 This grism produces a Y offset on the CCD see section 2 4 1 for details 5 Higher throughput volume phased holographic grisms are available on FORS2 6 The selection of filters GG375 or GG435 grism 600V is only important for offset P MOS slits FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 13 Longslits of FORS1 2 slit offsets FORSI in 24um pixels FORS2 in 151m pixels sky CCD SR mode HR mode CCD SR mode CCD HR mode Table 2 6 Slit widths of the FORS1 2 longslits and approximate offsets relative to the central slit in pixels on CCD The exact values are slightly different for both instruments and depend also on the reproducibility of the CCD position after maintenance 2 4 5 Multi Object Spectroscopy with Movable Slitlets MOS Mode MOS Concept in the MOS mode up to 19 objects can be observed simultaneously by means of slitlets which are formed each by two blades mounted on opposite carriers The slitlets can be moved by linear guides to any position along the dispersion direction in the field of view The slit width of the single MOS slits can be adjusted to any user defined value B
26. sorting filter 465 250 special U filter Table 2 2 FORS2 standard configuration of opto mechanical components FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 7 2 3 Direct Imaging IMG and OCC modes 2 3 1 Basic Characteristics of the Imaging Optics Field of View Pixel Resolution Transmission Image Quality FORS reduces the VLT Cassegrain image scale of 528 um arcsec to 0 2 pixel with the standard resolution collimator and 0 1 pixel with the high resolution collimator and the 24 um pixels of the 2048x2046 Tektronix CCD detector of FORSI In case of FORS2 to 0 125 pixel and 0 0625 for the 15 um pixels of the CCD mosaic Please take the accurate scales and informations about the image field distortion from section 4 2 Sky concentration effects will be small and negligible for flat field and photometric calibrations 1 Standard Resolution High Resolution Image Quality 80 in 072 80 in 01 within 4 0 within 20 collimator focal length 1233 mm 616 mm camera focal length 280 mm 280 mm final f ratio 3 13 6 25 FORSI Pixel Scale 0 2 pixel 0 1 pixel Field of View 6 8 6 8 FORS2 Pixel Scale 0 126 pixel 0 0632 pixel Field of View 6 8 6 8 42 42 Table 2 3 Optical properties of FORS Field vignetting and detetor geometry with the larger FORS2 CCD The field of view of FORS2 with MIT CCDs is restricted by the MOS unit in the focal plane of the unit telescope to about 6
27. spectral calibration lamps in appendix D The red internal flat field lamps can t be used anymore after the installation of the external calibration units The control electronics of the respective lamps is now used by the external units External Calibration Units the flatfield lamps in the old internal calibration units have produced parasitic light in MOS and LSS flatfield exposures see section 2 9 1 Therefore new external calibration units ECUs have been installed which are located above the LADC in the Cassegrain tower The new calibration units consist of two blue and two red lamp which are linked to the Cassegrain tower with a fiber bundle One of each red and blue lamps will be projected into the fiber bundle in focus high illumination level while the other lamps are out of focus of the projection optics Only one of the two red and one of the blue lamps can be used at a given time The ECUs are the only calibration units used for spectroscopic flats fields since March 2003 for 1 and April 2003 for FORS2 respectively Actually we use the faint red lamp together with the bright blue lamp such that there is a secondary peak in the flat field spectrum which may appear odd on the first view Since the data reduction requires to normalize the flat fields anyway there should be no negative consequences from the little bump but far more light in the blue Nighttime Calibrations For technical reasons the arcs and flats are only taken
28. time to optimize the strategy and to estimate if all your OBs can be done in the limited number of nights or service mode hours 3 7 Visitor Mode 3 7 4 final package The final package needed at the telescope will typically consist of e finding charts e observing blocks e the fims output files and the pre imaging data on which the fims preparation was done fims modes In most cases the meteorological conditions will be fine but there are also bad nights with bad seeing or clouds and sometimes strong wind which will come typically from the North 3 7 2 Atthe telescope The telescope and instrument operation is done by the staff personal A good finding chart and a close collaboration between staff and visiting astronomer is the fastest way to the slit The incoming data will be displayed on real time displays which will allow only very basic assessment of the data and automatically transferred to an offline workstation with data reduction software packages iraf Midas and idl The basic observing modes will be pipeline reduced but sky subtraction and target extraction has to be done interactively The working environment is described on the Science Operation WEB page http www eso org paranal sciops At the end of the night an automatic procedure calobBuilt will be started which will create a complete calibration OB for all modes and setups used during the night The calibration OB will be executed during the morning hours 3
29. 0 59 1650 Grism central Arange dispersion A Aa Iter A A A mm A pixel at Acentral FORS2 standard GRIS_600B 22 3300 6210 50 0 75 GRIS_6001 25 5 6630 9390 44 0 66 0G590 32 GRIS_300V 20 1 3300 6600 112 1 68 GRIS_300V 20 1 3850 7500 112 1 68 GG375 80 GRIS 300V 20 4450 8700 112 1 68 GG435 81 GRIS_3001 21 6000 11000 108 1 62 0G590 32 GRIS_2001 28 2 5600 11000 162 2 43 GRIS_1501 27 1 3300 6600 230 3 45 GRIS_1501 27 1 3850 7500 230 3 45 GG375 80 GRIS_1501 27 1 4450 8700 230 3 45 GG435 81 GRIS_1501 27 6000 11000 230 3 45 0G590 32 FORS2 volume phased holographic GRIS_1400V 18 4 4560 5860 20 8 0 31 GRIS_1200R 93 5750 7310 25 0 0 38 GG435 81 GRIS_1028z 29 7730 9480 28 3 0 42 0G590 32 GRIS_600RI 19 4 5120 8450 55 0 83 GG435 81 GRIS_600z 23 7370 10700 54 0 81 0G590 32 FORS2 2nd order GRIS_600z 23 3 4660 3890 5460 25 0 38 1530 FILT_465_250 82 Table 2 5 Characteristics of the FORS grisms The table lists the resolution 2 AA achieved for a 1 slit in case of the standard resolution collimator and for a 0 5 slit in the case of the high resolution collimator at the given central wavelength in column 2 The wavelength range corresponds to a slit which is located in the field center see Table 2 6 for long slit x offsets A value in parenthesis indicates the approximate wavelength at which order overlap occurs Off center slit positions for instance with MOS MXU
30. 00 1543 E 2 LSS Orientation The orientation of the FORS image in LSS mode is given below for rotator position 0 deg Note that the sky directions in this schematics change for different rotator angles while the orientation on the CCD remains unchanged The orientation of the images on the CCD is given in parenthesis LSS Orientation Roator angle 0 sky position angle 0 north pixel 20060 2000 4 1 6 5 9 4 1 3 slit width in arcsec N RIGHT 97531246 8 lt internal slit number east LEE west IE ep peo pe ab 7 do p ep pl S d le X 12 5 1 0 0 3 0 7 2 0 slit width in arcsec pixel 1 1 BOTTOM Long Slit Decker TOP north pixel 20060 2000 4 qu lt 155 gt X l LEFT East West Vy lt HIT pixel 1 1 south BOTTOM The x distance between HIT decker and LSS decker is half the distance between two LSS slits HIT decker enables 3 slit the LSS decker will enable the large pinhole close to the center of the field of view Appendix World Coordinate System Information The header of the FITS file used for preparing a FORS target mask with FIMS should contain the following keyword
31. 1 lt FORS2 CCD ro ts EE 0 9 H 0 8 H 0 7 H 0 6 0 5 F 0 4 F 0 3 0 2 0 1 0 3000 4000 5000 6000 7000 8000 9000 10000 A in Figure 2 5 Quantum efficiency of the FORS CCDs Tektronix and the two slightly higher readout noise Pickup noise is clearly visible for the fast imaging modes and in some exposures of the slow spectroscopic mode with the MIT CCDs of FORS2 Linearity Full Well Capacity Dark Current etc some more characteristic data of the CCDs are given in Table 2 11 None of the CCDs will saturate before reaching the numerical truncation limits 65535adu Read out Speed and Times detector readout speed is 50 kHz for the Tektronix and Site detectors but 100 kHz and 200 kHz for the spectroscopic and imaging modes with the MIT mosaic detectors Approximate readout times for various modes are given in Table 2 12 Please note also that window readout is possible only in single port readout through port A of FORS1 and only for FORSI In window readout mode the typical readout time will depend on the position and size of the window For simplicity the readout time including all overheads should be estimated by the following equation 30s 92s DET WINI NX DET WINI NY 2080 2048 with the size of the window NX and NY as specified by the users CCD Defects at a number of positions on the FORS Site and Tektronix CCDs the e
32. 100 2309 The knowledge of these manuals is essential for the preparation of proposals and observations with FORS1 and FORS2 1 2 More Information on FORS The FORS1 2 User s FIMS and Templates Manuals are published on the FORS instrument WEB page Further links to FORS related informations are set on the top of the FORS page http www eso org instruments fors Information and software tools for the preparation of service and visitor mode observations with FORS1 and FORS2 are given under http www eso org observing p2pp Visiting astronomers will find instructions and hints on the Paranal Science Operations WEB page and the Paranal Observatory home page http www eso org paranal http www eso org paranal sciops 2 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 1 3 Contact Information In case of specific questions related to Service Mode observations and proposal preparation please contact the ESO User Support Group usg help eso org For visitor mode observations please contact the Paranal Science Operations Team pso eso org 1 4 Acknowledgements The first edition of this User Manual was delivered by the FORS Consortium which was formed by the Landessternwarte Heidelberg the University Observatories of G ottingen and Munich in the scope of the FORS contract and finally com piled and edited by G Rupprecht Later editions were edited by B ohnhardt until June 2002 and Szeifert until June 2004
33. 2 SDSS coordinates This method is limited by the accuracy of the astrometric positions of the stars The measurements were done with FORSI in the Bessel V band A third order polynomial was fitted to the measured data The formulas to determine the deviation in pixel of the position measured on the detector from the real astrometric position r in pixel are given in the table The measured distortion is in agreement with the design data SR 0 30 HR 1 55 at the corner of the field The residuals of the fit were 0 05 pixels in SR and 0 06 pixels in HR mode FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 35 FORSI SR 2 091 1079 2 1 228 1079 0 360 or FORSI HR Ar 9 515 1079 73 3 605 10 r 1 001 1073 r The radial offset derived from the equations above has to be subtracted from the measured position on the CCD The radius is calculated from the reference pixel fits keywords CRPIX1 and CRPIX2 of the world coordinate system From the optics design it was estimated that the chromatic and thermal effects are of the order of 10 of the distortion The radial field distortion of FORS2 was measured with a pinhole MXU mask The offsets are expressed in units of 24 micron pixels even though measured with 15 micron pixels of the new MIT detectors Ar and r in pixels measured on the MIT FORS2SR 2 113 1079 73 2 158 10 6 72 0 537 10 3 FORS2HR 7 133 107 r 3 782 1076 r 0 160 1073
34. 20000 M 10000 Si 0 al i 1 r 0 500 1000 1500 2000 pixels Figure D 20 Calibration spectrum taken with the SR collimator and grism GRIS_150I 17 FORS1 65 66 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 30000 T T T T T T T T al T T T T T T SES F SS EE SE S ODOMTR AD OHPOAMPATAUMOANMOR o Q LL J UE 20000 E D 4 Wei n A 0000 bal lee La LA TEM 0 500 1000 1500 2000 pixels Figure D 21 Calibration spectrum taken with the SR collimator and grism GRIS 1501 27 FORS2 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 67 ESO ech helium fits Objectname 21 20 00 000 90 00 00 00 LINEAR astro Rtd Jun 09 2000 at 07 27 43 Figure D 22 Calibration spectrum of the Echelle grism configuration EGRIS 110B XGRIS 600B taken with the SR collimator and He lamps Line identification list see Table D 2 Wavelength 3888 6460 3964 7290 4026 1912 4387 9282 4471 4790 4713 1455 4921 9312 5015 6797 5047 7378 5875 6211 ae D OO mp Dom Table D 2 Line identification list for Echelle grism configuration EGRIS_110B XGRIS_600B HgCd lines see Figure D 23
35. 3 ee 20000 10000 ke 0 500 1000 1500 2000 pixels Figure D 17 Calibration spectrum taken with the SR collimator and grism GRIS_300I 11 FORS1 30000 h L 1 1 Fo a 5109 8 QW CO 5 EN d r 5 D Ox se P MES 5 Ses 8 8 I g 88 QNS Be S 2 oo C Oo Q L J u 20000 9 M D e J o L 4 n L 4 0000 o We 0 i JL JL UL 0 500 1000 1500 2000 pixels Figure D 18 Calibration spectrum taken with the SR collimator and grism 5 3001 21 FORS2 FORS1 2 User Manual Issue 2 8 arc lines _1501 17 VLT MAN ESO 13100 1543 30000 E EE sued oc 4 POO Z TO LAN TA Te 3 8 Olt gt T COD HOO o Quo 1000 e DPN Q Wo M O Qu CC Q L 20000 n L E N o L E 0000 o L bal Figure 2 19 500 Calibration spectrum taken with the SR collimator grism 5 2001 28 FORS2 1000 pixels 1500 30000 T T T T I T T T T L T T T T eo D CO x e Du d E Sg 9 288585 9 355 D e 000 00 quo Q e 0 N oorrrry oo o
36. 36 L BESS437 g GUNN 40 Quarter wave plate mosaic Half wave plate mosaic Wollaston prism Bessel U filter Gunn u filter Gunn v filter Gunn r filter Gunn z filter Grism 600V Grism 300V Grism 300I Grism 600B Grism 600R Grism 600I Grism 150I Grism 1200g Order sorting filter GG375 Order sorting filter GG435 Order sorting filter OG590 Bessel B filter Bessel V filter Bessel R filter Bessel I filter Gunn g filter Table 2 1 FORS1 standard configuration of opto mechanical components Exchangeable Components up to 7 and 6 interference filters can be installed in FORS1 and FORS2 respectively in addition to the standard configuration set up For visitor mode observers the appropriate filter set up request referring to the available filters of Table 2 4 has to be submitted to the Paranal Science Operations Group at least one day before the start of the observing program For service mode Paranal Science Operations will take care of the proper instrument set up for the observations Only one copy of each exchangeable interference filter is available Conflicting requests if any will be decided upon by the Observatory Special rules and recommendations apply for the use of user provided filters see section 2 3 3 Filter and Grism Combinations In general only 1 filter can be used per instrument setup for imaging and 1 grism plus the recommended order separation filters in table 2 5 if needed in spectroscopic modes C
37. 4 slit shapes rectangular circular and curved slits Acquisition Accuracy With the improved astrometry of FORS2 with the MIT CCDs the targets can be properly placed on the slits all over the unvignetted field of view in standard resolution mode Collimator Constraints only observations with the SR collimator are supported Target Acquisition with MXU The MXU mask design has to be prepared with FIMS The alignment of the mask on the sky is done with user defined reference stars and with pre defined reference slits on the bottom of the upper CCD 2 4 8 Echelle Spectroscopy with FORS2 ECH mode Echelle Spectroscopy with FORS2 FORS2 allows Echelle spectroscopy in the blue spectral range The Echelle grisms are located in the grism wheel the cross disperser grisms in the Wollaston wheel A cut off filter is used to suppress the scattered infra red light The filter is placed in the converging beam below the camera Echelle Crossdisperser Orders Wavel range Dispersion RS OSF nm mm pixel EGRIS 110B498 XGRIS_600B 92 345 590 17 37 041 0 89 1530 FILT465 Table 2 7 Parameters of the Echelle grism and cross disperser Transmission Characteristics the Echelle efficiency curve for the blue grism is given in Figure C 4 The cross dis persers are optically identical to the standard grisms of the same name the efficiency curve of the only cross disperser used so far is given in Figure C 4 The spectral format for the E
38. 7 3 At the very end Finally after the last night a package of all science calibration and test data is prepared by the data handling admin istrators optionally on CD ROM DND or DAT and only copy Reduced data no matter if pipeline or interactive reduction are not on the package but DAT tapes are available to help yourself Please send us your end of mission reports with evaluations and suggestions available from WEB page http www eso org paranal sciops 3 8 FORS and the Unit Telescopes 3 8 1 Guide Stars Telescope Offsets All FORS science observations will require a guide star in the unvignetted field of view of the Unit Telescope The guide star is used for the alignment of the telescope relative to the guide star coordinates for the wave front sensor of the active optics system and for fast off axis guiding with typical tip tilt corrections of the M2 of greater than 20Hz The guide stars are automatically found from the USNO catalog by the telescope control system TCS during the acquisition of the field Due to the limits of the Cassegrain field of view and vignetting constraints for the FORS instruments the optimum distance range for guide stars from the field center is 4 7 4 arcmin for the SR collimator and 2 7 4 arcmin for HR collimator Depending the seeing the guide star brightness should be between 10 13 mag For small telescope offsets a few arcsec to a few arcmin the telescope may keep the sam
39. 8 arc minutes for the standard resolution collimator In case of the high resolution collimator the corners of the field of view are vignetted by the camera lenses The two CCDs are mounted slightly offset by 33 arc seconds for operational reasons The center of the field of view will fall on y pixel 260 of the upper master CCD for unbinned standard resolution mode Images showing the respective vignetting pattern for the standard MOS and high resolution collimator mode can be found in appendix G of this manual High resolution imaging with the FORS2 CCD mosaic With the higher sampling of the MIT CCDs of 0 125 pixel for the unbinned 15ym pixels it will be possible to operate with the standard resolution collimator down to seeing values of about 0735 without performance losses in respect to observation with the high resolution collimator Below seeing values of 0 3 the high resolution collimator is expected to improve the image quality in a significant way 2 3 3 FORS Filter Set Standard Broadband Filters FORS provides positions for 7 broadband filters in any of the three wheels of the parallel beam section and for 8 interference filters in two wheels of convergent beam section Presently available standard filter sets for FORSI are Bessel U B V R I Gunn u v r z and some order separation filters see Table 2 1 FORS2 has Bessel B V I and R pass band filters Gunn z as well as order separation filters see Table 2 2 The spe
40. DME file Wavelength calibration exposures are done during the day only with the telescope in zenith and the instrument in calibration position 38 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Grim eei GG435 00590 00590 F465 250 GG435 00590 F465 250 GG375 GG435 00590 10G590 GG375 GG435 Echelle Table 4 5 Approximate exposure times and switch on times of calibration lamps seconds for FORS2 wavelength calibrations with FORS2 Any slit width SR collimator high gain readout 2x2 binning The update with the integration times for FORSI is still pending 4 6 Calibrating Polarimetric Measurements 4 6 1 Circular polarimetry The amount of circular polarization V can be determined observing with the quarter wave retarder plate at two retarder plate angles of 6 45 by the equation ales Url ee EE f f being the ordinary and extraordinary beam of the object measured for a given retarder plate angle 9 One could determine the circular polarization observing at one retarder plate position but two observations are required to eliminate the strongest observing biases in the first order approximation e the improper flat field correction err e the color dependent offset to the nominal retarder plate zero angle e the incomplete and color dependent retardation of 90 4 degree of the quarter wave plate Observations at only one retarder plate angle wou
41. Imaging Polarimetry 16 2 5 Quantum efficiency of the FORS CCDs Tektronix and the two 23 4 1 Zero angle chromatism of the half wave plate 39 Bessell filter transmission curves 46 B 2 Gunn filter transmission 47 FORS intermediate band filter transmission curves 49 Efficiency curves of the low resolution 2115118 ooo 50 C 2 Efficiency curve of the medium resolution 51 Efficiency curve of the medium resolution 52 C 4 Efficiency curve of Echelle grism 110 53 D 1 Calibration spectrum SR 5 1400 56 D 2 Calibration spectrum SR 5 1200 56 D 3 Calibration spectrum SR 5 12006 57 DA Calibration spectrum SR 5 10282 57 D 5 Calibration spectrum SR GRIS 600B 58 D 6 Calibration spectrum SR 5 600 58 D 7 Calibration spectrum SR GRIS 600V 59 D 8 Calibration spectrum SR GRIS 600R
42. MG mode e photometric zero points IMG mode e master screen flats LSS mode e flat fielded science images only for a few slit grism combinations LSS mode 42 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Appendix Abbreviations and Acronyms The following abbreviations and acronyms are used in this manual ACQ ADU BOB CCD DDTC DSS ECH ESO ETC FIERA FIMS FITS FORS FWHM HIT HR IDL IMG IPOL IRAF ISF LADC LSS MIDAS MOS MXU OB OSF OT PMOS PSF P2PP RMS RON SR S N TBC TBD TCS Acquisition Analogue to Digital Unite Broker of Observation Blocks Charge Coupled Device Director s Discretionary Time Committee Digital Sky Survey Echelle Spectroscopy European Southern Observatory Exposure Time Calculator Fast Imager Electronic Readout Assembly FORS Instrumental Mask Simulator Flexible Image Transport System Focal Reducer Low Dispersion Spectrograph Full Width Half Maximum HIgh Time resolution High Resolution Interactive Data Language Imaging Imaging Polarimetry Image Reduction and Analysis Facility Instrument Summary File Longitudinal Atmospheric Dispersion Compensator Long Slit Spectroscopy Munich Image Data Analysis System Multi Object Spectroscopy Mask eXchange Unit Observation Block Order Separation Filter Observing Tool Polarimetric Multi Object Spectroscopy Point Spread Function Phase 2 Proposal Preparation Root Mean Square Read Out Noise
43. ORS1 The FORSI polarization optics allow the determination of the degree of polarization to a relative error of lt 3 x 107 and of the position angle depending on the target polarization to about 0 2 For observation in the center of the field no instrumental polarization was found at the detection level of our measurement of lt 3 x 10 4 For off axes measurements 3 arminutes offset spurious polarization of up to 8 x 1074 was detected in some measurements circular polarization in this case Important update on the instrumental polarization We have found a strong linear instrumental polarization in the corners of the field of view This spurious polarization field shows a high degree of axial symmetry and smoothly increases from less than 3x10 on the optical axis to 7x10 at a distance of 3 arcmin from it V band In case of the other filters and spectro polarimetric measurement there is no data available yet The corrective functions can be estimated with an observation of a globular cluster with the respective filters Such work is under way and you will find more information on the FORS webpage http www eso org instruments fors pola html Please note that there should be no problem for spectro polarimetric observation of single targets in the center of the field of view or single targets in imaging polarimetry in the center of the field of view In case of the circular polarization the spurious polarization was found an order of mag
44. acq_fast target acquisition FORS2_hiti_obs slit_fast through slit image FORS2 hiti obs exp fast Science exposures Grism XGRIS 600R was announced in earlier versions of the document but we have learned later that there are technical problems using this grism as cross disperser grism 20 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Very similarly in case of the HITS spectroscopic mode FORS2 hits acq fast target acquisition FORS2 hits obs slit fast through slit image FORS2 hits obs exp fast Science exposures The detailed description of the template functionalities and parameters will be available in due time for the phase 2 proposal preparation 2 6 5 Calibration plan The bias frames of the normal spectroscopic modes can be also used for modes HITI and HITS This is not the case for flats fields and arcs of cause Please note that only the CCD columns are used to detect the incoming light onto which the slit or square aperture is projected Pixel to pixel variations of the detector response can not be corrected The flat field frames and arcs should not depend on the selected readout speed The observatory staff will define an appropriate readout speed for which well exposed calibration frames can be achieved For the other readout speeds it is typically impossible to get the exposure level right Night time calibrations are not possible Night time standard stars are to be selected by the HIT mode users and the respective ob
45. adout Table 4 3 gives typical exposure times for screen flats for the SR collimator and Bessell filters The numbers are in dicative only since they are subject to changes due for instance lamp replacements The observatory staff has updated values at hand and takes also care of proper adjustments of the calibration exposure times for delivered service mode OBs unless otherwise stated in the readme file of the program 4 4 2 Spectroscopic Modes For the spectroscopic modes one will use internal screen flats in most cases These flats are taken during daytime with the telescope pointing to zenith and the instrument in calibration position Spectroscopic flats on the sky in twilight are not supported by the FORS standard templates A guide to exposure times is given in Table 4 4 In MOS mode some bleeding from zero order may occur for low dispersion grisms and unfavorable i e wide spread in dispersion direction object geometry The numbers are indicative FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 37 BER Blue 1 Blue 2 Red 1 Red 2 mp me 39 4 4 Po 10 Table 4 3 Approximate exposure times seconds for FORS2 imaging screen flat calibrations for the Bessell and special broadband filters SR collimator high gain readout only since they are subject to changes due for instance lamp replacements The observatory staff has updated values at hand and takes also care of proper adjustments of the calibration exposur
46. ale structure and small scale noise in sky flats 36 Exposure times for FORS2 imaging screen flat calibrations 37 Exposure times for spectroscopic screen flat calibrations 37 Exposure times and switch on times for FORS2 wavelength calibrations 38 FORSI half wave plate calibration 39 Characteristics of the FORS1 2 broadband filters 45 Characteristics of FORS interference filters 48 FORS arc lamp wavelength table 2e 55 He line identification list for Echelle grism configuration EGRIS 110B XGRIS 600B 67 HgCd line identification list for Echelle grism configuration EGRIS 110B XGRIS 600B 68 vi Chapter 1 Introduction 1 4 Scope The FORS1 2 User s Manual is intended to cover all aspects of the VLT instruments FORS1 and FORS2 and to give comprehensive information on the following topics e Overall description of the FORS instruments e Observing with FORS e Calibrating and reducing FORS data e Supplementary Data and Informations about CCDs filters and grisms The informations about observation block preparation for FORS with p2pp and mask preparation with FIMS are given in the following supplementary manuals e FORS1 2 FIMS Manual ESO document VLT MAN ESO 13100 2308 e FORS1 2 Templates Manual ESO document VLT MAN ESO 13
47. arcseconds Note these plots are indicative only since minor shifts of the wavelength pixels may occur between the two FORS instruments and due to different dewar mounting after instrument and CCD maintenance The x scale is in units of binned pixels in case of FORS2 54 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Wavelength Element Wavelength Element 3610 500 3650 144 3654 840 3663 274 3888 646 3964 700 4026 200 4046 557 4077 831 4347 500 4358 343 4471 479 4678 160 4713 200 4799 920 4916 070 4921 929 5015 675 5085 824 5341 100 5400 562 5460 742 5764 419 5769 598 5790 656 5852 488 5875 620 5881 900 5944 830 5975 534 6029 977 6074 338 6096 160 6143 063 6163 594 6217 281 6266 495 6304 790 6334 428 6382 991 6402 246 6438 470 6506 528 6532 880 6598 953 6678 149 6678 300 6717 040 6907 160 6929 468 6965 431 7032 413 7065 200 7081 880 7091 990 7147 041 7173 939 7245 167 7272 930 7281 349 7346 200 7383 900 7383 981 7385 300 7438 900 7488 870 7503 868 7514 652 7535 800 7635 106 7724 210 7948 176 8006 157 8014 786 8103 693 8115 311 8264 523 8300 326 8377 367 8408 210 8424 648 8495 360 8521 442 8591 259 8634 648 8654 384 8667 944 8681 900 8704 150 8853 867 8919 500 9122 968 9201 800 9224 499 9300 850 9354 218 9425 380 9657 784 9784 501 10140 000 10394 600 10830 171 Table D 1 Wavelengths of the arc lamp lines with the corresponding
48. at day time with the telescope guide probe LADC parked and the beam shutter identical with the calibration screen closed 2 9 1 Parasitic Light Longslit and MOS PMOS Flatfields As a reference for the data reduction of older data taken with the internal calibration units we still want to provide the following informations Longslit Mask due to multiple scattering in the longslit mask parasitic light from the flatfield lamps of the instrument has reached the CCD detector This light appeared as high spatial frequency horizontal wave or ripple pattern in the flatfield exposures The position on the chip varied with the use of the calibration unit MOS PMOS Set ups due to multiple reflections of light from the calibration lamps at the LADC and the side walls of the MOS slit carriers flatfield exposures have shown a few higher exposed pixel rows at the upper or lower edge of the slit image on the CCD depending on the calibration unit used for the exposures Two sets of flatfields using alternatively the lamps of only one calibration unit were taken in order to compute a clean flatfield with the parasitic light partly removed from the calibration flatfield For the wavelength calibrations the parasitic light is of little to no concern 26 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 10 Forthcoming New Equipment and Observing Modes GRIS 1200B to replace the Echelle mode The Echelle mode with the two conventional grisms is
49. based 28 3 5 OB preparation Fast modes 29 3 6 Estimate execution time and optimize 29 3 7 Visitor Mode esu Ruso Oe UA eon m ROBUR Soo dom eoo v Rue e woe EORR es 31 3l Thefimalpackage Re eR RR Ex cR 31 2752 At E EE MAE Seg REESE EE E 31 227 3 Atthe wernyend enn veo AE WAS up AT NS 31 3 8 FORSandtheUnit Telescopes 31 3 8 1 Guide Stars Telescope 5 31 3 8 2 Telescope and Instrument Focus 32 3 8 3 Instrument Rotation and Position Angle 55 32 3 8 4 Atmospheric Dispersion Compensation 32 Calibrating and Reducing FORS Data 33 44 Calibration Plan om fom UR ar gui et RYE 33 4 2 Image Field Distortion and Scales 34 4 3 Data Reduction of Pre Imaging Data for the Mask Preparation 35 44 Hat ibireldmg o sasiga ap seco Sate d dead a Ble RO ie ee e ee Sep 36 44 1 Amaging Modes Bos Cae RR DU OM eed ed ba CEES 36 442 Spectroscopic Modes onk 4E eh 36 4 5 Wavelength Calibration 222525222222 RR tores Rose E p rv Rer Ae ee Sen 37
50. chelle mode is shown in Figure 2 3 Echelle Slits via MOS the slit for this mode will be produced by the MOS slit 10 but can be extended by the adjacent MOS slit 9 The Echelle slit is placed in the MOS field center Target Acquisition for Echelle Spectroscopy Only fast target acquisition mode is available for Echelle spectroscopy e the fast mode where the object is identified by mouse click in an acquisition image followed by a telescope offset to put it into the MOS Echelle slit This mode is suitable for brighter objects Given the low efficiency of the Echelle grisms the fast mode may actually satisfy most observations in this mode e for faint sources the acquisition can be done with blind offsets in fast mode the offsets will be executed after centering a reference star on the slit template FORS1 2 ech_obs mosSlit_fast Note In almost all cases the same data can be obtained quicker with a long slit For instance we advise the use of GRIS 600B in LSS mode with a slit width two times smaller 0 5 A holographic grism GRIS 1200B will replace the Echelle mode in the near future 2 4 9 Slitless Spectroscopy Slitless spectroscopy can be performed in MOS mode with all slits open Please not that the sky background will be the same as for imaging mode observations and jitter offsets between the exposures must be applied to achieve a good sky subtraction For the preparation of observations in slitless spectroscopy a very goo
51. cial U and the Gunn g filters are interference filters and have to be mounted into the interference filter wheels The special R band filter and the Bessel I filter of FORS2 show internal fringes at a faint level In case of the Bessel I the internal fringes can be only seen with the IR optimized MIT detectors In both cases the typical shape of the pattern is circular and off axes The complete list of filters together with the transmission curves are presented in appendix B of this manual Order Separation Filters the order separation filters are foreseen for spectroscopic applications in the first place but they are also available for imaging exposures They have an edge shape transmission curve with cut off wavelength designed to match the respective grisms for spectroscopy The order separation filters are installed in the parallel beam of the instruments except of the box shape filter FILT 465 250 82 which is in convergent beam of FORS2 Interference Filters the standard interference filters available for FORS1 and FORS2 are centered on important emis sion lines and on wavelengths 5 and 10 longer The interference filters are located in the convergent beam in the camera and have a diameter of 115 mm Their wave front error is less than 4 4 within 25 mm 8 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 P2PP entry Filter type 44 filter 4000 45 filter redshifted by 4000 km s OII 8000 46 O II filter redshift
52. d filter the targets should be more than half the length of spectra off the zero order and the field vignetting 16 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 5 Polarimetry with FORSI Polarimetry Concept with FORSI the polarimetric modes are implemented FORSI only They allow the measure ment of linear and circular polarization both for direct imaging and spectroscopy The polarization optics are located in the parallel beam section of FORSI and consists of a Wollaston prism as beam splitting analyser and two superachro matic phase retarder plate mosaics 9 individual plates arranged in a square mosaic frame to measure linear and circular polarization Both mosaics are installed in rotatable mountings on a dedicated swing arm which can be moved in and out of the light path The Wollaston prism is inserted in the uppermost wheel of the parallel beam section 2 5 1 Imaging Polarimetry IPOL mode Strip Mask for Imaging Polarimetry IPOL for imaging polarimetry IPOL of extended objects or crowded fields strip mask is produced in the focal area of FORSI to avoid overlapping of the two beams of polarized light on the CCD When using the standard resolution collimator the strip mask is formed by placing every second MOS slit jaw carrier arm odd numbered MOS slits across the field of view of the instrument A full coverage of the imaging field of view is then achieved by taking two frames displaced by 22 in direction of t
53. d understanding of the instrument optics is essen tial Note that the Oth order of grisms 150I and 2001 will fall into the field of view of FORS and contaminate 790 and FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 15 e ORDER BLUE CEN RED m 9 380 435 508 8 425 485 568 475 550 590 6 550 590 9 12 345 380 1 345 365 415 S tn 10 350 395 450 1 gt 2 m ORS2 ECHELLE SPECTRAL FORMAT FORS2 SR WITH HD11 B CD600B ANDESS R RIE THU TUL 22 15 2 7 1999 KOENICSTI 12 D 69117 CONFIGURATION 7 OF 7 Figure 2 3 Spectral format of the EGRIS_110B 98 Echelle grism in FORS2 The plot shows the positions of the Echelle orders on the CCD the corresponding wavelength range of each order is tabulated in the upper right corner of the figure 300 pixels on the left blue side of the field of view of FORSI 24um pixels and 1260 and 480 unvignetted pixels of FORS2 unbinned 15 pixels Any observation with filters of wavelengths which are off the central wavelength of the grism will cause field vignetting which can cut the field on both sides depending on the sign of the wavelength offset between filter and grism Depending on the length of the spectra the requeste
54. e guide star otherwise it will automatically try to find a new one Whether or not such telescope offsets cause a change of the guide star depends on the offset amplitude and direction and on the position of the original guide star in the field If the guide star is kept 32 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 during an offset the offset accuracy will be better than 0 1 arcsec If the guide star is changed larger offset errors can be introduced by the uncertainties of the guide star positions 3 8 2 Telescope and Instrument Focus The telescope focus is automatically set by the active optics system No intervention is required by the observer Defo cussing of the telescope is not possible during the observations The instrument focus is corrected automatically for the different thickness of the various filters for the grisms collimator and for varying instrument temperature autofocus For user provided filters visitor mode only the instrument focus will be determined by the observatory engineering and operations staff which requires the provision of these filters to the observatory at least 6 weeks before the scheduled observing run 3 8 3 Instrument Rotation and Position Angle on the Sky FORS can be rotated independently from the guide probe The allowed range for rotator presets with FORS is 180 to 180 deg while the operational range with FORS is 270 to 270 deg Please note that the rotator offset angle of the te
55. e interference filters used with for FORS1 and FORS2 Their characteristics are given with the FORS SR and HR collimators central wavelength peak transmission and FWHM Due to their location in the converging beam the filter characteristics depend on the collimator used The filter bandwidths are wider the central wavelength is blue shifted and the peak transmission is lower than in a parallel beam With the SR collimator the effect 15 larger than with the HR collimator The filters are centered on important emission lines and on 5 and 10 longer wavelengths ER FWHM nm 00 44 372 7 7 0 011 4000 45 5 8000 46 10 Hell 47 Hell 468 6 0 Hell 3000 48 6 5 Hell 6500 49 10 50 500 7 0 3000 51 5 OIII 6000 52 1096 Hel 53 Hel 587 6 7 0 Hel 2500 54 5 1 5000 55 10 O1 56 630 0 0 01 2500 57 01 4500 58 H_Alpha 83 656 3 _ 1 2500 60 H_Alpha 4500 61 1 62 SII 672 4 SII 2000 63 SII 4500 64 5 65 511 953 2 SIII 1500 66 1500 67 FILT 485 37 FILT 691 55 FILT 815 13 FILT 834 48 7 SPECIAL FILT 917 6 FILT_5005 500 7 FILT_5035 FILT 530 25 Table B 2 Characteristics of FORS interference filters is the central wavelength in nm the peak transmission FORS1 2 User Manual Issue 2 8 transmission transmission VLT MAN ESO 13100 1543 49
56. e scale gradients of the order of 1000 pixels In service mode twilight flats are provided as standard calibration frames Screen flatfields can be taken see section 2 9 with the internal lamps and the screen in the telescope guide to approximate exposure times is given in Table 4 3 Screen flats should be used only for removing the high frequency component of the flat field However this can be equally well achieved using sky flats since the exposure levels in both are comparable Furthermore screen flats contain artificial reflections off the LADC 2 3 dots close to the image center which need to be removed before applying Screen flats are not provided as standard calibration frames in service mode but need to be requested Table 4 2 lists results from the analysis of the flatfields including master flats produced by the pipeline taken during the past periods The sigma values scale as sqrt exposure level other values scale with the exposure level sigma in masters goes down by a factor sqrt N where is the number of raw files contributing diff AB is the fractional gain difference between ports A and B which is removed by the flattening gradient is the large scale gradient measured in a window of size 200x200 pixels Typical exposure sigma sigma diff AB gradient level noise photon noise fixed ADU raw pattern Table 4 2 Large scale structure and small scale noise in sky flats high gain CCD re
57. e times for delivered service mode OBs unless otherwise stated in the readme file Grim OSF Exposure time Ls Fors Fons GG435 00590 66435 00590 F465 250 GG435 00590 F465 250 GG375 GG435 06590 0G590 GG375 GG435 Echelle Table 4 4 Approximate exposure times seconds for FORS2 spectroscopic screen flat calibrations with FORS2 Flat field lamps of one calibration unit switched on Approximate exposure level is 30000 ADU Slit width 1 SR collimator high gain readout 2x2 binning 4 5 Wavelength Calibration For the wavelength calibration one may use the and Ar lamps at the lowest spectral resolution grism 1501 and in addition the Ne lamp at higher resolution Note that the exposure time of the Ne lamp should be reduced by a factor of 5 at least switch on times can be defined individually for each lamp in the corresponding calibration template For grism 600B the HgCd lamp must be used Approximate exposure times for well exposed spectra are given in Table 4 5 for the different grisms and lamps for slit width of 1 Calibration spectra taken with the different grisms are plotted in figures D 4 D 21 The numbers are indicative only since they are subject to changes due to e g lamp replacements The observatory staff has updated values at hand and takes also care of proper adjustments of the calibration exposure times for delivered service mode OBs unless otherwise stated in the REA
58. ed Bessel U Bessel B Bessel V Bessel R Bessel I Table 4 6 Calibration of the FORSI half wave retarder plate in imaging mode from the spectroscopic measurements with the Glan Thompson prism These values will depend slightly on the color of the observed targets 40 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 In this case the Stokes parameters Q and U can be derived via Fourier transformation N 1 Q YS Fea 4 5 i 0 N 1 2 U gt ef sin 46 4 6 0 In principle two observations at different retarder angles 2 are sufficient to calculate U At least four measurements at angle 0 0 to 67 5 are needed to suppress the impact of the improper flat fielding of the data Best results will be obtained if observations at all the rotation angles of the retarder plate N 16 will be carried out Although a super achromatic half wave plate is used with FORS the zero angle of the plate is not negligible Therefore all raw measurements of polarization position angles are rotated by an angle of a few degrees For the half wave plate the chromatic dependence of the zero angle was determined with an aligned Glan Thomson prism The tabulated values of the zero angle as displayed on figure 4 1 can be obtained on request For imaging polarimetry the offset angles can be determined by convolving the filter response curves with the color dependence of the half wave plate The results are given i
59. ed by 8000 km s 47 He II filter 3000 48 He II filter redshifted by 3000 km s 6500 49 He II filter redshifted by 6500 km s 50 filter 3000 51 O filter redshifted by 3000 km s 6000 52 O filter redshifted by 6000 km s 1 53 He I filter 1 2500 54 He I filter redshifted by 2500 km s 1 5000 55 He I filter redshifted by 5000 km s OI 56 O I filter OI 25004 57 O I filter redshifted by 2500 km s 1 4500 58 I filter redshifted by 4500 km s H_Alpha 83 H Alpha filter replacement for H_Alpha 59 1 2500 60 Alpha filter redshifted by 2500 km s 1 4500 61 H Alpha filter redshifted by 4500 km s SIL 62 II filter SII 2000 63 II filter redshifted by 2000 km s 51 4500 64 II filter redshifted by 4500 km s 65 S filter SIII 1500 66 III filter redshifted by 1500 km s SIII 3000 67 III filter redshifted by 3000 km s FILT_485_37 68 special intermediate band filter FILT 691 55 69 special intermediate band filter FILT_815_13 70 night sky suppression filter FILT_834_48 71 night sky suppression filter zSPECIAL 43 Special z band filter width 20nm FILT_917_6 88 Special z band filter width 6nm FILT_530_25 84 Munich intermediate band filter FILT_5005 85 Munich filter FILT_5035 86 Munich filter redshifted by 1800 km s Table 2 4 Exchangeable filter set for both FORS instruments The intrinsic transmission curves of the narrow band fil
60. ed master sky will give quite reasonable results even at z band wavelengths where observations without jitter or nodding will be very hard to calibrate Most applicants will observe fainter targets with 8m class telescopes while the sky will be as bright as with any other telescope 2 8 3 Shutter FORS contains a rotating half segment exposure shutter which guarantees uniform illumination of the CCD to the 1 level or better for exposure times as short as 1 sec the shortest possible exposure time is 0 25 sec 2 8 4 CCD Contamination The CCD dewars of both FORS1 and FORS2 showed contamination effects from the very beginning of science oper ations with the instruments The contamination affects mostly the relative sensitivity across the CCD i e it changes toward the edges and appears as a diffuse quadrangle in the outer part of the chip The contamination is most pronounced in the UV wavelength region and it is growing over time It can be cured i e removed by a special warm up procedure currently done every 6 months A major CCD dewar maintenance in October 1999 considerably improved the contamination of FORS1 but did not remove it completely The FORS2 deward was also contaminated and a similar maintenance of the dewar was exercised in January 2000 The measurements of the CCD contamination indicate that the growth rate of the contamination slows down with each decontamination cycle of the dewar The impact on the scientific applications
61. eld of view 2 47 Multi Object Spectroscopy with masks on FORS2 MXU mode FORS2 has a Mask eXchange Unit MXU built into its top section This MXU is a magazine holding up to 10 masks made of black painted stress relieved invar sheets of 0 21 mm thickness laser cut by the Mask Manufacturing Unit MMU of the VIMOS instrument The purpose of the MXU mode is to allow more objects to be observed simultane ously than with the 19 slitlets MOS unit Furthermore it gives more freedom in choosing the location size and shape of individual slitlets MXU spectroscopy is only offered in the standard resolution mode of FORS2 It is recommended that observers in Visitor Mode prepare the masks design or get familiar with MXU mask preparation before their arrival on Paranal usually 3 days before the start of their observation run Mask manufacturing and installation is only done at day time Therefore the mask manufacturing has to be initiated 1 day before starting the observations Only up to 10 masks can be stored in the magazine and observed in one night 2 reason is alternating light traps which prevent sky light from falling between the slit blade carriers 14 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 MXU Slits boundary conditions for the MXU slits are 1 slit width 071 minimum to 40 2 slit length up to 40 3 available field of view X minus 15mm at either end this is indicated by FIMS Y full field of view
62. en zenith distances zenith distance COLL SR 0 15 lt 0 3 1 lt 0 15 1 30 lt 0 5 1 lt 0 25 1 45 lt 0 7 1 lt 0 35 1 60 lt 0 9 1 lt 0 45 1 org In all standard configurations telluric emission lines will fall into the wavelength range of FORS However with GRISM 600B and off axes slits toward the right red side of the instrument the telluric lines may fall out of the wavelength range Another effect which will shift the dispersion solution is the mask positioning error which will be much smaller The mask positions can be reproduced within 5 to 10 microns compared to the telescope image scale of 528 microns per arc second While observing point sources the centering of the target on the slit will be the main source of uncertainty 2 4 4 Longslit Spectroscopy LSS mode Longslit Mask LSS A mask providing 9 longslits with high quality slit edges is available for the focal area of FORS they have a common slit length of 6 8 and fixed slit widths as given in Table 2 6 The approximate offsets of the slits to the central slit of 0728 are given in the same table as offsets on the sky and on the CCD collimator dependent The actual slit for the observation is selected by a decker mask See appendix E 2 for the orientation The actual LSS slitpositions on the CCD depend also on the mounting reproducibility of the CCD dewar and may change slightly when the CCD dewar is mounted back to the instr
63. erstanding of the instrument is required before starting the preparation of the observing blocks It is possible to define observing sequences which don t make any sense both within FIMS and within p2pp Inconsistencies should be eliminated by the user although a cross check of the OBs will be done both in visitor and service mode by verification scripts or the staff astronomers It will be one of the first steps to define the instrument setups chapter 2 and to calculate the exposure times with the exposure time calculator 3 4 OB preparation FIMS based modes Get your pre imaging data or other astrometrically corrected images see section 2 4 2 Select the observing mode the instrument setup and calculate the exposure times with the exposure time calculator Prepare your masks with FIMS and keep the fims output file with suffix fims to reload the mask if needed and the output files with extensions p_targ p_focf and p_gbr for MXU mode for the OB preparation These files will be saved by FIMS in directory fims SET Make a hard copy of the mask configuration within FIMS on which the reference stars and slits are well visible and a few hard copies of the same masks with high magnification This will be the typical set of finding charts needed at the end Prepare the observing blocks a typical OB in imaging mode with occulting bars OCC mode will consist of two templates FORS img acq align target acquisitio
64. erview table of all grisms 12 wavelength calibrations 35 52 wavelength calibrations in Echelle mode 64 y offsets of grisms 600RI and 1400V 10 standard instrument configuration 5 exchangeable components 5 filter combinations 5 FORSI 5 FORS2 6 waivers 5 telescope 30 atmospheric dispersion corrector 30 focus 30 guide stars 30 LADC 30 paralactic angle 30 rotator offset angle 30 template manual 1 visiting astronomers general informations 1 observing with FORS 25 on the site 29 WEB page 1 world coordinate system 68 VLT MAN ESO 13100 1543
65. ference star on the slit In case that you ask for special calibrations not included in the FORS calibration plan section 4 1 a calibration OB has to be prepared which would look like the following scheme FORS ima cal coll collimator selection FORS Iss cal scrflat fast screen flats FORS Iss cal wave fast Screen arcs where the first template is only used to select the collimator There are a few important points to be verified now a don t mix observing modes in one OB b make sure that the same slits are used in LSS mode for all templates within an OB c verify that the offsets for blind offset acquisitions are correct in size and sign 3 6 Estimate execution time and optimize overheads In the following example in MOS mode we presumed that the reference stars for the target acquisition were bright enough to be seen in 5 seconds fims mode or blind acquisition typically with broad band filters and that there were some targets on the slits which can be seen in 60s on the through slit image which is ideally done without filters in case of FORSI atmospheric dispersion corrector or with a broad band filter in case of FORS2 to reduce the sky brightness in case of the IR sensitive MIT detector No further acquisition overheads are required for the imaging mode after the preset and the start of the active optics correction There is in most cases no need to repeat the acquisition procedure in the spectroscopic modes The through slit ima
66. fficiency is significantly reduced Although the effect is linear and can be corrected by flat fielding the data it should be avoided to place the science target onto these spots with quantum efficiencies below the expected level The typical size of the spots is about 5 pixels but some spots are much larger as summarized in Tables for FORS1 Note that in the tables the deficit flux is given for the worst pixel only Vertical bleeding of the CCD occurs for overexposure levels beyond 5 times saturation level Horizontal charge memory effects over a few pixels are seen for extremely bright sources Cross talks in 4 port read out is detected to be of the order of 0 01 percent between ports C and D by a factor of 4 less between D and C and negligible between ports A and B Bad columns are occasionally seen at X 1357 for CCD and at X 1226 1228 for the old FORS2 Site CCD 2 8 2 Fringes Tektronix and SITE CCDs The fringe amplitudes of the FORS detectors are relatively low and only observed at wavelength greater 750nm MIT mosaic detector The amplitudes of the internal CCD fringes are strongly reduced in respect to the Site and Tektronix CCDs For Bessel I imaging fringes are hardly visible circular fringes from the filters are however visible for 5 and R SPECIAL filters For z Gunn imaging the fringe amplitudes are below 1 and in the strongest telluric 24 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 E old
67. for the MIT CCDs The default readout modes will be 200kHz 2x2 low for imaging 2x2 binned low gain mode read with 200kHz 100kHz 2x2 high for spectroscopy For special applications such as high resolution imaging or deconvolution techniques an unbinned mode is supported 200kHz 1x1 low Read out FORS1 default modes ABCD high 1 1 imaging A high 1 1 spectroscopic FORS1 other modes A low 4x4 200kHz 2x2 low imaging Table 2 9 Detector set ups of FORS for normal CCD modes Standard Operation Modes of the CCDs only the following standard CCD set ups are offered for service mode observations e ABCD 1x1 high FORSI direct imaging IMG OCC and imaging polarimetry IPOL e A Ixl high FORSI spectroscopy LSS MOS PMOS e 200kHz 2x2 low FORS2 direct imaging IMG OCC e 100kHz 2x2 high FORS2 spectroscopy LSS MOS MXU Visitor mode observations allow the full complement of CCD read outs However it is strongly recommended to use the CCD standard operations read out modes whenever possible for instance to benefit from the calibration data taken in the context of the FORS instrument calibration plan Noise Gain and Conversion Factors the read out noise RON and conversion factors K as measured on the site for all CCDs are given in Tables 2 10 Please note that low gain denotes high charge conversion factors in e adu and FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 23 FORS
68. ges taken with the targets on the slits typically have to be repeated in case of corrections of the order of 1 pixels Two loops are require to verify safely that the targets are on the slits 30 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Telescope telescope preset guide star acquisition active optics LADC resetting Interactive Acquisition excluding exposure time one loop IMG occulting IPOL 1 5 min per loop one loop MOS MXU PMOS 2 0 min per loop one loop LSS ECH HIT 1 5 min per loop two loops through slit exposure 2 0 min per loop Instrument instrument setup collimator exchange retarder plate setup Exposure integration time read out A 1x1 high read out ABCD 1x1 high read out ABCD 2x2 high FORS2 read out 100kHz binned FORS2 read out 200kHz binned FORS2 read out 200kHz unbinned user defined 122s 49s 25s Als 31s 62s Table 3 1 Operational overheads with FORS on the VLT The through slit exposure is typically executed twice It is important to include the overhead times while preparing proposals and service mode observations packages FORS1 mos acq FORS2 mos acq telescope preset 180s telescope preset 180s guide star acquisition 45s guide star acquisition 45s active optics 2 loops 120s active optics 2 loops 120s acq image integration time 55 acq image integration time 5s acquisition procedure 120s acquisition procedure 120s FORS mos obs slit 2 loops FORS mos
69. gs The dispersion direction is along the X direction of the CCD in all spectroscopic modes The camera focus is set automatically depending on the grism filter combination in the optical path of the instrument Usable Field for Spectroscopy for objects close to the edge of the field of view in the direction of dispersion a part of the spectrum will not reach the CCD Therefore the typical usable field of view for spectroscopy with the standard and high resolution collimators will be reduced in dispersion direction 2 41 Grisms and Order Sorting Filters Normal Grisms two sets of normal grisms are available for the two instruments which cover the full operational wavelength range of FORS with essentially three different resolutions 230 A mm 110 A mm 45 to 50 A mm see Table 2 5 Each instrument has a baseline set of identical grism replica which support the same spectroscopy options GRIS_600B 12 22 GRIS_6001 15 25 GRIS_300V 10 20 GRIS 3001 11 21 GRIS_150I 17 27 In addition 3 more normal grisms exist in single copies GRIS_600V 94 GRIS_600R 14 in FORS1 and 2001 28 in FORS2 All grisms are mounted in the grism or Wollaston wheels of the parallel beam section Holographic Grisms in addition to the normal standard grisms some medium resolution high throughput grisms are available with FORS2 GRIS_1400V 18 GRIS_1200R 93 GRIS 10282 29 GRIS_600RI 19 and GRIS 600z4 23 and FORS1 GRIS_1200g 96 These grisms are based on volume
70. he MOS slitlets For the high resolution collimator a separate pre manufactured strip mask of slits of 11 is moved into the focal area of FORS1 MOS stripes 3 5 7 hhh polarisation MEE optics zx ss split stripe into and o ray EZ EE 2 2 4 4 6 6 8 8 Figure 2 4 For imaging polarimetry IPOL of extended objects or crowded fields strip mask is produced in the focal area of FORS1 to avoid overlapping of the two beams of polarized light on the CCD x stripe pairs on the CCD E NN SAX D ll Wing inm Nl i D Field Coverage since with IPOL observations only half of the full field of view of the FORSI instrument is imaged the CCD in one exposure the complete field coverage can only be achieved by off setting the telescope accordingly in between exposures Retarder Plate Angles the retarder plate angles can be selected from a set of fixed predefined angles see Table 2 8 Filters for IPOL all imaging filters see section B can be used except the ones of the FORSI instrument standard configuration see section 2 2 which are located in the Wollaston wheel The use of the latter ones is in principle possible but requires a reconfiguration of the instrument This howeve
71. he Observing Mode The first step is to select the best observing mode according to the scientific needs In some cases there will be a choice between eg MOS and MXU mode for example to observe 10 targets well distributed over the FORS field of view and in this case the optimization of the strategy will start at this point In most cases the observing modes will be pre defined and only a limited number of observing templates are needed and have to be studied with the help of the FORS template manual in detail 3 2 Fast modes or FIMS mask preparation multi object observations in modes MOS MXU and PMOS will require the preparation of mask with FIMS Occult ing bar imaging and slitless spectroscopy is only supported with fims based modes Typically the mask design has to be ready before starting the preparation of the observing blocks Meanwhile all observations in modes IMG IPOL LSS and ECH are done without using FIMS as well as single target observations in PMOS mode For faint targets we support blind offset acquisition modes for all the fast modes this is done with the through slit templates The astrometric requirements are similar for blind fast acquisitions and FIMS acquisitions In general the OB execution in fast mode won t be much faster than the FIMS mode but the OB preparation will be 27 28 3 3 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Selecting the Instrument Setups and Exposure Times A good und
72. ith an accuracy as listed in Table 4 1 Applicants have to request additional observation time including overheads if much higher accuracy is required than given below or if the mode is not supported by the calibration plan In this case the respective observation blocks must be provided by the users The FORS Calibration Plan will ensure that ESO provides dark frames biases flat field frames and arc lamp spectra with the exceptions given below Observations of standard stars in broad band filters are executed to obtain photometric Zero points atmospheric extinction coeficients and first order color terms for the UBVRI filters For the other filters only one flux standard star close to airmass 1 is taken Spectra of spectro photometric standard stars with 5 arcsec slit width will provide response functions for the flux calibration of spectroscopic data The standards for the spectroscopic modes are all observed with the MOS slits in the center of the field to avoid additional target acquisition overheads Neither the longslits nor the MOS or MXU slits of the science setups are in the center of the field of view Therefore some part of the spectra won t overlap with the derived response function Please request special calibrations send OBs if this is problematic for your scientific data reduction Visitor mode observers are welcome to use calibration data taken in the framework of the FORS Calibration Plan They should expect about half an hour per nigh
73. l FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 29 3 5 OB preparation Fast modes 1 Get any imaging data and good target coordinates and very good astrometry in case of blind offset acquisitions see section 2 4 2 and prepare finding charts with targets slit positions and reference stars for blind offset acquisitions 2 Selectthe observing mode the instrument setup and calculate the exposure times with the exposure time calculator 3 Prepare the observing blocks a typical OB in imaging mode fast IMG mode will consist of two templates FORS img acq target acquisition FORS img obs crsplit Science exposure or similar for imaging polarimetry FORS ipol acq fast target acquisition FORS ipol obs off fast Science exposures For all spectroscopic modes a through slit image is required to verify the proper position of the target on the slit For fast observing modes LSS ECH SPECPHOT or PMOS the OB would typically consist of the following three template FORS Iss acq fast target acquisition FORS Iss obs slit fast through slit image FORS Iss obs off fast Science exposures here for LSS mode but very similarly for the other spectroscopic modes For blind acquisitions in fast modes LSS ECH and PMOS the coordinates of the reference star will be required for the target acquisition The offset from the reference star to the target will be executed from the through slit image template after fine adjustment of the re
74. l appear brighter while the residual image motion is in direction of the moving charges on the CCD High accuracy photometry can not be done with the HIT modes unless a nearby star can be used as a reference source 2 6 2 High Time Resolution Mode Imaging HITI The MOS slits will be placed to the extreme left side 3 arcminutes and opened to a user defined typically broad slit width The mode HITI can be used with any available FORS2 filter of the FORS2 standard configuration and the exchangeable interference filters Accurate photometry on a 1 level is only possible if there is a nearby star observed simultanously on the slit as a flux reference Another requirement is to select a slitwidth which is larger then the actual seeing The residual guiding offsets would reduce the performance to about the 10 level without the differential measurement of a reference star The atmospheric effects on the image motion would be only corrected in case of a reference star within the isoplanatic angle This effect is however thought to be relatively small for large telescopes This parameter is still to be optimized FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 19 2 6 3 High Time Resolution Mode Spectroscopy HITS The readout direction is for FORS2 in spectral direction for the standard FORS2 grisms Only the cross disperser grisms XGRIS_600B and XGRIS_300I can be used for the the HITS mode There are 7 masks available with slit widths
75. l the complete detector is used as storage for the data A part of the FORS2 MIT detector mosaic is vignetted by the FORS2 top unit Therefore not all 4096 columns can be used to store the data but only 3548 columns The charges are moved over this number of columns in the user specified times of 1s 4s 16s 64s 256s and 1024s The resulting frequencies of 0 28 to 289 milliseconds are not the effective time resolution the time resolutions is reduced by the seeing or the slit width in units of pixels For an image scale of 0 125 pxl and a seeing of 1 the time resolutions would be between 2 3 milliseconds and 2 3 seconds for the fastest and slowest modes HIT mode name one shifttime time resolution HIT OS1 1sec 15 0 00235 HIT OS2 4sec 4s HIT OS3 16sec 16s HIT OS4 64sec 64s HIT OS5 256sec 256s HIT OS6 1024sec 1024s 2 35 readout mode 100kHz 2x2 high was selected to get the lowest possible readout noise level frames will be binned at the readout time The CCD parameters like the binning are deeply hidden in the CCD configuration file and can not be changed during normal operation About 40 seconds overhead time is expected to readout the full mosaic detector and to handle the data files The fundamental problem with the HIT modes is that even smallest image motions due to atmospheric effects or residual guiding offsets will strongly compromise the photometric accuracy of the measurements Note that the respective targets wil
76. lates described in the FORS Template Reference Guide More information on the RRM can be found on the USG Phase II webpages http www eso org observing p2pp rrm html 22 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 8 Detectors 2 8 General Properties Chip Characteristics Pixel Number and Size CCD Control the FORSI detector is a 2048x2048 Tektronix CCD thinned and anti reflection coated The pixel size is 24 x 24 um The surface of the curved CCD can be fitted with sphere of a radius of 2300 mm FORS1 The curvature is taken into account by the camera field lens The new detector mosaic of FORS2 consists of two 2kx4k MIT CCDs thinned and anti reflection coated The detectors are flat and the bottom detector is rotated by 0 08 degree and shifted by 30um with respect to the upper master detector The gap between the two detectors is 480um The CCDs are controlled by FIERA controllers The detector systems are not interchangeable between both instruments Read out Modes the FORS1 CCD can be read in single port A and four port read out mode with amplifier ports ABCD the four ports are located at the corners of the chips Two ADU level settings are implemented high and low Three pixel binnings are possible 1x1 2x2 4x4 The allowed combinations of read out ports ADU level settings and pixel binnings are listed in Table 2 9 The CCD is read with a speed of 50kHz In case of FORS2 only the 2 port readout is supported
77. ld cause hardly correctable Stokes parameter cross talks in the case of objects with non negligible linear polarization The color dependence of the retarder angle would cause an additional polarization of AV 260 and the incomplete retardation 90 degree quarter wave would cause the additional polarization of AV 00 p amp eg in radians UVQ being the Stokes parameters One would get fo rye Fe F lo 45 The difference between the two observations yields V while the small deviations have the same sign in the two equations and are therefore eliminated for small angles 62 e 5 V err 260 52 9 45 V 2e U ep 4 3 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 39 4 T T T chromatic zero angle A 2 plate 4 1 1 1 1 1 1 1 1 3000 4000 5000 6000 7000 8000 9000 10000 11000 A Figure 4 1 Zero angle chromatism of the half wave plate 4 6 2 Linear Polarimetry After the pre reduction of the spectroscopic data and integration of the ordinary and extraordinary target spectra or flux f 6 and f 6 the normalized flux differences F 6 must be calculated fe f 8 fO EEN where 6 i 22 5 is the angle of the retarder plate 0 lt i lt 15 F 4 4 If the polarimetry is obtained from the normalized flux differences no absolute flux calibration of the data is requir
78. lescope is minus the position angle of the targets on the sky Note that a value of 9999 can be used to set the position angle to the parallactic angle 3 8 4 Atmospheric Dispersion Compensation Atmospheric dispersion is partially compensated by a linear atmospheric dispersion compensator LADC which is built into the MI cell of the telescope in front of the Cassegrain focus It is designed to maintain the intrinsic image quality of FORS for zenith distances between 0 and 45 and to significantly reduce the effects of the atmospheric dispersion at higher airmass The LADC position is automatically set when the telescope is preset to the target position and can not be corrected during the exposure It is recommended to reset the LADC after significant changes in airmass during long series of exposures At zenith distance larger then 45 degree the LADC prisms remain however always at the maximum separation Although placed in front of the polarization optics there are no negative impacts instrumental polarization for polarimetric measurements expected or known l The LADC is described G Avila G Rupprecht J Beckers Atmospheric Dispersion Correction for FORS Focal Reducers at the ESO VLT Optical Telescopes of Today and Tomorrow A Ardeberg ed Proc SPIE 2871 1135 1997 4 Calibrating and Reducing FORS Data 4 1 Calibration Plan The VLT observatory aims at providing calibrations of the FORS instruments w
79. lter is located in one of the interference filter wheels as it is physically designed as an interference filter 4 these are intended as order separation filters for spectroscopy 45 46 transmission transmission transmission FORS1 2 User Manual Issue 2 8 0 55 100 T 80 F 60 7 SEEN jns 4 L ch 40 20 NEN 300 350 400 450 500 wavelength nm V BESS 100 5 80 L 4 60 F S 40 Kag ER Sai Le A 20 F 0 id fe p t jns Ps Ee 400 450 500 550 600 650 700 wavelength nm H SPECIAL 100 EE EE L E a d 4 80 sj 60 40 20 4 0 E KEEN 550 600 650 700 750 800 850 wavelength nm transmission transmission transmission VLT MAN ESO 13100 1543 B_BESS 100 80 r zl SN P d d Y A 40 20 1 0 SCH 300 350 400 450 500 550 600 wavelength nm R BESS 100 TE xU d ET d S 40 BN 20 E 4 L mm EL LI 550 600 650 700 750 800 850 wavelength nm I_BESS 100 EE 80 4r 20 L 0 Ji Lilli 650 700 750 800 850 900 950 wavelength nm Figure B 1 Bessell filter transmission curves U BESS and R BESS are only available for FORSI R SPECIAL is a
80. lts for the mask preparations In general 1st fsmosaic and then imcombine Pipeline support The quality control group is planning to deliver reduced science frames to applicants which have requested pre imaging runs with the MIT mosaic The reduced and merged files can be combined with the standard tools The description of the functionality of the fsmosaic plug in is given in the fims manual see section 1 4 4 Flat Fielding 4 4 1 Imaging Mode Best results for flat fielding are obtained if the illumination is as similar as possible to that of the science frames This can be achieved from 4 science frames with adequate S N of the sky background taken with offsets of 75 fields should not be too crowded as well This observing mode is supported by the corresponding templates In order to achieve a suitable S N of the resulting super flatfield a larger number of science frames may be needed if the sky level is low If this is not guaranteed twilight sky flats should be taken in addition Night flats need to be carefully checked for remaining stars Master night flats are processed by the reduction pipeline Templates are also available which for any desired filter generate sky flats during dusk or dawn automatically determin ing the required exposure time from a brief windowed exposure and taking into account the decreasing or increasing sky brightness in the evening or morning Flat fielding from these exposures will however not remove larg
81. m Acentral Arange dispersion filter A mm A pixel Acentral XGRIS_600B 92 4452 3300 6012 50 0 75 780 XGRIS_3001 91 8575 6000 11000 108 1 62 660 0G590 32 XGRIS_3001 91 8575 5032 6600 108 1 62 660 XGRIS_600R announced in earlier versions can not be used The central wavelength is defined as the wavelength Acentra in the center of the field of view The gap between the two CCDs will cause a gap of about 7 pixels in the spectra at a wavelength of approximately Jana 267pxl dispersion Visitor mode only cross disperser grisms not included in the FORS2 standard configuration There will be no instrument setup changes according to the service mode rules and accordingly the spectroscopic HITS mode is only offered in visitor mode HITI imaging mode is offered both in visitor and service mode 2 6 4 OB preparation The HIT mode templates for modes HITI and HITS are all of fast mode target acquisition type There is no mask preparation required for the phase 2 observation block OB preparation There are special templates available for the two modes Three observations templates for the night time science observations for target acquisitions through slit images and the science observation Additionally flat field templates for HITI and HITS mode and an arc line spectral template for HITS mode For the HITI imaging mode the OB would consist of three templates in the following order FORS2_hiti_
82. n FORS img occ crsplit Science exposure or similar for imaging polarimetry FORS ipol acq target acquisition FORS ipol obs off Science exposures For all spectroscopic modes a through slit image is required to verify the proper centering of the target on the slit For observing modes MOS MXU LSS or PMOS the OB would typically consist of the following three templates FORS mos acq target acquisition FORS mos obs slit through slit image FORS mos obs off Science exposures here the MOS mode as an example but with an identical sequence of observing templates for the other spectro Scopic modes In case you want special calibrations not included in the FORS calibration plan section 4 1 a calibration OB has to be prepared which would look like the following scheme again for the MOS mode example FORS ima cal coll collimator selection FORS mos cal scrflat screen flats FORS mos cal wave Screen arcs where the first template is only used to select the collimator There are a few important points to be verified now a don t mix observing modes in one OB b make sure that all fims input files belong to the same mask in general only one mask per OB is possible The keyword INS FIMS NAME on the top of the p focf p targ and p gbr files must be identical c be sure that the requirement for reference stars and reference slits in MXU mode are fulfilled the details about the reference star selection are explained in the fims manua
83. n Table 4 6 Measuring a polarization angle of e g 0 134 20 deg in the Bessel B filter one would correct this raw measurement to a final result of 0 132 66 deg The offset angles should be confirmed periodically by the observation of polarized standard stars 4 7 Pipeline Reduction A data reduction pipeline is operational for FORS1 and FORS2 4 7 Supported modes The FORS pipeline presently supports two instrumental modes imaging IMG and longslit spectroscopy LSS It provides e creation of master calibration products e reduction of science data e photometric zero points and spectral response For IMG data the raw data are bias subtracted and flat fielded The single frames taken within a sequence are not combined LSS data in addition to de biasing and flat fielding high spatial frequencies only are rebinned to wavelength space No correction for instrumental response is done No night sky subtraction and source extraction is applied and the single frames taken within a sequence are not combined 4 7 2 Quality Control Pipeline Service Mode Only data taken in service mode are reduced by the quality control group in Garching Master Calibration Data As part of the service mode concept the master calibration data set is provided to the service mode observer All raw calibration data from the pipeline supported modes of FORS regardless of whether they are obtained in visitor or in service mode are processed to
84. ncy curves of the low resolution grisms The vertical lines mark the approximate limits of the spectral range with the slit in the center of the field The cutoff wavelength is in most cases given by the order separation filters http www eso org instruments fors grisms fl html http www eso org instruments fors grisms f2 html http www eso org instruments fors grisms f2 2nd Order Let wavelength nm the red CCD limit or the 330nm limit of the FORS optics in the blue 50 GRIS_300V GRIS 300I 100 100 80 SEN 80 5 F g mW 1 60 4 o 60 So E d 8 xS Ne 404 4 amp 40 4 n n 74 d 4 4 20 F 20 0 da Pf 1 0 rene E 200 300 400 500 600 700 800 9001000 500 600 700 800 900 100011001200 wavelength nm wavelength nm GRIS 2001 GRIS_ 1501 100 T T 100 80 C 80 _ x Fw 4 Bx Te 7 60 60F e n S Fi D H J a 40 4 amp 404 4 a 7 J i d 4 4 a 2 20 F 20 0 1 l l l E ban pem 854 ce epe fy 600 700 800 900 1000 1100 400 500 600 700 800 900 1000 1100 wavelength nm response response 100 80 60 40 20 100 80 60 40 20 FORS1 2 User Manual
85. nitude smaller 18 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 6 High Time Resolution Modes 2 6 1 Overview The principle of the high time resolution HIT mode is to move the charges in positive x direction on the CCD while integrating the incoming light with the exposure shutter open The time resolved spectra or light curves are stored on the CCD which is then read out at the end of the sequence with the mode of lowest read out noise 100kHz 2x2 high The HIT mode allows spectroscopic observation for a single target on a square aperture and imaging light curves of one or two stars on a long slit For the spectroscopic mode the target is centered on an aperture on the extreme left side of the unvignetted field of view This aperture is punched on pre fabricated masks to be installed in the FORS2 mask exchange unit MXU There will be masks for various aperture sizes of up to 5 arcseconds For the imaging modes the movable slit blades of the FORS MOS unit are used with all slits opened by a user specified slit width and placed to the extreme left side of the field of view The position angle of the instrument can be selected such that a second target may be observed simultaneously Please note that the HIT mode observations were only configured for the standard resolution collimator COLL SR For the time being only one shift modes are offered One shift mode denotes that the charges are moved at constant speed on the detector unti
86. obs slit 2 loops instrument setup 30s instrument setup 30s through slit integration time 260s 120s through slit integration time 2 60s 120s through slit image 2 120s 240s through slit image 2 120s 240s FORS mos obs off NEXP 1 amp NOFF 1_ FORS mos obs off 1 amp NOFF 1 instrument setup 30s instrument setup 30s science integration 1 3000s 3000s science integration 1 3000s 3000s 1 port CCD readout 1 122s 122s 100kHz 2x2 CCD readout 1 41 Als all OB execution time 4012s all OB execution time 3931s FORS _img_acq FORS2 img acq telescope preset 180s telescope preset 180s guide star acquisition 45s guide star acquisition 45s active optics 2 loops 120s active optics 2 loops 120s FORS img obs crsplit NEXP 1 amp NOFF 5 FORS img obs crsplit 1 amp NOFF 5 instrument setup 30s instrument setup 30s science integration 5 600s 3000s science integration 5 600s 3000s 4 port CCD readout 5 49s 245s 200kHz 2x2 CCD readout 5 3 1s 155s all OB execution time 3620s all OB execution time 3530s There would be an additional overhead of 270 seconds to exchange the collimators but this setup is partly executed during the telescope preset and the guide star and active optics setup procedure Further overheads of 60 seconds per 51 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 31 template exist for the PMOS and IPOL science templates to setup the retarder plates This is now the
87. ode Imaging HITD 18 2 6 3 High Time Resolution Mode Spectroscopy HITS 19 26 4 gt x sels Ae a oe Re ADR ROSE EN ge 19 2 6 5 Calibration plan i cuu e pue PR IR EUR dE Goa as 20 26 6 Perfornanceonthesky 2 22 2254 ge Ro Ep EE 20 2 7 Rapid Response 21 2 8 Detectors Rex v MR be epe tee ead rut o 22 2 81 GeneralProperties bu deeem ie Se A DU 22 2 8 2 BP nges oou epe VERRE en 23 218237 SHUUCES dus ap Bed amp Roni fe wae eben s Ree 44 24 2 8 4 CCD Contaminationy 24 xs ete Spe REGUM WA S dt 24 2 9 The Cahlibration Unit s ue e tad RE oO RUE AY RR n 25 2 9 1 Parasitic Light in Longslit and MOS PMOS Flatfields 25 2 10 Forthcoming New Equipment and Observing Modes 26 iii iv FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 11 Retired Instrument Components llle 26 Observing with FORS 27 3 1 Selecting the Observing Mode 27 3 2 Fast modes FIMS mask preparation 27 3 3 Selecting the Instrument Setups and Exposure Times 28 3 4 FIMS
88. ombination of grism with 1 of the other filters then the order separation filters are supported if the two components are mounted in different wheels Combination of two filters at the same time are generally not supported in normal operation since these setups would require testing and software reconfiguration FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Enty Name MOS LSS MXU Focal area COLL SR 6 COLL_HR 7 GRIS 300V 20 GRIS 300I 21 GRIS_2001 28 _1501 27 6008 92 GRIS_600RI 19 GRIS_600z 23 GRIS_1028z 29 GRIS_1400V 18 GRIS_600B 22 GRIS_1200R 93 GRIS_6001 25 z GUNN 78 EGRIS 110B 98 GG375 80 GG435 81 0G590 32 B_BESS 74 V_BESS 75 R_SPECIAL 76 I_BESS 77 Collimator unit Wheel 1 Wollaston wheel Wheel 2 grism wheel Wheel 3 broadband filter Wheel 4 interference filter U_SPECIAL 73 FILT_465_250 82 19 slitlet multi object spectroscopy unit 9 slit longslit mask unit mask exchange unit for multi object spectroscopy with up to 10 masks Standard resolution collimator High resolution collimator Grism 300V Grism 3001 Grism 2001 Grism 1501 Cross disperser grism 600B Grism 600RI Grism 600z Grism 1028z Grism 1400V Grism 600B Grism 1200R Grism 6001 Gunn z filter Echelle grism 110B Order sorting filter GG375 Order sorting filter GG435 Order sorting filter OG590 Bessel B filter Bessel V filter Special R filter Bessel I filter Order
89. ometry SR HR annually dark current CCD check distortion scale 1 pixel Imaging Sky Flats SR weekly 1 Twilight normalized fht HR as needed Twilight normalized fht UBVRI photom std SR nightly 1 2 Night Zero points UBVRI photom std as needed Night Zero points Flux std Gunn amp other fi 1 as needed Night response ters AM 1 6 UBVRI std weekly 1 2 Screen Flats LSS MOS as needed MXU ECH Screen Arcs LSS MOS as needed MXU ECH Flux std spectroscopic as needed 5 Imaging Sky Flats with as needed out polarizers IPOL polarized std as needed IPOL unpolarized std annually IPOL unpolarized std annually PMOS arcs as needed PMOS fats 45 degree as needed PMOS polarized std as needed PMOS unpolarized std annually PMOS unpolarized std annually Night extinction coeff normalized fht 596 dispersion coeff 0 3 pixel 3 response 1096 Twilight normalized fht 2 Night zero angle lin 1 degree Night instr pol lin Night instr pol cir dispersion coeff 0 3 pixel 3 normalized fht 596 zero angle lin 1 degree instr pol lin instr pol cir Table 4 1 FORS Calibration Plan Tasks 1 only during FORS observing runs 2 for U BVRI filters only and under photometric conditions only 3 internal accuracy not considering instrumental flexures see section 2 4 3 4 Frequency as needed denotes that the calibration task is done if the subsequent mode was used 5 Please note that the flux std to calib
90. ordinate system 68 IMG mode 7 IPOL mode 16 instrument performance 17 restrictions 16 LADC 30 LSS mode 11 slit orientation 67 x offsets 13 manuals 1 MOS mode 13 fims only 13 movable slits 13 slit lengths 13 slit orientation 66 slitless spectroscopy 15 MXU mode 14 restrictions 14 SR collimator only 14 target acquisition 14 visitor mode arrival time 14 observing 25 fast or fims 25 OB preparation 26 OCC mode 8 overhead times 28 example 28 P2PP WEB page 1 Paranal Science Operations contact information 2 WEB page 1 pipeline data reduction 38 PMOS mode 16 instrument performance 17 restrictions 16 polarimetry 16 chromatism of the half wave plate 36 37 circular polarization 36 76 FORS1 2 User Manual Issue 2 8 imaging polarimetry 16 instrument performance 17 linear polarization 36 slitless spectro polarimetry 16 spectro polarimetry 16 replaced components 23 service mode observations contact information 2 observing with FORS 25 WEB page 25 slitless spectroscopy 15 SPECPHOT mode 13 spectroscopy 10 astrometric requirements 10 33 catalog of the HgCd He Ne and Ar lines 52 data reduction for pre imaging data 33 echelle grism response 51 field of view 10 flat fields 34 grisms holographic 10 grisms standard 10 grisms response 48 instrument flexures 11 lamp exposure times 35 longslits 11 order separation filters 10 other filters 10 ov
91. p For HR observations in imaging mode the MOS slit arms are also used to form a field stop mask to limit the field in the focal area of the instrument and thus to reduce stray light 2 3 5 Occulting Masks Individual arms of the MOS unit can be used in the direct imaging modes this includes also imaging polarimetry to block light from bright objects next to very faint ones In this case the use of the FIMS software tool is mandatory for the preparation of the observations for details see the FORS1 2 FIMS Manual 2 3 6 Image Motion due to Flexure Image motion due to instrument flexure under gravity is below 0 25 pixel over a 1 hour exposure with the standard and a 2 hour exposure with the high resolution collimator for zenith distances less then 60 10 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 2 4 Spectroscopy Spectroscopy Modes the FORS instruments offer four spectroscopic observation modes LSS MOS MXU and ECH With the exception of MXU and MOS spectroscopy of FORS2 SR collimator only all spectroscopy modes are sup ported for both collimators The HR collimator will project the slit image with the double size to the CCD with respect to the SR mode and the spectral resolution in HR mode will be therefore reduced by a factor of two variety of grisms with different wavelength ranges and dispersions is available see Table 2 5 The grisms can be combined with filters for order separation or more specialized settin
92. phased holographic gratings which are cemented between two glass prisms see Figures C 2 for the 1st order throughput measurements A special note about grisms 600RI and 1400V of FORS2 Due to manufacturing errors a tilt of the light beam is induced for grisms GRIS_1400V 18 and GRIS_600RI 19 which shifts the spectrum on the detector in Y direction by 111 and 272 pixels unbinned 15 micron pixels as compared to the object position in the through slit image There should be no part of the spectrum lost for grism 1400V since the MIT CCD mosaic is large enough to receive all the tilted light For grism GRIS 600RI 19 the expected consequences will be that the uppermost 21 arc seconds of the field of view will fall off the CCD in SR mode Order Separation Filters order sorting filters are available to allow for the suppression of spectral order overlaps in the spectra Order separation filters are installed in the broadband filter wheel with the exception of the blue band pass filter FILT_465_250 82 to be used for 2nd order observations and the suppression of scattered light in Echelle mode Other FORS Filters normal broad band medium and narrow band filters can also be combined with the grisms but only one filter at a given time and only filters which are not mounted in the same wheel as the user selected grism Grism and Filter Transmission efficiency curves of the available grisms are presented in Appendix C For the filter characteristics see A
93. ppendix B 2 4 2 Relative Astrometric Accuracy Requirements for Spectroscopy Highly accurate relative astrometry is required for any observing mode which will make use of FIMS or blind offset acquisitions The mask preparation with FIMS requires input images which are astrometrically corrected within the definitions and precision given below DSS images will in almost all cases not be suitable for the task In general the relative astrometry must be known better than 1 6 of the slit widths all over the field of view Relative astrometry here means that the slit positions must be known relative to those of reference stars in the field of view with the given precision To achieve such an astrometric calibration based on stars in your field is difficult It is recommended to cross check the values for the image scale and field distortion in other fields whenever possible in fields with astrometric standard stars these relative astrometric calibrations are not required if your FIMS preparation is based on pre images taken with I see eg Zacharias et al 2000 AJ 120 p2131 or SDSS Stoughton et al 2002 AJ 123 p485 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 11 any of the FORS instruments Itis strongly recommended to search in the VLT Science Archive http archive eso for released FORS imaging data Restrictions for pre images to be used for the mask preparations The target acquisition procedures were re
94. r with r r 15 24 binning Ar Ar 15 24 binning The images scale was determined using astrometric standard stars in the star clusters 47Tuc and Pal 3 in several nights during commissioning of the instruments The plate scales have also been measured in June 2004 when FORS2 was moved to Antu and FORSI to Kueyen In this case three fields of standard UCAC2 stars in the vicinity of the cluster Centauri have been used The measured values are given in the table For FORS2 the scale is given for unbinned 15 micron pixels in SR mode FORST FORSI Antu SR 47Tuc 0 20013 0 00005 mur 099754000008 FORS2 Yepun 0 12604 0 00003 FORS2 Yepun 0 12607 0 00003 FORS2 Yepun 0 06323 0 00003 FORS2 Antu SR Q Cen 0 12591 0 00002 FORS2 Antu SR Q Cen 0 12591 0 00002 FORS2 Antu SR Cen 0 12590 0 00003 4 3 Data Reduction of Pre Imaging Data for the Mask Preparation Pre imaging data delivery As soon as a pre image is successfully taken the data will be immediately transfered to the ESO data archive in Garching where it will be automatically reduced bias subtraction and flat fielding Reduced and raw data will then be available on a dedicated ftp account Detailed instructions on where to retrieve the data from as well as further information is send to the user by e mail typically the day after the pre image was taken Please note that the data must be fetched from its ftp location within a certain range of time usually
95. r is considered for visitor mode observations FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 17 Retarder Plate Position Angles deg circular 45 45 135 225 linear 0 22 5 45 67 5 90 112 5 135 157 5 180 202 5 225 247 5 270 292 5 315 337 5 Table 2 8 The table lists the angles of the predefined retarder plate positions which can be selected for imaging and spectropolarimetry with 1 To achieve the highest accuracy we are recommending to take exposures with the highlighted plate position angles only and needs a priory approval by the observatory before proposal submission Target Acquisition in IPOL Only Fast modes are available In the fast mode the object is selected at the instrument console by mouse click in an acquisition image and the telescope is then offset such that the target is at the center field position of MOS slit 10 FIMS can still be used PMOS mode with all slits open to simulate the focal field geometry in cases of rather complex target distribution 2 5 2 Spectropolarimetry PMOS mode MOS Slit Strip Mask for Spectropolarimetry PMOS spectropolarimetry PMOS using MOS slitlets is possible with the standard resolution collimator only In this mode the MOS slitlet arms with odd numbers are positioned to form the same strip mask as for imaging polarimetry The even numbered slitlets are available as in the normal MOS mode i e they can be positioned on the objects in the field of
96. rate LSS mode is taken with a MOS slit of 5 at the center of the field to include all the flux If you want the std to be observed with the same 1 55 slit you have to provide a special calibration OB 10 any IPOL day or night time calibrations with COLL HR The observatory staff will prepare a day time calibration OB in the morning with biases screen flats and arc lamp spectra for all spectroscopic and spectro polarimetric setups This is done with the semi automatic calobBuilt software Calibrations according to item 5 and 6 are hard to configure in an automatic tool and therefore not included in the calibration plan Calibrations according to item 8 and 9 are thought to be not very usefull for the data reduction and therefore not included In all other cases the respective calibrations are not supported by the calibration plan to keep the time for the calibration plan within some reasonable limits The daily maintenance activities of telescope and instruments must not be compromised by extensive calibration requests by visiting or staff astronomers We will have to keep it as short as possible or the calibrations must be interupted postponed or even partly canceled in case of scheduled or urgent maintenance and setup activities 4 2 Image Field Distortion and Scales The image distortion was measured on an astrometric standard star field in 47Tuc Tucholke 1992 A amp AS 93 293 for FORS1 and FORS2 and in the field of cluster Pal for FORS
97. ric zero points are routinely calculated for the standard Bessell or special broadband filters of the instrument They are provided to the users as part of the service mode package They will be published on the web instrument page Science Data science data are pipeline processed if they are obtained in service mode Any standard mode IMG observation either of the 4 CCD modes if 1x1 binning no window one of the 5 standard filters either collimator can expect a reduced file LSS observations are reduced if taken in single port readout mode 4 7 3 Paranal Science Operation Pipeline IMG LSS mode only In parallel to Garching the FORS pipeline is in operation on Paranal This allows the staff and visiting astronomer to better estimate the quality of the data The on site pipeline is operated with calibration data provided by the quality control group and therefore are not the most recent ones Note also that the calibration database can be incomplete in particular in longslit mode due to the high number of longslits grisms and filters combinations and therefore only a part of the data will be processed The Paranal pipeline works on a dedicated machine Reduced science data are computed shortly after they have been exposed and are transmitted for inspection to the off line user workstation The on site pipeline will deliver the following products e master bias frames e master twilight flats IMG mode e flat fielded science images I
98. s for a linear scale 1 CRVAL1 1 2 CRVAL2 CRPIX2 CDELT1 1 CDELT2 2 EQUINOX RA TAN 12 345678 512 0 DEC TAN 12 34567 525 5 3 234E 5 19 0 3 234 5 19 0 2000 0 tangential projection type x coord of reference pixel RA in deg X coord of reference pixel PIXEL tangential projection type y coord of reference pixel in deg y coord of reference pixel Pixel scale degrees per pixel rot in degrees from N to E y scale degrees per pixel rot in degrees from N to E equinox ESO instruments where PC keywords are the rotation matrix CTYPE1 CRVAL1 1 2 CRVAL2 CRPIX2 CDELT1 CDELT2 001001 001002 002001 002002 EQUINOX RA TAN 12 345678 512 0 DEC TAN 12 34567 525 5 3 234E 5 3 234E 5 0 9848 0 1736 0 9848 0 1736 2000 0 tangential projection type X coord of reference pixel in deg X coord of reference pixel PIXEL tangential projection type y coord of reference pixel DEC in deg y coord of reference pixel Pixel scale degrees per pixel y scale degrees per pixel cos CROTA Sin CROTA sin CROTA cos CROTA equinox A third notation for WCS FITS header keywords is the CDi_j notation Transformation formulae between the different keyword notations
99. servation blocks are to be prepared by the users 2 6 6 Performance on the sky The limiting magnitudes to reach a signal to noise ratio of S N 5 as obtained in every 2x2 binned pixels for the different grisms are given below The value was calculated for the center of the wavelength range at dark time The S N would drop strongly in the blue part of grism 600B For the spectroscopic mode the S N is independent of the seeing for the very wide slit but time resolution and spectral resolution would both become worse in case of a bad seeing Here for the slowest readout mode of 1024 seconds per one shift grism limiting magnitude XGRIS 600B 15 8 XGRIS 300I 15 9 The expected number of counts per binned pixels can be derived by the following equation for a 10th magnitude star a dispersion of 0 75 pxl response 0 17 0 75 A pxl and an OS time of 256s t counts flux zr R resp disp d bin 2 1 3548 1000 1079410 x 4052 0 17 0 75 0 00028 256 22 2 2 1757 photons 2 3 In case of the imaging modes the number of parameters like seeing night sky brightness and the number of filters 15 very high and it s hard to present a meaningful table with limiting magnitudes here The expected count rates integrated in spatial direction no slit losses for a filter width of 1115A are estimated by the following equation for a 15th magnitude star time ts fi R fwhm bi 24 counts ux zt R x re
100. sm wheel and the broadband filter wheel in the parallel beam Furthermore the camera the interference filter wheels in the converging beam and the exposure shutter in front of the CCD Observing Modes FORS offers the observing modes tabulated below While the main observing modes IMG LSS MOS IPOL and ECH are supported for both collimators some restrictions apply in the modes MXU and MOS of FORS2 and PMOS of FORSI FORSI direct imaging IMG imaging with occulting bars OCC multi object spectroscopy with movable slitlets MOS longslit spectroscopy LSS imaging polarimetry IPOL multi object spectro polarimetry PMOS SR collimator only FORS2 direct imaging IMG imaging with occulting bars OCC multi object spectroscopy with masks SR collimator only multi object spectroscopy with movable slitlets MOS SR collimator only longslit spectroscopy LSS medium dispersion Echelle spectroscopy ECH FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 el Seele Collimator Drive Unit Section Filterwheels 7 Filter Camera Interference Section Filterwheels i YYA Figure 2 1 Schematic view of the FORS instruments Standard Resolution High Resolution 258 Figure 2 2 Light paths for the standard and high resolution collimators of FORS1 and FORS2 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 5 2 20 Standard Instrument Configurations Both FORS instruments are operated in
101. sp x fwhm 3548 bin 2 4 10001079415 4052 0 3 1115 0 00028 256 2 2 5 17288 photons 2 6 You may have to distribute the 17000 photons over the PSF and to devide with the gain factor of 0 7e to estimate peak flux values and the integrated signal to noise ratio 51000 1070410 photons cm s at 5500 the 1000 photons at 5500 for a Oth magnitude star is just nice number to remember FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 21 2 7 Rapid Response Mode for FORS Starting in Period 74 a new mode the Rapid Response Mode RRM is offered for observations of transient phenom ena such as gamma ray bursts or supernovae in semi automatic mode The user PI or Co I of an approved target of opportunity program submits an ftp file with the coordinates of the target to a specific ftp server on Paranal A special program at the telescope continuously monitors this ftp directory when it detects a file it checks if the filename corre sponds to an approved activation code and if this is the case the on going observations are ended and a new BOB starts an OB with the same name as the ftp file The telescope automatically presets to the coordinates specified in the ftp file and the requested observations are performed straight away PIs of approved FORS ToO programs requesting this mode need to prepare their OBs in the usual way However these RRM programs use specific acquisition temp
102. ssue 2 8 VLT MAN ESO 13100 1543 9 2 3 3 User Provided Filters The installation of user provided filters in the FORS instruments is subject of approval by the Director of Paranal and will only be considered upon recommendation of the ESO program committees OPC and DDTC The filters and their mounts must comply optically and mechanically with the specifications of the standard FORS filters and mounts which can be requested from the Instrumentation Division The diameter of user provided filters shall not be smaller than 138mm parallel beam to avoid vignetting which would be equivalent to a reduction of the main mirror diameter Interference filters 115 0 25mm are used in the converging beam of the camera Their spectral resolution shall not exceed 100 SR mode or 400 HR mode There is a limited number of filter mounts for converging beam filters only available in Garching to be sent to the users on request The filters fully assembled in the mounts must be made available to the Paranal Observatory at the latest 6 weeks before the start of the respective observing program execution They will be installed in the instrument and tested for compatibility and focusing during this time The Observatory reserves the right not to allow special filters to be mounted for observations in case of technical and or operational problems User provided filters are usually not allowed for FORS service mode observing programs 2 3 4 HR Collimator Field Sto
103. t to be used by observatory staff for calibration exposures In most case the staff will observe one field with photometric standards for the performance monitoring and a spectro photometric standard with a 5 arcsecs MOS slit for the setups used in the respective nights The calibration plan does not support 1 night time standard stars and twilight flats for non standard CCD modes as a baseline only the read out modes 1 1 6126 imaging A 1x1 high spectroscopy for FORS1 and 200kHz 2x2 low imaging 100kHz 2x2 high spectroscopy for FORS2 will be supported 2 any standard star observations to correct for telluric absorption lines 3 radial velocity standards Spectro photometric standards for 2nd order spectroscopy with FILT 465 250 any day or night calibrations for slitless spectroscopy ON any day or night calibrations for spectroscopy with filters other then the recommended order separation filters GG375 GG435 06590 and FILT 465 250 7 any day or night time polarimetric calibrations for retarder plate angles different from 0 22 5 45 67 5 degree lin ear and 45 45 degree circular polarimetry 8 any PMOS screen flats at retarder plate angles different from 45 0 degree 9 any IPOL screen flats 33 34 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Calibration Mode Frequency 4 Number Accuracy Bias weekly bias level RON RON 2 Darks monthly Screen Flats UBVRI SR weekly Astr
104. ters has approximately Gaussian shape The central wavelengths of the interference filters depend on the tilt angle of the incident beam Therefore all interference filters of FORS are used in the convergent beam only to minimize the field dependence of the filter curves For the given focal ratio of FORS the minimum recommended filter resolution 4 A4 will be 100 SR and 400 HR collimator Filter curves more narrow than this will be convolved and only the transmission will drop down The measured transmission parameters of the narrow band filters for the convergent beam are summarized in Table B 2 Medium band Interference Filters a few intermediate band filters are available to be shared with both FORS instru ments Table B 2 lists the filter names and the transmission characteristics Figure B 3 shows the transmission curves of the filters Image Offsets The V_BESS 75 filter of FORS2 is known to show a residual wedge angle which would displace the images slightly This filter should not be used for target acquisitions Other sources of image offsets would be the relatively small flexures of FORS and the atmospheric dispersion The later is corrected by the atmospheric dispersion corrector such that there should be no significant image offsets between the telescope guiding system and the respective images taken with FORS for zenith distances of up to 45 degrees it is partly correcting at even higher zenith distances 51 2 User Manual I
105. the standard configurations with certain opto mechanical components perma nently mounted in fixed positions This instrument configuration is kept frozen for a given observation period to ensure that all observations in service or visitor mode can be taken at any time without delays due to configuration changes The summaries of the FORSI and FORS2 standard configurations are listed below in Tables 2 1 and 2 2 The interference filters given in Table 2 4 and up to 10 MXU masks will be mounted on user request Please note that the instrument standard configurations will be only modified in exceptional cases upon request and with the a priory approval by ESO The requests should be submitted to usg help eso org before the beginning of an ESO observing period with a brief justification for the changes PPP Emmy Name Focal area MOS 19 slitlet multi object spectroscopy unit LSS 9 slit longslit mask unit polarimmask Mask unit for imaging polarimetry with HR collimator Standard resolution collimator High resolution collimator COLL_SR 1 COLL_HR 2 Collimator unit Retarder swing arm Wheel 1 Wollaston wheel Wheel 2 grism wheel Wheel 3 broadband filter Wheel 4 interference filter RETA4 4 RETA2 5 WOLL 34413 U_BESS 33 u_GUNN 38 v GUNN 39 r GUNN441 z GUNN442 GRIS 600V 04 GRIS_300V 10 GRIS_300I 11 GRIS_600B 12 GRIS_600R 14 GRIS_6001 15 GRIS_1501 17 GRIS_1200g 96 GG375 30 GG435431 0G590 72 B_BESS 34 V_BESS 35 R BESS4
106. tting figures appendix G note about the Echelle mode Jun 30 2004 all manual under pdf format updates for new FORSI Grism 12002 96 update of gain and ron of FORS1 CCD update of the plate scales due to the FORSI and 2 move to UT2 and 1 new Rapid Response Mode notes about the instrumen tal linear polarization the pre imaging data delivery the slit along parallactic angle the calibration plan in LSS Mode Editors E Jehin O Brien T Szeifert Paranal Science Operations ESO email ejehin G eso org kobrien eso org Contents 1 Introduction 1 US De EENEG KREE demos 74 1 1 2 More Information om FORS e REA NM ey AE 1 1 3 Contact Information SC E IO UP We d H 1 4 Acknowledgements Ae EN eh E uem 40 2 2 Instrument Characteristics 3 2l Overview s eu ev eg uet p erem 3 2 2 Standard Instrument Configurations 5 2 3 Direct Imaging IMG and OCC 7 2 3 1 Basic Characteristics of the Imaging 7 213 2 i The FORS Filter 90g RE RECEN Veg t as 7 2 333 User Provided Filters enee wu OAR AO RE RRP EE OR eg Reb alodus 9 2 3 44 HROCollimatorFieldStop 9 23 53
107. ument after maintenance However the centering accuracy of the objects on the slits is not affected by these variations in the on chip slit positions Target Acquisition on Slit target acquisition on the LSS mask slit can be done in the following ways 1 in case of fairly bright objects the fast mode acquisition can be used This basically involves a direct image of the target field and a mouse click on the object 2 for faint sources the acquisition can be done with blind offsets fast mode the offsets will be executed after centering a reference star on the slit template FORS1 2 Jos obs sit fast 12 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 Grism Acentral Arange dispersion A Aa fi lter A A A mm A pixel at Acentral FORSI standard GRIS_600B 12 3450 5900 50 1 20 GRIS_600V 94 6 4650 7100 49 1 18 GG375 30 GRIS_600V 94 6 4650 7100 49 1 18 GG435 31 GRIS_600R 14 5 5250 7450 45 1 08 GG435 31 5 6001 15 5 6900 9100 44 1 06 00590 72 GRIS 300V 10 1 3300 6600 112 2 64 GRIS 300V 10 1 3850 7500 112 2 64 GG375 30 GRIS_300V 10 4450 8650 112 2 69 GG435 31 5 3001 11 6000 11000 108 2 59 00590 72 _1501 17 1 3300 6500 230 5 52 GRIS_1501 17 1 3850 7500 230 5 52 GG375 30 _1501 17 1 4450 8700 230 5 52 GG435 31 _1501 17 6000 11000 230 5 52 06590 72 FORSI volume phased holographic GRIS 1200g 96 4880 4310 5490 24 4
108. vailable only for FORS2 U SPECIAL for 52 is still to be measured see Table for some preliminary information FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 47 u GUNN 100 w BOF x 4 2 60 4 ae aot Nn ES es S 20b 4 ME 0 Loa 300 350 400 450 500 wavelength nm v GUNN g GUNN BOF PE 80 vou J 80 j um Sgr i g g je 60 We ie 60 r m 7 7 N 1 M 1 404 40 Nn ul e J S 20 5 20 Ge L 1 L _ 0 0 L LN 300 350 400 450 500 400 450 500 550 600 wavelength nm wavelength nm r_GUNN z GUNN 100 SSES UU 100 EE eg L N J L Pg J M 80 X BO 2 Tus 1 5 4009 60 n n 2 1 1 40 40 d 4 sot 8 20 4 0 pe SAL 4 0 s uh ud eei op 4 550 600 650 700 750 800 850 800 850 900 950 1000 1050 1100 wavelength nm wavelength nm Figure B 2 Gunn filter transmission curves The Gunn filters uvgr only available with FORS1 z GUNN filters are available for both instruments 48 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 B 2 Interference Filters Table B 2 lists all presently availabl
109. view Slitless Spectropolarimetry slitless spectropolarimetry can be implemented for SR collimator in a similar way as for MOS but keeping the odd MOS slitlets in close position It is also possible with the HR collimator using the special HR strip mask as for IPOL observations fast acquisition mode only See section 2 4 9 for general comments on slitless spectroscopy Grisms and Filters for PMOS all grisms but GRIS 600V 94 together with the recommended order separation filters can be used in PMOS mode GRIS 600V 94 is configured for the Wollaston wheel and can t be mounted in the grism wheel Other filters together with these grisms can be used if the filter is not mounted in the Wollaston wheel see section 2 2 Retarder Plate Angles the retarder plate angles can be selected from a set of fixed predefined angles see Table 2 8 Collimator Constraints spectropolarimetry PMOS is possible only with the SR collimator in FORSI Target Acquisition for PMOS Fast and FIMS based acquisition modes are available but fast mode can only be applied for single target observations Multi object spectro polarimetry will require mask preparation with FIMS The fast mode will put the selected object on MOS slit 10 moved to the field center The other MOS slits are set up to the same position and slit width like slit 10 and can serve for sky background measurements Blind offset acquisitions are supported 2 5 3 Performance of the Polarimetric Modes of F
110. viewed and based on the latest astrometric measurements there should be no more restrictions in using FORS1 FORS2 and other astrometrically corrected images with world coordinate systems defined in the fits headers to prepare masks for any FORS instrument The fits headers of FORSI images taken before March 22 2003 would need to be corrected in the fits headers This should be discussed with the observatory staff usg help eso org before submitting the respective masks instrument pre imaging source alignment quality FORSI 1 after March 22 2003 optimum FORSI FORS2 optimum FORSI other images amp catalogs optimum FORS2 FORS2 optimum FORS2 FORSI after March 22 2003 optimum FORS2 other images amp catalogs optimum 2 4 3 Instrument Flexures The image motion due to instrument flexure under gravity is less then 0 25 pixel over a 1 hour exposure with the standard and a 2 hour exposure with the high resolution collimator for zenith distances less then 60 Arcs and flat are however taken at daytime and at the zenith This will introduce an offset between night time calibration based on telluric emission lines and day time calibrations based on arc lines depending on the zenith distance and the absolute angle of the Cassegrain rotator The passive flexure compensation of the FORS instruments based on support struts on the camera section was optimized down to the following small but not negligible image motions between zenith and the giv
111. will be minor since the effect on photon response is very small and the typical time scales of the growth of the contamination pattern is now of the order of months FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 25 speed binning default mode for FORSI Tektronix 2kx2k 50kHz 1x1 122s spectroscopy 2kx2k 50kHz 1x1 51 imaging 2kx2k 50kHz 2x2 ABCD 25s window 100 100 50kHz 1x1 undefined FORS2 MIT mosaic 2x4kx2k 100kHz 2x2 2 port spectroscopy 2x4kx2k 200kHz 2x2 2 port imaging 2x4kx2k 200kHz 1x1 Table 2 12 Approximate CCD readout times in the different read modes The read out times include the overheads during the exposure execution for CCD wiping header compilation The new MIT mosaic detectors of FORS2 are clean 2 9 The Calibration Units Each FORS instrument contains two sets of internal calibration lamp units in its top section The light from a variety of calibration lamps is projected onto a calibration screen inside the telescope located approximately 2 5m above the instrument All lamps can be switched on and off individually and in several combinations by means of calibration templates see FORS1 2 Template Manual Blue and red flat field lamps as well as Neon and Argon arc lamps are installed in both calibration units He and HgCd arc lamps are only installed in one of the two calibration units A guide to approximate exposure times is given in sections 4 4 and 4 5 a spectral atlas of the FORS
112. within a week The data delivery process starts as soon as the first pre image is taken i e not only after the whole pre imaging run is completed Shift and add only The mask preparation for FORS MOS PMOS and MXU modes will require that the original scale and field distortion is the same in reduced data as it was for the raw data This is required since the fims tool will correct for the scale distortion in case of FORS pre images at the time when the masks are saved Advanced techniques to combine jitter images such as drizzle will require some distortion corrections before the techniques will be applied It is strongly recommended only to use clean shift and add techniques eg IRAF imcombine to reduce images which are thought to be used for fims mask preparation MIT mosaic don t cut the edges In case of pre imaging data taken with the MIT mosaic detector it will be required to keep the original file format of the pre images Vignetted parts of the images and pre and overscan regions must not be cut before using the files with fims The plug in function fsmosaic delivered with the fims software can be used to merge the two files safely fsmosaic RAW INPUT FILE OUTPUT FILE The merged output files could be now combined with standard software such as imcombine eg for IRAF imcombine a 36 FORS1 2 User Manual Issue 2 8 VLT MAN ESO 13100 1543 median of the jittered files with the offset parameter set to wcs should give satisfactory resu
113. y combining the linear positioning of the slitlets in the focal area with a rotation of the FORS instrument around its optical axis a wide variety of object configurations can be realized MOS Slitlets 19 movable slitlets are available per instrument Even numbered slitlets are 20 long odd numbered slitlets 22 projected on the sky The approximate Y position within which objects should be positioned is slightly decreased by parasitic light falling between the slitlets Collimator Constraints MOS mode with the high resolution collimator is only supported with FORSI The standard resolution collimator allows to use all 19 MOS slitlets with the high resolution collimator on FORS1 only slitlets 6 to 14 can be used Target Acquisition with MOS MOS observations must be prepared using FIMS Reference stars are used to position the telescope and instrument such that the spectroscopy targets are in the slitlets of the predefined MOS mask 2 46 Wide Slit Spectro Photometry SPECPHOT mode For high accuracy spectro photometry a supplementary mode SPECPHOT was introduced which is used mostly for the monitoring of the instrument response in the framework of the FORS calibration plan The MOS slits are opened to 5 arcsecs slit width By default the slits will be placed to the center of the field in dispersion direction Alternatively the slits can be set to to the position of the FORS longslits or to any user defined offset position to the edge of the fi
114. ywords of the templates eg grisms filters slits the preparations are done with the phase 2 proposal preparation tool p2pp Furthermore FORS masks will have to be prepared with the FORS instrument mask simulator FIMS The detailed information for the observation preparation are given in the p2pp manual the FORS template manual and the FIMS manual The instructions how to retrieve the manuals from the WEB pages are given in Section 1 The strategy behind observing blocks and templates is to prepare the observations well in advance to minimize any interactive steps during the observations optimization and service mode compatibility The execution of the OBs will be mostly automatic and the execution will be done by telescope and instrument operators or the staff astronomers Direct interaction at execution time is needed only for the target identification and the quality control of the data or for real time decisions In the following we summarize the steps from a successful application to the final access of the data The preparation of service mode observations will require special care some more rules and recommendations since unclear points in the service mode packages will significantly delay the execution of the project The additional require ments and instructions for service mode observations are available on WEB pages http www eso org observing p2pp http www eso org observing p2pp ServiceMode html 3 1 Selecting t

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