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FORS User Manual

1. lo Con 4o o oo ob nos o do o Tt b ENO MN Q AN Qoo 2 ANO O TW A MN 0 dp F 3 SSK Sr 2 a8 28 Se 8 S S a5 fa DOS Ht E do nn BOJ 5 OG Ola BE S M C Mat vt SU iU Qe gt ro oo o CH D L 4 30000 H o e9 A Uu m pc L 4 L 4 n L 4 v E 0000 r o KL L A bel 0 LJ o es WEM ANU L JL TNT 0 500 1000 1500 2000 pixels Figure D 15 Calibration spectrum taken with the SR collimator and grism GRIS 300V 20 useful also for GRIS 300V 10 30000 T T T T b T T T T In T T T T I T T T T I s 0 925 X3 QN CO E 19 5 b 0 o s an FF Ny o 3 5 Se 8 B 2595 QNS Be 3 A la or gt o Oo na DOD Q S L 4 E 20000 E S e D E J We L El un n L 0000 H m o S E z 0 PAL AM UL S i A L 0 500 1000 1500 2000 pixels Figure D 16 Calibration spectrum taken with the SR collimator and grism GRIS 300I 21 68 FORS User Manual VLT MAN ESO 13100 1543 30000 E role da Q cO gt TROS LI to LAN Eu TS F 19 328 Oly m Sst DS o 4 DU 00 slo wo NV ND e OX HIV Wo NM Qr o QU t co oco DO DO o J co Q L 4 20000 e ci Ya pe L 4 Oo L 4 n b 4 0000 o S L E o
2. 0 500 1000 1500 2000 pixels Figure D 17 Calibration spectrum taken with the SR collimator and grism GRIS 20011 28 30000 OH T T T T T T T T T T vbosoomo q o doo OTNFFTNONOOM I I TANANAN ToD VPWODT TELO DOMOM 7 CDoDoDTOITU ONNO sp DN S GO o A00 sri 0 QU MOO HOPS PROMOM Late DCI e ODOMNTER AO HO 0NPO TONO 102001 o DO qq U KOD P P U O MO GOGO GO DODNDO Q N Ge J 20000 4 ive 3 un n D st L Nn n J E E 40000 o Ka E 4 bel 0 geen kal VL pet L MU Uy A 4A 0 500 1000 1500 2000 pixels Figure D 18 Calibration spectrum taken with the SR collimator and grism GRIS 150127 Appendix E FORS Image Orientation E 1 MOS Orientation The orientation of the FORS image in MOS mode is given below for rotator position O 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 0 position angle on sky 0 TOP MOS 1 north pixel 2000 2000
3. GRIS_600B 22 GRIS_600V 94 10077 r d m T 100 vm r TT 90r 4 90r 1 80r 80 L d Se 70H i 4 Se 7or g Z r 1 oo 1 2 sob 1 8 50r 1 5 40r 1 5 40r H 4 i i i 5 i D S i i E 30r 1 E 350r f 7 20 1 20r 5 10 E 10F 4 oLa 1 1 G 0 auch 1 iii 300 400 500 600 700 800 300 400 500 600 700 800 900 1000 1100 Wavelength nm Wavelength nm GRIS_600R 24 GRIS_600I 25 100 T 1 r T 100 r 1 T 1 T r 90r 4 90r 4 soh 1 80 J ze 7or i 1 Se 70 1 5 60r q 60 C 1 2 50r 4 50b 1 5 40r i 1 5 40r 7 2 e 5 i i 5 E Sor i i 7 E Sor 1 20r i 1 1 20r 1 10 J 10F 4 0 i 1 1 i d L 0 L L 1 L 1 400 500 600 700 800 900 1000 400 500 600 700 800 900 1000 1100 Wovelength nm Wavelength nm 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 FORS User Manual VLT MAN ESO 13100 1543 Transmission 72 Transmission 2 Transmission 100 90r 80 70 60 50 40 30 20 10 GRIS_1400V 18 VT 1 L 400 500 600 Wavelength nm GRIS_1200R 93 100 90r 80r 70r 60r 50 40r 30 20r 10 1 L 500 600 700 Wavelength nm GRIS_600RI 19 800 100 90r s0r 70r 6or 5or 40r 30 C 20r 10 D T 400 500 600 700 Wavelength
4. 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 6 The table lists the angles of the predefined retarder plate positions which can be selected for imaging and spectropolarimetry To achieve the highest accuracy we are recommending to take exposures with the highlighted plate position angles principle possible but requires a reconfiguration of the instrument This however is considered for visitor mode observations only and needs a priori 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 objec
5. The CCDs are controlled by FIERA controllers The obvious difference between the detector mosaics is the response for the MIT mosaic it is optimised for wavelengths gt 600 nm with low fringing level while for the E2V mosaic the response is optimised for wavelengths lt 550nm especially below 450 nm For this reason users should consider carefully which detector is best suited to their scientific goals remembering that the E2V detector is available in Visitor Mode only Read out Modes The default readout modes for both mosaics are 200kHz 2x2 low for imaging 2x2 binned low gain mode read with 200kHz and 100kHz 2x2 high for spectroscopy For special applications such as high resolution imaging or deconvolution techniques the following modes are supported 200kHz 1x1 low Imaging and 100kHz 1x1 high Spectroscopy Standard Operation Modes of the CCDs the following standard CCD set ups are offered for service mode observations e 200kHz 2x2 low direct imaging IMG OCC and imaging polarimetry IPOL e 100kHz 2x2 high spectroscopy LSS MOS PMOS and MXU Additional Operation Modes the following CCD set ups are available in service mode only with a pre approved waiver e 200kHz 1x1 low direct imaging IMG OCC and IPOL e 100kHz 1x1 high spectroscopy LSS MOS PMOS and MXU Visitor mode observations allow the full complement of CCD read outs However it is strongly recommended to use the standard read out modes whe
6. 2x4kx2k 200kHz 2x2 low 28s imaging 2x4kx2k 200kHz 1x1 low 56s high resolution imaging MIT mosaic 2x4kx2k 100kHz 2x2 high 41s spectroscopy 2x4k x2k 100kHz 1x1 high 82s high spatial resolution spectroscopy 2x4kx2k 200kHz 2x2 low 31s imaging 2x4kx2k 200kHz 1x1 low 62s high resolution imaging Table 2 10 Approximate CCD readout times in the different read out modes The read out times include the overheads during the exposure execution for CCD wiping header compilation 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 9 The Calibration Units FORS2 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 http www eso org sci facilities paranal instruments fors docs Blue and red flat field lamps as well as Neon and Argon arc lamps
7. ES EUROPEAN SOUTHERN OBSERVATORY Organisation Europ ene pour des Recherches Astronomiques dans l H misph re Austral Europ ische Organisation fir astronomische Forschung in der siidlichen Hemisph re ESO European Southern Observatory Karl Schwarzschild Str 2 D 85748 Garching bei M nchen Very Large Telescope Paranal Science Operations FORS User Manual Doc No VLT MAN ESO 13100 1543 Issue 89 1 Date 22 10 2011 I Saviane Prepared a ar d EE EEN EEN RR UA EN Date Signature A Smette Sela WEE Date Signature C Dumas Released spas 6 o4 2 13 AEN AOV Bue hie AN US Date Signature This page was intentionally left blank Change Record Issue Rev Date Sections affected Reason Remarks draft July 9 1998 all Draft delivered by VIC 1 0 Feb 11 1999 some first release 1 1 March 25 1999 all LADC setting recommended 1 2 August 5 1999 all document re arranged page and section numbering changed 1 3 Sept 1 1999 all more information on FORS2 1 4 Feb 1 2000 all revision for SM P65 amp proposals P66 1 5 July 10 2000 all revision for SM P66 amp proposals P67 2 0 Sept 17 2000 all revision after MXU commissioning and split up of manuals FORS1 2 User s FORS1 2 FIMS FORS1 2 Templates 2 Dec 27 2000 all revision for SM p67 and proposals P68 2 2 June 27 2001 all all sections restructured SM P68 amp CFP P69 2 3 Jan 5 2002 all MIT CCD mosaic detectors 2 4 May 22 2002 all upd
8. equinox A third notation for WCS FITS header keywords is the CDi_j notation Transformation formulae between the different keyword notations 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 CTYPE1 CRVAL1 CRPIX1 CTYPE2 CRVAL2 CRPIX2 CDI 1 CDI 2 cD2 1 CD2 2 EQUINOX RA TAN 12 345678 512 0 DEC TAN 12 34567 525 5 3 185E 5 5 616E 5 5 616E 5 3 185E 5 2000 0 a e ANS SNS e Se Tac ASS FORS User Manual VLT MAN ESO 13100 1543 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 DEC in deg y coord of reference pixel Pixel partial derivative partial derivative partial derivative partial derivative equinox Appendix G Field vignetting with the MIT CCD File View Graphics Go Data Servers FORS2 TOP 1 FORS2 fsmosaic_1007 x 1046 0 270 Value 5847 a 00 47 19 575 Bitpix 16 Low 5000 High i0000 Auto Set Cut Levels Scale Z 2 SI ZH s BOTTOM 13 img i image W select object scroll image measure WCS Control select region Figure G 1 The field of view of FORS2 with M
9. pixel Field of View 6 8x 6 8 42x42 Table 2 2 Optical properties of FORS Field vignetting and detector geometry with the FORS2 CCD mosaics The field of view of FORS with the MIT E2V 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 In case of the high resolution collimator the corners of the field of view are vignetted by the camera lenses The two CCDs of each mosaic are mounted slightly offset from the optical axis by 33 for operational reasons The center of the field of view will fall on y pixel 260 of the upper chip1 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 mosaics With the high sampling of the MIT E2V CCDs of 0125 pixel for the unbinned 15 um pixels it is possible to operate with the standard resolution collimator down to seeing values of about 0 35 without performance losses in respect to observation with the high resolution collimator Below seeing values of 073 the high resolution collimator is expected to improve the image quality in a significant way 2 3 2 The 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 i
10. 2 2 2 2 244585445 EE 402 2 03 E O BE 4 6 Wavelength Calibration oo lt doc cionado s 9 xoxo x VA A 4 6 Calibrating Polarimetric Measurements o o eee eee eee eee Go Cireular polarimetry lt se ans 22288 ES 3 VASE Roe baee BES 25 2 Liar Pola BY oci 2 r kk Ben uoa X 3 ES et e ADR A we a7 Pipeline Reduction 2 2 44 rias b b SAD E eee mco e Geese dds vom 9 P bd Ao BApLOTE ea dE aa eRe RE Ehe uo uk O I UR E f f l 452 Quality Control Pipeline o soc c egea gie fae aa A ee gy Boe 4 7 3 Paranal Science Operation Pipeline IMG LSS and MOS MXU modes only A Abbreviations and Acronyms B FORS Filter Characteristics B1 Broadband Milter ccs 24 0 4224 X xe BEES CR A Re xw e AA we B2 Interference Filters aaa zo ana pune eee eda den A E RA ohh ER Reo raed as C Efficiency Curves for the FORS Grisms Gl FORSI G sms cc cs oc cd oh anda 3 30 3 eu 3e X3 Lae ee eee RR D Wavelength Calibration Spectra for the FORS Standard Grisms E FORS Image Orientation EJ MOS Orientation e ec 622254442 45 eo Pee EEE vcr wo3 3 09 RE Re aa 34 E LSS Orientation 22m pape bebe eee Oe a de d r de Xo EEE Ee EES F World Coordinate System Information G Field vignetting with the MIT CCD Retired Instrument Components Modes 29 29 29 30 30 31 31 33 33 33 33 33 33 34 34 34 35 35 36 38 39 39 39 40 41 41 41 42 42 43 44 47 49 49 53 55 55 58 69 69 70 71 73 75 List of
11. Interference Filters the standard interference filters available for FORS2 are centered on important emission lines and on wavelengths 5 and 1096 longer The interference filters are located in the convergent beam in the camera and have a diameter of 115 mm Their wavefront error is less than A 4 within 25 mm The intrinsic transmission curves of the narrow band filters have 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 to minimize the field dependence of the filter curves For the given focal ratio of FORS the maximum recommended filter resolution A AA will be 100 SR and 400 HR collimator Filter curves that are narrower than this will be convolved and will only lead to a decrease in transmission 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 Table B 2 lists the filter names and the transmission characteristics Figure B 2 shows the transmission curves of the filters Image Offsets The sources of image offsets are the small flexures of FORS and the atmospheric dispersion both of which are relatively small effects The later is corrected by the atmospheric dispersion corrector such that there should be no significant image offsets between t
12. PMOS mode For faint targets we support blind offset acguisition modes for all the fast modes this is done with the through slit templates The astrometric reguirements are similar for blind fast acguisitions and FIMS acguisitions In general the OB execution in fast mode won t be much faster than the FIMS mode but the OB preparation will be 29 30 FORS User Manual VLT MAN ESO 13100 1543 3 3 Selecting the Instrument Setups and Exposure Times A good understanding 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 1 Get your pre imaging data or other astrometrically corrected images see section 2 4 2 2 Select the observing mode the instrument setup and calculate the exposure times with the exposure time calculator 3 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 The
13. RON e K e ADU A high 5 13 0 11 1 37 0 02 5 2 0 1 1 85 0 03 B high 5 57 0 11 1 70 0 04 5 510 1 2 00 0 02 C high 5 76 0 10 1 62 0 02 5 3 0 1 1 90 0 02 D high 5 87 0 15 1 73 0 03 5 510 1 1 8310 05 A low 5 89 0 16 2 74 0 06 5 6 0 1 2 62 0 03 B low 6 23 0 17 3 3510 06 5 9 0 1 2 81 0 05 C low 6 27 0 10 3 17 0 05 5 8 0 1 2 68 0 04 D low 6 40 0 15 3 31 0 06 5 7 0 1 2 61 0 03 Table H 2 The old FORS1 and FORS2 CCDs Detector readout noise and conversion factors of the old Tektronix and SITE FORS1 and FORS2 CCDs Port A of FORS1 is the lower left B the lower right C the upper left and D the upper right quadrant e Table H 1 lists the instrument components which are no longer offered for FORS and the time period during which they were used The Echelle mode with the two conventional grisms was known to perform bad in terms of its instrument response It was officially decommissioned in P77 To replace it a new volume phased holographic grism has been installed whith a response 3x higher than the Echelle mode or the respective 2nd order observations which also show relatively low performance The wavelength range is 3730 to 4970 with the central wavelength at 4340A and a dispersion of 0 61 A pxl The spectral resolution is equal to 1420 for a slit width of 1 The Second order observations were not supported beyond P82 as there was little or no demand for them and the space was needed in th
14. nm 800 900 1000 1100 Transmission Transmission Transmission GRIS_1200B 97 57 100 TT 1 La 500 Wavelength nm GRIS_1028z 29 100 L 1 800 900 Wavelength nm GRIS 6002423 1000 100 eor 50r 40r Sor 10 70 800 900 1000 Wavelength nm 1100 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 Appendix D Wavelength Calibration Spectra for the FORS Standard Grisms This Appendix gives a wavelength table for the calibration lamps used in 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 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 58 FORS User Manual VLT MAN ESO 13100 1543 Wavelength A Element Wavelength A Element 3610 500 Cd 7065 200 He I 3650 144 Hg 7081 880 Hg 3654 840 Hg 7091 990 Hg 3663 274 Hg 7147 041 Ar I 3888 646 He I 7173 939 Nel 3964 700 He I 7245 167 Nel 4026 200 He I 7272 930 A
15. 1000 10704 15 4 m x 405 x 0 3 x 1115 0 00028 256 x 2 17288 photons counts Hus x R resp fwhm gt bin 2 4 You may have to distribute the 17000 photons over the PSF and to divide with the gain factor of 0 7e7 adu to estimate peak flux values and the integrated signal to noise ratio HIT MS Since the exposure time and slit width are free parameters the limiting magnitude for the HIT MS mode is the same as that for the traditional spectroscopic modes The only difference is that the charge remain stored on the detector for longer and are therefore subject to correspondingly higher dark current and cosmic ray events For this reason the HIT MS mode is best suited to short exposures which would otherwise be impossible or prohibitively expensive in terms of overheads 3chap2 foot3 FORS User Manual VLT MAN ESO 13100 1543 23 2 7 Rapid Response Mode RRM for FORS The Rapid Response Mode RRM is offered for observations of transient phenomena 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 corresponds to an approved activation code and if this is the case the on going observations are ended and a new BOB starts a
16. 14 wavelength calibrations 40 58 y offsets of grisms 600RI and 1400V 11 standard instrument configuration 5 exchangeable components 5 filter combinations 5 FORS2 5 waivers 5 telescope 33 atmospheric dispersion corrector 34 focus 34 guide stars 33 LADC 34 paralactic angle 34 rotator offset angle 34 template manual 1 visiting astronomers general informations 1 observing with FORS 29 on the site 33 WEB page 1 world coordinate system 71
17. 900 Appendix C Efficiency Curves for the FORS Grisms C 1 FORS2 Grisms This appendix contains the efficiency curves of all standard grisms available for FORS2 and the approximate wavelength range for a slit which is located in the field centre Tables of the measured efficiency values are available on the WEB page GRIS_300V 20 GRIS_3001 21 10077 r T T T 100 T T T T T T 90r J 90r 4 80 q 80r 7 s 70 7 I 1 5 SF 1 5 1 S 50rl 1 E 1 5 4045 4 5 4 e E 5 5 E 30 7 E 7 20r S 10r J J OL 1 1 1 fi f 1 o 1 1 1 1 1 1 300 400 500 600 700 800 900 1000 500 600 700 800 900 1000 1100 1200 Wavelength nm Wavelength nm GRIS_2001 28 GRIS_1501 27 100 TT T T T T T 100 T7 T T T T T T T sop 1 sob E sob sob 4 g 70b d s ot 4 s eor 1 5 9 E 1 9 50 1 2 50r 1 5 40 3 5 40r 3 o o E zob J 2 sob EB 20r 1 20 i 4 10r J 10 MER 0 L 1 1 L L L OL 1 f 1 f 1 1 1 L 500 600 700 800 900 1000 1100 1200 300 400 500 600 700 800 900 1000 1100 1200 Wavelength nm Wavelength nm Figure C 1 Efficiency 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 the red CCD limit or the 330nm limit of the FORS optics in the blue 55 56 FORS User Manual VLT MAN ESO 13100 1543
18. Antu SR wCen I 0 12591 0 00002 FORS2 Antu SR wCen I 0 12591 0 00002 FORS2 Antu SR wCen I 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 transferred 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 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 Note that pre imaging data are automatically reduced only for standard read out mode SR collimator and the following filters 1_BESS v_HIGH b_ HIGH or R_ SPECIAL Other filters might be supported if twilight flats are taken within a reasonable time range but this is not guaranteed If these conditions do not apply then the pre imaging package will contain only raw frames 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
19. 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 requested filter the targets should be more than half the length of spectra off the zero order and the field vignetting 16 FORS User Manual VLT MAN ESO 13100 1543 2 5 Polarimetry with FORS2 Polarimetry Concept the polarimetric modes allow the measurement of linear and circular polarization both for direct imaging and spectroscopy The polarization optics are located in the parallel beam section and consists of a Wollaston prism as beam splitting analyser and two superachromatic 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 a strip mask is produced in the focal area of FORS2 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 M
20. FORS2 standard configuration of opto mechanical components 5 Optical properties ot F RS lt ae ones 2 See E A Dao E EK 7 Exchangeable filter set o e040 ss aa a A 9 Characteristics of the F RS grisms c dsccchsocdh dd aha co doc a 14 The F RS longslits o mo cci ioei oi aoon a e a a AT d ai a aa A d e a a a a E 15 Retarder plate angles for circular and linear spectro polarimetry 17 Mlosal GERE sece amp coe a AR AA a a Y T 24 Detector readout noise and conversion factors 25 Basic characteristics of the FORS CCDs ce RRR RR RR di Kd sd bor eee 26 Approximate CCD readout times 2 202 a a a e a ee 26 Operational overheads with FORS2 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 oco 2 22 5l moo A LA RR a ab aa 32 F RS Calibration Plan Tasks o A OA PAS ES AA RR c9 xod 37 Large scale structure and small scale noise in sky flats 39 Exposure times for FORS2 imaging screen flat calibrations ooo oo 39 Exposure times for spectroscopic screen flat calibrations 40 Exposure times and switch on times for FORS2 wavelength calibrations 40 FORSI half wave plate calibration e 43 Characteristics of the broadband filters 0 0 22 49 Characteristics of the FORS interference filters 53 FORS arc lamp wavelength table 59 Ret
21. Figures 2 1 Schematic view of the FORS instruments o a 4 2 2 Light paths for the standard and high resolution collimators 4 2 3 FORS2 standard broadband filters 8 2 4 Results of flexure measurements as a function of the rotator position for the SR collimator at zenith distance of 40 The panels show the flexure in un binned pixels across X and along Y the slit The solid green circle represents zero flexure 12 2 0 Strip Mask for Imaging Polarimetry 2 200 00400444 A eho a NEEN a 16 2 6 Quantum efficiency of the MIT red and E2V blue CCDs The individual curves show the slight difference in QE of the 2 detectors in each mosaic The dashed line shows where the fringing will limit the S N achievable with the E2V detector 0 25 4 1 Chromatism of the half wave plate 42 BT Filter transmission CUrv lt aaa 41202024 2 a aa ede al 30 B 2 Additional Bessell filter transmission curves 51 B 3 Gunn filter transmission CUEV S lt lt aae a e coc PRAVA AA A 52 BA FORS intermediate band filter transmission curves 00 eee eee ees 54 C 1 Efficiency curves of the low resolution grisms 0 00002 e ee eee eee 55 C 2 Efficiency curve of the medium resolution grisms 2 000000 eee eee 56 C 3 Efficiency curve of the medium resolution grisms 0000000080 e 57 D Calibration spectrum SR GRIS H400V oc oc oh bee ee bebe n dd ps ba b b 60 D 2 Calibration spectrum
22. IC DI L sil MOS A __ MOS B east west LEFT RIGHT A Bl 4 pixel 1 1 MOS 19 south BOTTOM 69 70 FORS User Manual VLT MAN ESO 13100 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 O sky position angle 0 9 longslits in north south direction TOP north pixel 2000 2000 EE EE 1 6 0 5 0 4 1 3 lt slit width in arcsec N l l ol o LEFT LEE EL EL g gg gl RIGHT 975312468 lt internal slit number east LEE O Gg g 1 west PITTA LLL EL ttt N Nd 12 6 1 0 0 3 0 7 2 0 lt slit width in arcsec pixel 1 1 BOTTOM Long Slit Decker TOP north pixel 2000 2000 4 des lt LSS AY LEFT RIGHT East West deeg gt X lt HIT l 4 pixel 1 1 south BOTTOM The x distance between HIT decker and LSS decker is half the distance between two LSS slits If HIT decker enables 0 3 slit the LSS decker will enable the large pinhole cl
23. SR GRIS 1400 lt de eee a om RR eae ae ee 60 D 3 Calibration spectrum SR lt gt GRIS DOOR s ouo cee we ee A ee xw 61 DA Calibration spectrum SR GRIS 1200g 61 D 5 Calibration spectrum SR GRIS 10282 o cccog opas eee ee ee eee 62 D 6 Calibration spectrum SR GRIS 600B so i 0 e ede eee eae ee eee es 62 D Calibration spectrum SR GRIS 00V ENN aaa a Bl BOR RE E A 63 D 8 Calibration spectrum SR GRIS 600R gt 22 s eee be see eee ee 63 DS Calibration spectrum SR 4 GRIS O00RBI salsa a EE eee A a 64 D O Calibration spectrum SR GRIS 6001 sic o nrto cada 9 ba wwe RUE ee es 64 D 11 Calibration spectrum SR GRIS GOOL s os soe 1 ko EEN ee R R RN a 65 D 12 2nd order Calibration spectrum SR GRIS 6001 or 65 D 13 Calibration spectrum SR GRIS 6002 s s nic X ee ee Ra 66 D 14 Calibration spectrum SR GRIS 6007 ss ss ss sosse 66 D 15 Calibration spectrum SR GRIS 300V o o a 67 DAG Calibration spectrum SR GRIS 30013 a 2 4 0 ncc Ek RE eee ROA REEL A 67 D 17 Calibration spectrum SR GRIS 2001 gt acca caac e cdo do caci ENN 68 DAS Calibration spectrum SR GRIS BOL e a cc cac s s ss AA RO EUR RR E Rs 68 G 1 Vignetting of FORS2 CCD by MOS standard resolution model 73 G 2 Vignetting of the FORS2 CCD in high resolution mode sosse oso oo 74 vil List of Tables 2 1 2 2 2 3 2 4 2 5 2 6 27 2 8 2 9 2 10 ol 4 1 4 2 4 3 4 4 4 5 4 6 B 1 B 2 D 1 H 1
24. flat 2 HIT I Screen Flats SR every 5d 5 Day normalized flat 5 HIT S Screen Flats SR every 3d 5 Day normalized flat 5 HIT S Screen Arcs SR every 3d 5 Day dispersion coeff 0 3 pixel 3 HIT MS Screen Flats SR every 3d 5 Day normalized flat 5 HIT MS Screen Arcs SR 3d 1 Day dispersion coeff 0 3 pixel 3 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 calibrate LSS mode is taken with a MOS slit of 5 at the position of the used slit to include all the flux 38 FORS User Manual VLT MAN ESO 13100 1543 stars in the vicinity of the cluster wCentauri 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 FORS UT Coll Target Filter Scale arcsec pix FORS1 Antu SR 47 Tuc I 0 20013 0 00005 FORS1 Antu HR 47 Tuc I 0 09975 0 00004 FORS2 Yepun SR 47 Tuc I 0 12604 0 00003 FORS2 Yepun SR Pal 3 I 0 12607 0 00003 FORS2 Yepun HR Pal 3 I 0 06323 0 00003 FORS UT June 2004 Coll Target Filter Scale arcsec pix FORS1 Kueyen SR wCen I 0 20036 0 00008 FORS1 Kueyen SR wCen I 0 20047 0 00007 FORS2
25. intended as order separation filters for spectroscopy 3 not available after P86 49 Transmission Transmission Transmission FORS User Manual VLT MAN ESO 13100 1543 u High b High 100 100 i 90 L 90 L 1 80 L g 80r 1 70 L 70 L J 60 L 1 S 60h 1 50r J 50r 401 1 5 40r 1 30 L 30f J 20 L E 20b o i 500 400 500 400 500 600 Wavelength nm Wavelength nm v_High R_Special 100 100 j i 90 L 90 L J 80 L g 50b J 70 L For J eor S 60h 7 50r J 50r 1 40r 7 E 40r 7 30 L 5 30F J 20 L E 20f J 101 101 0 1 1 0 1 1 1 400 500 600 700 500 600 700 800 900 Wavelength nm Wavelength nm Bessel 100 4 T s 100 i 90r 1 90r 80r 1 80r 70r 7 Op 60 60b 50r 1 50r 40r 1 E 40h 301 S 30h 20 L E 20F 10r le 1 10F 0 y Ss 0 i 600 700 800 900 1000110012001300 700 800 900 1000 1100 Wavelength nm Wavelength nm Figure B 1 Filter transmission curves for the standard instrument broadband filters FORS User Manual VLT MAN ESO 13100 1543 U Bessel U Special 100 100 90 F J 90 F 1 se BUL 1 s BUL id 7057 J 70F 7 S 60h 7 S 60r B sob J o 50 5 40r al 5 40r 1 5 30F b 5 30F 7 E 20 L 20L 1 10f 1 104 0 0 300 400 500 300 400 Wavelength nm Wavelength nm B_Bessel V_Bessel 100 100 90
26. manual see section 1 FORS User Manual VLT MAN ESO 13100 1543 39 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 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 determining 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 large 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 A guide to approximate exposure times is given in Table 4 3 Screen flats should be used only for removing the high f
27. note above about sky subtraction if the appro priate calibration data are available bias lamp flat arc lamp frame 4 7 3 Paranal Science Operation Pipeline IMG LSS and MOS MXU modes 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 Therefore a special recipe was developed for the spectroscopic data which allows to get quick look results without dedicated calibrations 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 IMG mode e photometric zero points IMG mode FORS User Manual VLT MAN ESO 13100 1543 45 e normalized master screen flats LSS MOS MXU mode e wavelength calibration products LSS MOS MXU mode e reduced science data quick look only LSS MOS MXU mode 46
28. the SR collimator and grism GRIS 600RI 19 288988 v6 4998 F8 vS98 so ves 921698 vvloc8 9 96v8 S9 vovg 1288078 LE AER ee ooeg 25 v928 LTE SILO 69 018 64 b108 919008 IK UA r FESE9Z 08 Sec2 SY VISA L PREOGL 48 88v4 06 8 v4 96 EBEL LOVTeee 62224 ZUSP2L PEELIL PO ASIA oz s902 MAS v S969 Y 6g69 30000 61151009 SIHD seul ade 2000 1500 1000 500 pixels Figure D 10 Calibration spectrum taken with the SR collimator and grism GRIS 6001 15 65 VLT MAN ESO 13100 1543 FORS User Manual 0e ve26 468816 280988 v6 2998 se Fong 99 E98 1698 vy 1298 9 S6v8 S9 vcvg Te sore 4 44 8 00 8 29928 I SIT8 69 018 64 v108 81 8v64 Te ved 8094 08 SESZ S9 VISA v9 094 48 88v4 06 8 v4 62434 4VeveL VOELIL VOLVIL De er s969 4v 6269 PO ALA 1 30000 20000 10000 S6 1009 SIYO Saul ade 2000 1500 1000 900 pixels Calibration spectrum taken with the SR collimator and grism GRIS 6001125 Figure D 11 L03 14 p OSTAPP Hreecer OS A2VEP WBO 9S 9 0b 02 9207 04
29. 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 requirements and instructions for service mode observations are available on the web http www eso org sci observing phase2 SMGuidelines FORS html 3 1 Selecting the 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 in detail with the help of the FORS template manual 3 2 Fast modes or FIMS mask preparation All multi object observations in modes MOS MXU and PMOS will require the preparation of mask with FIMS Occulting 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 and LSS are done without using FIMS as well as single target observations in
30. 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 useful 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 interrupted 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 47 Tuc Tucholke 1992 A amp AS 93 293 for FORS1 and FORS2 and in the field of cluster Pal 3 for FORS2 SDSS coordinates This method is limited by the accuracy of the astrometric positions of the stars The measurements were done with FORS1 in the Bessel V band A third order polynomial was fitted to the measured data The formulae 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
31. v96 02 888 6000 4000 2000 S 1009 SIND Soul 049 Japo pug 2000 1500 1000 500 pixels Figure D 12 2nd order calibration spectrum taken with the SR collimator and grism GRIS 600I 25 66 FORS User Manual VLT MAN ESO 13100 1543 30000 Cd dh GGA e Dot AO GOUD 54 oo Q o o o SPO KV OM NANNAO FFINDONA o Quo a Q Mou O DO FINO y o MA so DU gr e ew E s bud 398529 3 Po 8855295995 sa 8 3 8 8 PEERED 00 COD ODM So o 0 Oo 20000 10000 are lines GRIS_600z 23 0 500 1000 1500 2000 pixels Figure D 13 Calibration spectrum taken with the SR collimator and grism GRIS 600z 23 6000 T T T T T T T T T T T T T I T T T T ox o o q o o G 06 8 a g o No et e Kei 19 19 E tw q o H So OM y x o o o 4 y wt Ki N wo v L 4 Q N L 4 o e n 4000 a o E 224 I n v L ul E 2 L 4 nd 2000 To s o M b 4 amp Q cal 0 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 FORS User Manual VLT MAN ESO 13100 1543 67 30000
32. with two 5 long slits allowing 41 pairs of spectra per CCD readout or a single 10 long slit and a user defined slit width The position angle of the rotator is determined so that the target and comparison slits fall onto adjacent x pixels on the CCD The y pixel positions of the two slits are determined by the offset between the target and the comparison star This offset also determines the relative spectral coverage of the two spectra Offsets in the range 5 300 are possible but it is recommended to choose a comparison with a separation of lt 60 to ensure a reasonable flux calibration Please contact the FORS instrument scientist fors2 eso org for updates and further information FORS User Manual VLT MAN ESO 13100 1543 21 Visitor mode only The cross disperser grisms are not included in the FORS2 standard configuration There will be no instrument setup changes according to the service mode rules and accordingly the spectroscopic HIT S and HIT MS modes are only offered in visitor mode HIT I imaging mode is offered both in visitor and service mode 2 6 5 OB preparation The HIT mode templates for modes HIT I and HIT S are of fast mode target acquisition type whereas the HIT MS mode is fims based There is no mask preparation required for the phase 2 observation block OB preparation except for the HIT MS mode There are special templates available for the modes Three observation templates for the night time science obser
33. 0 4 661 5 0 77 0 83 6 4 6 1 5 H Alpha 4500 61 666 5 667 6 0 72 0 77 6 5 6 1 10 5111 62 SII 672 4 672 8 673 9 0 77 0 82 6 6 6 3 0 SII 2000 63 677 4 678 5 0 77 0 82 6 8 6 5 5 SII 4500 64 683 2 684 3 0 72 0 78 6 4 6 0 10 SIII4 65 SIII 953 2 952 3 953 9 0 68 0 80 5 9 5 2 0 SIIT 15004 66 957 2 958 8 0 72 0 84 6 3 5 6 5 SIII 1500 67 962 1 963 7 0 70 0 83 5 9 5 2 10 FILT 485 37 485 0 89 37 FILT 691 55 691 0 93 55 FILT 815 13 815 0 90 13 FILT 834 48 834 0 90 48 z SPECIAL 915 0 94 20 FILT 917 6 917 0 85 6 FILT 500 5 OI 500 7 500 0 81 5 FILT 503 5 503 0 83 5 FILT 530 25 530 0 85 25 Table B 2 Characteristics of the FORS interference filters Ag is the central wavelength in nm Ty the peak transmission 34 Transmission Transmission TI LT_485_37 FORS User Manual VLT MAN ESO 13100 1543 100 90r 80r 70r eor SOP 40r 50 F 20r J 400 500 Wavelength nm FULT ME 15 600 100 90r 80r 70r eor 50 Ff 40r 30r 20r 700 Waveleng 800 e nm 900 Transmission Transmission LT_691_55 100 90r 80r 70r eor SQF 40 F 30 F 20r J 600 700 Wavelength nm FILT_834_48 800 100 90r 80r 70r eor SOT 40r 30r 20r J 700 Wavelengt 800 Figure B 4 FORS intermediate band filter transmission curves h nm
34. 00RI and 1400V 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 CCD mosaics are 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 CCDs 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 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 Please note that such set ups are not covered by the FORS Calibration Plan see Section 4 1 for details Grism and Filter Transmission efficiency curves of the available grisms are presented in Appendix C For the filter characteristics see Appendix 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 prep
35. 1 GRIS 300V 10 Grism 300V Wheel 3 broadband filter u HIGH 1112 Standard u band filter GG435 81 Order sorting filter GG435 0G590 32 Order sorting filter OG590 b HIGH 113 Standard b band filter v_HIGH 114 Standard v band filter R_SPECIAL 76 Standard R band filter I BESS 77 Standard I band filter Table 2 1 FORS2 standard configuration of opto mechanical components Exchangeable Components up to 8 interference filters can be installed in FORS2 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 3 has to be submitted to the Paranal Science Operations Group at least one day before the start of the observing program For service mode a maximum of two interference filters per OB can be asked Paranal Science Operations will take care of the proper instrument set up for the observations 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 4 if needed for spectroscopic modes The combination of 1 grism with 1 filter other than the order separation filters is only supported if the two components are mounted in different wheels However these non standard configurations are not supported by the calibra
36. 101027A_G1200B_free_S10_C1hi_2x2s fits Row 4100 8 o o o oo o o 19 3 e o oc a a E E y 8 E x x a o ej o a n a o Ue Ue eo o Q ps o e on E o o lt e N e u L L s 8 8000 S u tj L H v E a D L 8 gos z 6000 9 o L E a S s L lt gt S E oc L N E 4000 N 2000 0 D 4000 4500 5000 zx Position LINEAR d Figure D 1 Calibration spectrum taken with the SR collimator and grism GRIS 1200B 97 30000 T T T T I T T T T T T T T I T T T T I o e ooo o oo a o Coo e 19 Quo i e a or wo E o oo F Ei G a 0 o Od gt o oso o a ER oo U D ou L gt 20000 r o o x L I sa L D i L n o L d A 10000 5 E 0 t pd pp t t t ph A S SE i 0 500 1000 1500 2000 pixels Figure D 2 Calibration spectrum taken with the SR collimator and grism GRIS 1400V 18 FORS User Manual VLT MAN ESO 13100 1543 o So 0 OM ooo Q o y x adoj P Qu O dc Ou W QA AP no Q o oo AAs q o S JO cO a o ab Q ow x o9 CO Ud r o i5 Q Xo 78 o 09 on Q te TRH Ed QO O OO 22 D Qoo M QUO Q Ue o NANI Ye DO DO oo do dodo So rt DEN e N m o 20000 o o Q L 4 l un Y Lo 4 DZ o I n v zi E 10000 o K To 0 500 1000 1500 2000 pixels
37. 20 8450 55 0 83 1000 GG435 81 GRIS 6007423 9010 7370 10700 54 0 81 1390 0G590 32 GRIS 300V 10 1 5900 3300 6600 112 1 68 440 GRIS 300V 10 5900 4450 8700 112 1 68 440 GG435 81 GRIS 3001 11 8600 6000 11000 108 1 62 660 0G590 32 GRIS 1501 27 1 7200 3300 6600 230 3 45 260 GRIS 1501 27 1 7200 4450 8700 230 3 45 260 GG435 81 GRIS 1501 27 7200 6000 11000 230 3 45 260 OG590 32 other FORS2 grisms only available in Visitor Mode GRIS 1200g 96 4 4880 4170 5640 24 3 0 36 1605 GRIS 600V 94 4 5850 4430 7370 49 0 74 990 GG435 81 GRIS_600R 14 4 5 6270 5010 7690 45 0 68 1160 GG435 81 GRIS 600125 4 5 7950 6630 9390 44 0 66 1500 0G590 32 GRIS 2001 28 2 4 7450 5600 11000 162 2 43 380 Table 2 4 Characteristics of the FORS grisms The table lists the resolution A 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 5 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 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 An order separation filter OG550
38. AN ESO 13100 1543 FORS User Manual v c969 4v 6269 PO ALA 969699 882099 ES 90c9 0G8 v9 Se 20v9 Ev reco DG 6949 00920909 04 STOS 26 126b 06 664 HRA 30000 20000 10000 vV6 A009 SIYD Salt o4 2000 1500 1000 500 pixels Figure D 7 Calibration spectrum taken with the SR collimator and grism GRIS 600V 94 EG SALSA LF A4USP2L v6EZIZ PO ADIA 025904 MaS Ev S969 25 6269 vOLTLO KI 9882099 EG 90G9 Sz 20v9 6628 9 v vtE9 6L v0 9 059939 924129 6S E919 90 EP19 919609 V 209 866209 S SL6S Le8 vv6S 293499 GP 208G 6906429 666949 Se 0945 30000 20000 10000 VI 4009 SIYO soul oJe 2000 1500 1000 500 pixels Figure D 8 Calibration spectrum taken with the SR collimator and grism GRIS 600R 14 VLT MAN ESO 13100 1543 FORS User Manual 64 sovere T 128078 22409 00 8 es vozes Testis 69 018 IK 30000 20000 10000 61114009 SIND seu oJe 2000 1500 1000 500 pixels Figure D 9 Calibration spectrum taken with
39. FORS User Manual VLT MAN ESO 13100 1543 Appendix A 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 RQE SR S N TBC TBD TOS 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 Responsive Quantum Efficiency Standard Resolution Signal to Noise To Be Confirmed To Be Defined Telescope Control System 47 48 UV VIMOS VLT WCS FORS User Manual
40. Figure D 3 Calibration spectrum taken with the SR collimator and grism GRIS 1200R 93 30000 T T T T T T T T I T T T T T T T T I o o o o LZ o o G e x a o o m i i o j 5 S 5 8 3 8 acs i lt s rJ te wo wo e L J O amp 20000 H o o Di L E I Ya L A D o L J N D L J B 6 10000 H S la IM 0 PM t E31 d ap E Ep 0 500 1000 1500 2000 pixels Figure D 4 Calibration spectrum taken with the SR collimator and grism GRIS 1200g 96 61 arc lines GRIS 10282 29 30000 20000 10000 7948 18 8006 16 8014 79 8103 69 8115 31 FORS User Manual 8264 52 8300 33 8377 37 8408 21 8424 65 8521 44 8771 70 8853 87 VLT MA N ESO 13100 1543 9201 80 9224 50 9354 22 0 500 1000 1500 2000 pixels Figure D 5 Calibration spectrum taken with the SR collimator and grism GRIS 10287429 arc lines GRIS_600B 22 30000 T T T T I T T T T I T T T T I T T T T re o on o ooo ooo oo uN a 5 09 a XO no L S oi ON o gei o 3 Go ao J o o Ro o AO 00 S 9 SS Y SX S SBB F 20000 r 10000 gt 0 l r yhy pi i L t t 0 500 1000 1500 2000 pixels Figure D 6 Calibration spectrum taken with the SR collimator and grism GRIS 600B 22 63 VLT M
41. IT 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 FORS User Manual VLT MAN ESO 13100 1543 File View Graphics Go Data Servers FORS2 fsmosaic_1007 x 1866 0 v 23160 Value 201 ax 00 24 07 536 8 72 08 5143 Equinox 2000 Bitpix 16 Low 100 High 1000 Auto Set Cut Levels Scale Z zl Sl zin E image B 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 Appendix H Retired Instrument Components Modes 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 FORS1 1 4 00 31 3 01 low response GRIS 600z4 26 GRIS 600R4 24 FORS2 1 4 00 31 3 02 low response GRIS 600RI 19 XGRIS 600V 4 90 FORS2 no red Echelle none XGRIS 3001191 FORS2 no red Echelle none 2kx2k Site CCD FORS2 1 4 00 31 3 02 red optimization MIT mosaic GRIS 6002426 FORS1 2 1 4 00 31 9 02 low response none GRIS 600115 FORS1 1 4 00 31 3 07 low response none GRIS 300115 FORS1 1 4 00 31 3 07 low response none Table H 1 Retired instrument components old Tektronix FORS1 CCD old SITE FORS2 CCD port gain RON e7 K e ADU
42. MIT CCDs mosaic CCD read out mode RON e K e ADU RON e7 K e ADU chipl low 200kHz 5 2 24 41 1 25 chip2 low 200kHz 5 2 60 4 2 1 25 chipl high 100kHz 3 0 55 2 7 0 70 chip2 high 100kHz 3 0 60 3 0 0 70 Table 2 8 Detector readout noise and conversion factors 2 8 2 Fringes MIT CCDs The amplitudes of the internal CCD fringes are strongly reduced with respect to the old Site and Tektronix CCDs and the E2V mosaic For Bessel I imaging fringes are hardly visible circular fringes from the filters are however visible for I BESS and R SPECIAL filters For z Gunn imaging the fringe amplitudes are below 196 and in the strongest telluric lines in spectroscopic modes fringe amplitudes were found to be of the order of 596 in the worst cases The MIT mosaic is most suited for any observation gt 650nm E2V CCDs The fringe amplitude of the new E2V mosaic at gt 650nm is much larger than for the MIT detector The fringes will remain in the night sky background in imaging modes with filters of a central wavelength gt 650nm even after the flat fielding of the data For the further data reduction it will be required to subtract the scaled night sky background from the frames In spectroscopic modes only a partial correction of the fringe can be obtained from the flat fielding At wavelength gt 650nm signal to noise ratio of gt 15 may not be obtained due to the residual of the fringe corrections Sky subtract
43. OS 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 the MOS slitlets For the high resolution collimator a separate pre manufactured strip mask of slits of 11 is moved into the focal area of FORS2 MOS stripes polarisation Optics LLL CEE ESSA o VA OU O IKK Z split stripe into e ray and o ray stripe palrs on the CCD NN 8 7 7 E 2 2 4 HA 6 6 8 8 Figure 2 5 For imaging polarimetry IPOL of extended objects or crowded fields a strip mask is produced in the focal area of FORS2 to avoid overlapping of the two beams of polarized light on the CCD Field Coverage since with IPOL observations only half of the full field of view of the instrument is imaged on 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 6 Filters for IPOL all imaging filters see section B can be used except the ones that are located in the Wollaston wheel in the instrument standard configuration see section 2 1 The use of the latter ones is in FORS User Manual VLT MAN ESO 13100 1543 17
44. OT 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 the position of the FORS longslits or to any user defined offset position to the edge of the field of view see the FORS2 Template Manual for further details 2 4 7 Multi Object Spectroscopy with masks 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 2The reason is alternating light traps which prevent sky light from falling between the slit blade carriers 14 FORS User Manual VLT MAN ESO 13100 1543 Grism Acentral Arange dispersion A AX filter A mm A pixel at Accntral FORS2 standard grisms GRIS 1200B 97 4350 3660 5110 24 0 0 36 1420 GRIS 1400V 18 3 5200 4560 5860 20 8 0 31 2100 GRIS 1200R 93 6500 5750 7310 25 0 0 38 2140 GG435 81 GRIS 10282429 8600 7730 9480 28 3 0 42 2560 0G590 32 GRIS 600B 22 4650 3300 6210 50 0 75 780 GRIS 600RI 19 3 6780 51
45. VLT MAN ESO 13100 1543 Ultraviolet Visible Multi Object Spectrograph Very Large Telescope World Coordinate System ngstr m Electron Centimeter Hour KiloPixel Minute Millimeter Nanometer Pixel Second Micrometer Appendix B FORS Filter Characteristics B 1 Broadband Filters Table B 1 lists all presently see issue date of this document available FORS2 broadband filters The trans mission curves are given thereafter Tables of the measured transmission values are available via the ESO web pages http www eso org sci facilities paranal instruments fors inst Filters curves html http filters ls eso org efs index htm Filter A nm FWHM nm U HIGH 112 361 50 5 B HIGH 113 437 102 0 V_HIGH 114 555 123 2 e HIGH 115 467 160 3 R_SPECIAL 76 655 165 0 I BESS 37 77 768 138 0 Z SPECIAL 43 916 18 4 U BESS 33 3 366 36 0 U_SPECIAL 73 3 362 29 0 B BESS 34 474 3 429 88 0 V_BESS 35 75 3 554 111 5 R_BESS 36 3 657 150 0 u GUNN 3 3 359 33 5 v GUNN 39 3 398 46 0 g GUNN 40 1 3 506 79 5 r GUNN441 3 653 81 5 z GUNN 78 910 130 5 GG435 81 2 edge filter n a n a 0G590 32 2 edge filter n a n a FILT 465 250 82 2 3 465 250 Table B 1 Characteristics of the FORS2 broadband filters Ag is the central wavelength in nm 1 this is located in one of the interference filter wheels as it is physically designed as an interference filter 2 these are
46. aller We intend to repeat the analysis for FORS2 and will report the results once they are known We expect the results to be qualitatively the same for FORS2 as they were for FORSI FORS User Manual VLT MAN ESO 13100 1543 19 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 Imaging observations of one or more targets as well as spectroscopic observations of up to two targets The imaging mode uses the MOS to create a pseudo longlit on the extreme left hand side of the unvignetted field of view Whereas the spectroscopic mode uses re defined in the case of HTT S mode or user defined in the case of the HIT MS mode masks installed in the FORS2 mask exchange unit MXU The position angle of the instrument can be selected such that a second target may be observed simultaneously in the HIT MS mode and two or more in the imaging mode Please note that the HIT mode observations are only configured for the standard resolution collimator COLL SR One shift mode denotes that the charges are moved at constant speed on the detector until the complete detector is used as storage for t
47. an be stored in the magazine and observed in one night MXU Slits boundary conditions for the MXU slits are 1 slit width 0 1 minimum to 30 2 slit length up to 30 3 available field of view X minus 15mm at either end this is indicated by FIMS Y full field of view 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 Slitless Spectroscopy Slitless spectroscopy can be performed in MOS mode with all slits open Please note 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 good understanding of the instrument optics is essential Note that the Oth order of grisms 1501 and 2001 will fall into the field of view of FORS and contaminate 1260 and 480 unvignetted pixels on the left blue side of the field of view of FORS2 unbinned 15 um pixels
48. aration 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 All these relative astrometric calibrations are not required if your FIMS preparation is based on pre images taken with any of the FORS instruments It is strongly recommended to search in the VLT Science Archive http archive eso org for released FORS imaging data Isee eg UCACI Zacharias et al 2000 AJ 120 p2131 or SDSS Stoughton et al 2002 AJ 123 p485 12 FORS User Manual VLT MAN ESO 13100 1543 Restrictions for pre images to be used for the mask preparations The target acquisition procedures were reviewed 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 an
49. 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 spectral calibration lamps in appendix D The red internal flat field lamps FlatRed 1 and FlatRed 2 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 see below External Calibration Units the flatfield lamps in the old internal calibration units have produced parasitic light in MOS and LSS flatfield exposures 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 lamps 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 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 Nighttime Calibrations For technical reasons the arcs and flats are only taken
50. ast target acquisition FORS2 lss obs slit fast through slit image FORS2 lss obs off fast science exposures here for the LSS mode but very similarly for the other spectroscopic modes For blind acquisitions in fast modes LSS 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 reference 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 FORS2 ima cal coll collimator selection FORS2 lss cal daycalib screen flats amp arcs where the first template is only used to select the collimator There are a few important points to be verified now 1 don t mix observing modes in one OB 2 make sure that the same slits are used in LSS mode for all templates within an OB 3 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
51. at day time with the telescope guide probe LADC parked and the beam shutter identical with FORS User Manual VLT MAN ESO 13100 1543 the calibration screen closed 27 28 FORS User Manual VLT MAN ESO 13100 1543 Chapter 3 Observing with F RS All observations with FORS are done via observing blocks OBs OBs contain 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 keywords of the templates eg grisms filters slits All 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
52. ates for SM P70 2 5 Dec 24 2002 2 3 chapters 2 and 3 re sorted small changes elsewhere 2 6 July 12 2003 2 V BESS offset ECU pre image policy and small changes in the other chapters 2 7 January 5 2004 2 appendix HIT mode POL figure FORS2 vignetting figures appendix G note about the Echelle mode 2 8 June 30 2004 all manual under pdf format updates for new FORS1 Grism 1200g 96 update of gain and ron of FORS1 CCD update of the plate scales due to the FORS1 and 2 move to UT2 and 1 new Rapid Response Mode notes about the instrumental linear po larization the pre imaging data delivery the slit along parallactic angle the calibration plan in LSS Mode 3 November 28 2004 2 4 new integer manual issue numerotation update of FORS1 gain and ron MOS supported by the pipeline 4 August 30 2005 2 new FORS1 Grism 1200B 97 FORS2 Echelle Mode decommis sioned new FORS2 HIT MS mode 79 June 8 2006 all New pdf template MXU slit sizes Fig D22 23 C4 and table D2 3 about Echelle mode removed 80 March 1 2007 all New FORS1 E2V blue sensitive CCD mosaic new broad band high throughput filters 81 August 10 2007 Updated read out modes available in SM 81 2 October 25 2007 Updated information on geometry of mosaic s 82 1 February 27 2008 updated FORS1 standard configuration 83 0 July 14 2008 all First version post merger 84 0 February 26 2009 2 updated for P84 85 1 December 20 2009 some updated for P85 K O Brien up
53. ctions and hints on the Paranal Science Operations WEB page and the Paranal Observatory home page http www eso org sci facilities paranal http www eso org sci facilities paranal sciops 2 FORS User Manual 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 Department usd 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 G6ttingen and Munich in the scope of the FORS contract and finally compiled and edited by G Rupprecht Later editions were made by H B hnhardt T Szeifert and E Jehin We are very grateful 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 The P88 version has benefited by a careful scrutiny by H Boffin Chapter 2 Instrument Characteristics 2 1 Overview Instrument Concept FORS is the visual and near ultraviolet FOcal Reducer and low dispersion Spectrograph for the Very Large Telescope VLT of the European Southern Observatory ESO Two versions of FORS were built and installed at the Cassegrain foci They have been moved to diff
54. d MXU data Spectrophotometric standard star observations are reduced like LSS science data but no response curve is determined The night sky is subtracted However since the pipeline uses a median to determine the sky it does not correct for any spatial gradient along the slit It will also not work properly if more than 50 of the pixels along the slit at a given wavelength contain signal from sources This can cause problems for extended objects and crowded regions or for very short slits in the case of MXU The sources are extracted but the single frames taken within a sequence are not combined More information about the spectroscopic FORS pipeline can be found at http www eso org observing dfo quality FORS1 pipeline fors_calib html calibration data and http www eso org observing dfo quality FORS1 pipeline fors_science html science data The pipeline can be downloaded from http www eso org sci software pipelines 4 7 2 Quality Control Pipeline All data taken in both modes are reduced by the quality control group in Garching and they can be accessed through the User Portal 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 obtain master calibration data These are optimized for e g lo
55. d for the the HIT S mode There are 7 masks available with slit widths between 0 5 and 5 arcseconds The absolute photometric accuracy will be poor since it is not possible to do a differential photometric measurement 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 3001 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 8000A depending on the color of the target The following slit masks are available mask name slit width HITS 0 5 900015 0 5 HITS 0 7 900016 0 7 HITS 1 0 900017 1 0 HITS 1 31900018 1 3 HITS 1 6900019 1 6 HITS 2 0 900020 2 0 HITS 5 0 900021 5 0 The respective rotated grisms are identical copies to the standard FORS2 grisms 600B 3001 The wavelength range of the grisms are however slightly different from the standard ones 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 HIT S mode Grism Acentral Arange dis
56. d to be prepared for the spectrophotometric standard 2 6 7 Performance on the sky HIT S 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 drops 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 3001 15 9 22 FORS User Manual VLT MAN ESO 13100 1543 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 px response 0 17 0 75 A pxl and an OS time of 256s time 3548 1000 107910 a x 4052 0 17 0 75 0 00028 256 x 2 1757 photons counts flux r R x resp disp bin 2 1 HIT I In case of the imaging modes the number of parameters like seeing night sky brightness and the number of filters is 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 3548
57. dated graph in Sect E 2 first LyX version 85 2 January 25 2010 2 2 Note on interference filters 86 0 March 08 2010 none P86 no changes 86 1 July 2010 D Note added to Fig D 86 2 July 2010 2 1 Sec 2 1 moved to Appendix H 87 0 August 27 2010 4 7 1 4 7 2 Modes supported by pipeline QC data 87 1 November 2 2010 Tab 2 1 4 1 Revised calibration plan 87 2 December 11 2010 Fig D 1 Grism 1200B line atlas 88 0 February 25 2011 some Minor edits and fixes 88 1 March 9 2011 4 3 Note on pre imaging setup and data package 88 2 March 10 2011 Tab B 1 Added note 3 89 0 September 01 2011 none P89 several URL updated 89 1 October 22 2011 4 1 Intro and Table 4 1 updated with new calplan This page was intentionally left blank Contents 2 1 Introduction 1 ME o TTT 1 12 More Information om FORS sc oce ceca oc de 4840 koe da weed x o 1 TO EE ouo O a ee PE D P P Ee AR 2 14 Acknowledgements 25244 4 E E K eee whee eae D Ce 2 Instrument Characteristics 3 PI Overiew ca fase ee A A AAA AA uu 3 2 2 Standard Instrument Configuration o e ee osa 5 23 Direct Imaging IMG and OCC modes iiu diee x cc RRR eee 7 2 3 1 Basic Characteristics of the Imaging Optics llle 7 23 2 The FORS Milter Seb zu ose e de a a A a 7 Zao User Provided Filters 2o su sow a Ra ee Ye Bee E Wo 9 234 HR Colimator Field Stop s su srs mn ck kgRoe e Rm Rok ee bee Ee XR REOR Ro 3 10 43 0 Guling Masks a c
58. e a ab Ro RO 21 20 0 Performance on EENHEETEN 21 2 7 Rapid Response Mode RRM for FOR 23 R a cia RN 24 281 General Properties 22s kg PRX 448 688 ee AA Rb ES 24 2 POS a a eas d CE AN dl AR dr ng O HAr oo VAL A GR EE 25 Pee MEY HT 26 29 The Calibration Units oo 253406222 22 2635444084 ee 0 deser woe e EGGS 26 3 Observing with FORS 3 1 Selecting the Observing Made i acasa spurst aaraa daiya A 3 2 Fast modes or FIMS mask preparation 3 3 Selecting the Instrument Setups and Exposure Times cos sn 3 4 OB preparation FIMS based modes 000 0 eee eee ee 45 OB preparation Fast modes a 4 404 ss ox oo A a A A A ee NNN ADS E ees 3 6 Estimate execution time and optimize overheads lll wal EENEG aL Whe tinal a T TTT dice At the telescope c c Ad A TEE a R 4 dO A A AR ADA A UR 20 IRE VEDY EN 3 8 FORS and the Unit Telescopes 050 A 3 8 1 Guide Stars Telescope Offsets 3 8 2 Telescope and Instrument Focus so es 3 8 3 Instrument Rotation and Position Angle on the Sky o o o 3 8 4 Atmospheric Dispersion Compensation a a a s s e a a a e e e e e a e e a a 4 Calibrating and Reducing FORS Data dal lG ri Pl ooo aa fr der dr RA E EE See be dr E 4 2 Image Field Distortion and Scales aaoo ee 4 3 Data Reduction of Pre Imaging Data for the Mask Preparation 43d A H ON 4 4 1 Imagine Mode cc css ms ox Se aaa DO a a 4 4 2 Spectroscopie Modes
59. e instrument for more frequently used Grisms and filters 75 76 FORS User Manual VLT MAN ESO 13100 1543 e The MOS was not offered with the HR collimator following the upgrade of FORS1 to the E2V mosaic Users should use the un binned mode instead for applications requiring such observations e Table H 2 lists the readout noise and conversion factors of the old FORS1 and FORS2 CCDs Index calibration plan 35 overview table 37 unsupported modes 35 calibration units 26 new calibration units 26 night time calibrations 26 calibrations 35 CCD 24 conversion faction 24 dark current 24 exposure shutter 26 fringes 25 linearity 24 readout modes standard modes 24 readout noise 24 readout time 24 26 window readout 24 data reduction 35 pre imaging data 38 exposure shutter 26 filters 49 broad band filters 7 49 combinations 5 exchangeable components 5 interference filters 7 53 fims manual 1 FORS instrument components 3 instrument overview 3 observing modes 3 WEBpage 1 FORS upgrades new calibration units 26 replaced components 75 grisms 14 55 high time resolution modes 19 HIT mode 19 imaging 7 broad band filters 7 49 filters 49 fat fields 39 instrument flexures 10 interference filters 7 53 occulting masks 10 TT order separation filters 7 scale and field distortion 36 user provided filters 9 world coordinate system 71 IMG mode 7 IPOL mode 16 ins
60. e noida a a Aa wh A LA PER e ih dye e 10 23 6 Image Motion due te Plexure occ R Ree ee oe ee ee 4 A 10 2A Spectroscopy css oa T sora 4000222 se se 448480 8 Oe EE dee ee ed 11 241 Grisms and Order Sorting Filters s c ccc 22445744 e446 44444444 omy 11 2 4 2 Relative Astrometric Accuracy Requirements for Spectroscopy 11 24025 Instrument PIEXHPS ch N ee be Pa Bee dew 3 4L a X Oed a 12 24 4 Longslit Spectroscopy LSS mode 2 22 66 8a sk Ra 13 2 4 5 Multi Object Spectroscopy with Movable Slitlets MOS Mode 13 2 4 6 Wide Slit Spectro Photometry SPECPHOT mode lr 13 2 4 7 Multi Object Spectroscopy with masks MXU mode lll rn 13 2M Sntless Spectroscopy i e ec us 1G PAL e a b be 15 25 Polrmey E F RS2 ie EN o Y fr fd seb e cR OR CA ON Pee Ard wow x 16 2 5 1 Imaging Polarimetry IPOL mode 2 k 664844 88 FeV VE DRE RE eee 16 2 5 2 Spectropolarimetry PMOS mode 17 2 5 3 Performance of the Polarimetric Modes sovr 17 2 6 High Time Resolution Mode 2 sez olg RRR RRR ee oe oe 19 2 0 1 JONBPVIBW Zei l ka be bee YS po EE er EM ew Too 19 2 6 2 High Time Resolution Mode Imaging HIT 19 2 6 3 High Time Resolution Mode Spectroscopy HIT S o o o 20 2 6 4 High Time Resolution Mode Multiple Shift Mode HIT MS 20 2 0 b OB preparatiom l9 KR pl ss es PRG Ee OR OE eee eee E ere ld 21 2 6 0 Calibration DAN cs 856 2044444045564
61. e photometric accuracy of the measurements when used in conjunction with a wide slit The respective targets will 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 HIT 1 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 HIT I 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 than 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 20 FORS User Manual VLT MAN ESO 13100 1543 2 6 3 High Time Resolution Mode Spectroscopy HIT S The readout direction is for FORS2 in the spectral direction for the standard FORS2 grisms Only the cross disperser grisms XGRIS_600B and XGRIS 300I can be use
62. ed In this case the Stokes parameters Q and U can be derived via Fourier transformation N 1 Q F 0 cos 40 4 5 zs td 2 0 F 0 sin 40 4 6 i o z w 42 FORS User Manual VLT MAN ESO 13100 1543 4 T T T T T T T T T T T T chromatic zero angle A 2 plate 2 ls pd s OF 1 w 2 L 4 4 fi 1 1 1 1 1 fi 1 3000 4000 5000 6000 7000 8000 9000 10000 11000 M Figure 4 1 Chromatism of the half wave plate Tn principle two observations at different retarder angles N 2 are sufficient to calculate Q and 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 in Table 4 6 Measuring a polarization angle of e
63. en 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 is larger than with the HR collimator The filters are centered on important emission lines and on 5 96 and 10 96 longer wavelengths Ao To FWHM nm Filter Line SR HR SR HR SR HR Ao Shift 011144 011 372 7 371 7 372 9 045 0 48 7 3 6 9 0 OTI 4000 45 371 6 378 8 0 37 0 40 6 5 6 1 5 OII 8000 46 381 4 382 6 0 43 0 47 6 5 6 1 10 Hell 47 Hell 468 6 468 4 469 1 0 79 0 82 6 6 6 4 0 Hel1 3000 48 472 6 473 4 0 76 0 79 5 8 5 6 5 Hel1 6500 49 478 1 478 9 0 78 0 81 6 8 6 6 10 OITI 50 OI 500 7 500 1 500 9 0 76 0 80 5 7 5 5 096 OTII 3000 51 504 5 505 3 0 76 0 80 5 9 5 7 5 OIII 60004 52 510 5 511 3 0 74 0 78 6 1 5 9 1096 Hel 53 Hel 587 6 586 6 587 6 0 79 0 84 6 0 5 7 0 Hel 2500 54 592 0 593 0 0 77 0 81 6 8 6 5 5 Hel 5000 55 597 5 598 5 0 85 0 89 7 4 7 2 10 01156 OT 630 0 629 5 630 6 0 75 0 79 7 2 6 9 0 OT 2500 57 635 4 636 4 0 75 0 81 5 9 5 5 596 OT 4500 58 640 4 641 4 0 77 0 83 6 3 6 0 10 H Alpha 83 Ha 656 3 656 3 657 4 0 70 0 76 6 1 5 7 0 H_Alpha 25004 60 66
64. erent telescopes over the years As of P83 FORS1 was decommissioned to make way for the second generation of VLT instrumentation FORS was designed as an all dioptric instrument for the wavelength range from 330 nm to 1100 nm and provides an image scale of 0 25 pixel and 0 125 pixel with the standard resolution collimator and the high resolution collimator respectively and in the default binned 2x2 read out mode of both FORS detectors Since April 2009 FORS2 has been available with a choice of components from both of the original instruments FORS2 is equipped with a mosaic of two 2k x4k MIT CCDs pixel size of 15x15 um and particularly sensitive in the red part of the spectrum up to 1100 nm The detector previously mounted on FORSI is also available in Visitor Mode only It consists of a mosaic of two 2kx4k E2V CCDs pixel size of 15x15 um particularly sensitive in the blue range of the spectrum 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 the longslits the polarimetry mask the MXU mask exchange unit 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 the wheel for the Wollaston prism and optional optical analyzers filters and or grisms the grism wheel and the broadband filter wheel in the pa
65. evel RON RON 2 Darks monthly 3 Day dark current Screen Flats UBVRI SR every 4d 2 Day CCD check Astrometry SR HR annually 1 Night distortion scale 1 pixel Imaging Sky Flats SR every 3d 1 5 Twilight normalized flat 2 HR every 3d 5 Twilight normalized flat 2 BVRI photom std SR nightly 1 2 1 Night zero points lt 2 extinction coeff lt 8 colour coeff lt 30 high AM BVRI std SR as needed 1 2 1 Night zero points lt 2 extinction coeff lt 8 colour coeff lt 30 BVRI photom std HR as needed 1 2 1 Night zero points 5 Flux std Gunn amp other SR HR as needed 1 Night response 10 filters Screen Flats LSS SR HR every 3d 5 Day normalized flat 5 MOS MXU Screen Arcs LSS MOS SR HR every 3d 1 Day dispersion coeff 0 3 pixel 3 MXU Flux std spectroscopic SR weekly 5 1 Night response 10 Imaging Sky Flats SR weekly 5 Twilight normalized flat 2 without polarizers IPOL polarized std SR as needed 1 Night zero angle lin 1 degree IPOL unpolarized std SR annually 1 Night instr pol lin IPOL unpolarized std SR annually 1 Night instr pol cir PMOS arcs SR every 3d 1 Day dispersion coeff 0 3 pixel 3 PMOS flats Olin SR every 3d 5 Day normalized flat 5 45circ PMOS polarized std SR as needed 1 Night zero angle lin 1 degree PMOS unpolarized std SR annually 1 Night instr pol lin PMOS unpolarized std SR annually 1 Night instr pol cir HIT I Twilight Flats SR every 5d 5 Twilight normalized
66. g 0 134 20 deg in the Bessel B filter one would correct this raw measurement to a final result of 9 132 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 FORS2 4 7 1 Supported modes The FORS pipeline supports four instrumental modes imaging IMG longslit spectroscopy LSS multi object spectroscopy MOS MXU and spectro polarimetry PMOS It provides e creation of master calibration products e reduction of science data FORS User Manual VLT MAN ESO 13100 1543 43 zero angles imaging mode Filter Co Bessel U 2 07deg Bessel B 1 54deg Bessel V 1 80deg Bessel R 1 19deg Bessel I 2 89deg Gunn u 2 03deg Gunn v 0 47deg Gunn g 3 10deg Gunn r 1 31deg Gunn z 1 64deg Table 4 6 Calibration of the FORS1 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 e photometric zero points For IMG data the raw data are bias subtracted and flat fielded and SExtractor is used to detected and classify sources The single frames taken within a sequence are not combined LSS MOS and MXU data in addition to de biasing and flat fielding high spatial frequencies only are rebinned to wavelength space The spatial curvature is corrected for MOS an
67. he 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 1 4 16 64 or 256 seconds The resulting frequencies of 0 28 to 73 pixels millisecond are not however the effective time resolution the time resolutions is reduced by the seeing in the case of a wide slit or the slit width in units of pixels This corresponds to the number of shifts a pixel remains in the exposed region for For an image scale of 0 125 px and a seeing of l the time resolutions would be between 2 3 milliseconds and 0 56 seconds for the fastest and slowest modes respectively HIT mode name one shift time time resolution HIT OS1 1sec 1s 0 0023s HIT OS2 4sec 4s HIT OS3 16sec 16s HIT OS4 64sec 64s HIT OS5 256sec 256s 0 56s The readout mode 100kHz 2x2 high was selected to give the lowest possible readout noise level All frames will be binned at readout time The CCD parameters like the binning are deeply hidden in the CCD configuration file and cannot 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 I and HIT S modes is that even the smallest image motions due to atmospheric effects or residual guiding offsets will strongly compromise th
68. he telescope guiding system and the respective images taken with FORS for zenith distances of up to 45 degrees it gives a partial correction even at larger zenith distances FORS User Manual VLT MAN ESO 13100 1543 9 P2PP entry Filter type OIT 44 O II filter OIT 4000 45 O II filter redshifted by 4000 km s OIT 8000 46 O II filter redshifted by 8000 km s Hell 47 He II filter Hel1 3000 48 He II filter redshifted by 3000 km s Hell 6500 49 He II filter redshifted by 6500 km s OTII 50 O III filter OTII 3000 51 O III filter redshifted by 3000 km s OTII 6000 52 O III filter redshifted by 6000 km s Hel 53 He 1 filter Hel 2500 54 He I filter redshifted by 2500 km s Hel 5000 55 He I filter redshifted by 5000 km s 01156 O I filter OT 2500 57 O T filter redshifted by 2500 km s OT 4500 58 O I filter redshifted by 4500 km s H Alpha 83 H Alpha filter replacement for H Alpha 59 H Alpha 2500 60 H Alpha filter redshifted by 2500 km s H Alpha 4500 61 H Alpha filter redshifted by 4500 km s SII 62 S II filter SII 20004 63 S II filter redshifted by 2000 km s SII 4500 64 S II filter redshifted by 4500 km s SITI 65 S III filter SIII 1500 66 S III filter redshifted by 1500 km s SIII 3000 67 S III filter redshifted by 3000 km s FILT 485 374 68 special intermediate band filter FILT 691 554 69 special intermediate band filter FILT 815 13 70 night sky suppression filter FILT 834 48 71 night sky suppre
69. igher 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 coefficients and first order colour terms for the BVRI filters For the other filters only one flux standard star close to airmass 1 is taken The calibration plan for imaging has been revised in October 2011 following the detailed simulations of Bramich et al 2011 Their technical report VLT TRE ESO 13112 5429 FORS Zero Point Monitoring And Absolute Photometry concludes that it is possible to reach absolute photometric accuracies of 1 4 1 8 if a two standard star images per night are taken as close together in time as possible at airmasses of X 1 1 and 1 8 ensuring a range in airmass of 0 6 to 0 7 and if b at least 18 photometric nights are included in the photometric modeling of the data Such strategy also enables the monitoring of the atmospheric extinction coefficient and of the colour term coefficient to a precision of 6 8 and 20 30 respectively This strategy is now in place at UT1 and the data that are being collected will eventually be used to confirm these conclusions Additionall
70. ill be executed after centering a reference star in the method above and verification in the subsequent image of the slit template FORS2 Iss obs slit fast 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 By 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 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 the LSS mode is supported with both collimators but the MOS mode is only supported with the standard resolution collimator 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 4 6 Wide Slit Spectro Photometry SPECPH
71. ion at these wavelengths will require that the target is observed at offset positions nodding on the slit This will however not correct for the residual fringes in the extracted science spectra 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 scaled master sky will give quite reasonable results even at z 26 FORS User Manual VLT MAN ESO 13100 1543 Parameter E2V mosaic MIT mosaic photosensitive pixels Nchips X Ny X Ny 2 4096 2048 2 4096 2048 pixel size um 15 15 dark current at 120C e px h 5 9 0 8 4 5 0 8 2 1 0 4 14 0 7 linearity up to full well RMS 0 03 0 4 0 8 0 0 9 0 5 1 8 2 1 cosmic ray rate events min cm TBD TBD Table 2 9 Basic characteristics of the FORS2 CCDs The two values of the dark current refer to chip 1 upper and chip 2 lower of the mosaic The linearity values are given as chip1 chip2 and for high and low gain readout area speed binning gain total readout time default mode for E2V mosaic 2x4k x2k 100k Hz 2x2 high 39s spectroscopy 2x4k x2k 100kHz 1x1 high 78s high spatial resolution spectroscopy
72. ion units see section 2 9 Grism OSF Exposure time s E2V MIT 1400V 27 3 1200B 21 9 27 3 1200g 21 9 27 3 1200R GG435 11 5 1028z 0G590 9 1 600B 11 8 8 3 600V GG435 12 5 600R GG435 15 4 6001 0G590 34 0 7 0 600RI GG435 3 8 600z 0G590 5 2 300V GG435 5 2 9 0 300I 0G590 9 8 4 0 2001 1 2 1501 0G590 GG435 23 1 1 Table 4 4 Approximate exposure times seconds for spectroscopic screen flat calibrations with the 2 available mosaics Flatfield lamps of one calibration unit switched on Approximate exposure level is 30000 ADU Slit width 1 SR collimator high gain readout 2x2 binning Grism OSF He HgCd 2 Ar 2 Ne 1400V 100 25 0 100 1200B 100 100 0 0 1200R GG435 75 0 37 7 1028z 0G590 100 0 5 5 100 600B 100 25 0 0 600V GG435 50 50 10 10 600R GG435 0 50 10 10 600RI GG435 90 AO 4 5 4 5 600z 0G590 75 0 4 5 75 300V GG435 70 17 3 9 8 0 300I 0G590 100 0 5 0 2001 70 0 4 9 0 1501 0G590 GG435 60 18 6 0 Table 4 5 Approximate exposure times and switch on times of calibration lamps seconds for wavelength calibrations with FORS2 Any slit width SR collimator high gain readout 2x2 binning The update with the integration times for the E2V mosaic is still pending 4 5 Wavelength Calibration For the wavelength calibration one may use the He and Ar lamps at the lowest spectral resolution grism 1501 and in addition
73. ired instrument components 75 Detector readout noise and conversion factors of the old FORS1 Tektronix and FORS2 SITE A E O TET 75 viii Chapter 1 Introduction 1 1 Scope The FORS User s Manual is intended to cover all aspects of the VLT instrument FORS2 following the merging of FORS1 and FORS2 It is intended to give comprehensive information on the following topics e Overall description of the FORS2 instrument 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 FORS FIMS Manual ESO document VLT MAN ESO 13100 2308 e FORS Template Manual ESO document VLT MAN ESO 13100 2309 The knowledge of these manuals is essential for the preparation of proposals and observations with FORS2 1 2 More Information on FORS The FORS User s FIMS and Templates Manuals are published on the FORS instrument WEB page Addi tionally a version of the user manual from P82 can be found which has information on FORS1 and FORS2 prior to the merger http www eso org sci facilities paranal instruments fors Information and software tools for the preparation of service and visitor mode observations with FORS2 are given under http www eso org sci observing phase2 html Visiting astronomers will find instru
74. is cemented to the grism itself 3 This grism produces a Y offset on the CCD see section 2 4 1 for details 4 This grism is not part of the Standard Instrument Configuration and is therefore not available in service mode 5 Higher throughput volume phased holographic grisms are available on FORS2 FORS User Manual VLT MAN ESO 13100 1543 15 Longslits of FORS2 slit width slit offsets FORS2 in 15 um pixels sky CCD SR mode CCD HR mode 2 5 4573 362 723 1760 340 272 544 1 0 226 182 362 0 51 1173 91 182 0728 070 0 0 0 40 1173 91 182 070 226 182 362 1 31 34 0 272 344 20 45 3 362 723 Table 2 5 Slit widths of the FORS2 longslits and approximate offsets relative to the central slit in pixels on the CCD The exact values are determined after each dismounting of the Cryostat CCD objects to be observed simultaneously 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 c
75. 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 TRAF imcombine to reduce images which are thought to be used for fims mask preparation FORS1 2 mosaics don t cut the edges In case of pre imaging data taken with the FORS1 2 mosaic detectors 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 TRAF imcombine a median of the jittered files with the offset parameter set to wcs should give satisfactory results for the mask preparations In general first 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
76. kage 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 Visitors should thus have an approved backup programme 3 7 2 At the telescope The telescope and instrument operation is done by the ESO staff A good finding chart and a close collabora tion 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 envi ronment is described on the Science Operation WEB page http www eso org sci facilities paranal sciops At the end of the night an automatic procedure calobBuilt will be started which will create a complete cali bration OB for all modes and setups used during the night The calibration OB will be executed during the morning hours 3 7 3 At the very end Finally after the last night a single copy of all science calibration and
77. lescope 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 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 34 FORS User Manual VLT MAN ESO 13100 1543 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 Defocussing 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 telescope is minus the position angle of the targets on the sky The value 9999 can be used to set the po
78. mination 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 107 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 When the polarisation optics were mounted in FORSI we 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 3x107 on the optical axis to 7x107 at a distance of 3 arcmin from it V band There is currently no data available in case of the other filters and spectro polarimetric measurement The corrective functions can be estimated with an ob servation of a globular cluster with the respective filters and details of such work can be found on the FORS web page http www eso org sci facilities paranal instruments fors inst pola html There was no problem for spectro polarimetric or imaging polarimetry observations of single targets in the center of the 18 FORS User Manual VLT MAN ESO 13100 1543 field of view In case of the circular polarization the spurious polarization was found an order of magnitude sm
79. n 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 templates described in the FORS Template Reference Guide More information on the RRM can be found on the USD Phase 2 webpages http www eso org sci observing phase2 SMSpecial RRMObservation F RS html 24 FORS User Manual VLT MAN ESO 13100 1543 2 8 Detectors 2 8 1 General Properties Chip Characteristics Pixel Number and Size CCD Control The standard detector mosaic of FORS2 consists of two 2kx4k MIT CCDs the pixel size is 15 x 15 um thinned and anti reflection coated The E2V mosaic previously installed on FORS1 is a mosaic of two blue optimised 2kx4k E2V CCDs with the same pixel size 15 x 15 um In both mosaics the detectors are flat and the bottom chip 2 detector is rotated and shifted with respect to the upper chip 1 detector The most accurately determined values for this rotation and shift can be found in the header keywords for the respective detectors and they are summarised in Table 2 7 Detector Mosaic rotation X offset um Y offset um MIT 0 08 30 0 480 0 E2V 0 025 10 5 1390 Table 2 7 The detector geometry of the 2 mosaic detectors
80. n two wheels of convergent beam section The standard filter sets for FORS2 are the four new high throughput broad band filters previously mounted in FORS1 u HIGH b HIGH v HIGH g HIGH together with the R SPECIAL I BESS Gunn z see Figure 2 3 and some order separation filters see Table 2 1 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 only be seen with the IR optimized MIT detectors In both cases the typical shape of the pattern is circular and off axis 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 8 FORS User Manual VLT MAN ESO 13100 1543 Standard Filter Set TO E us MEE u_High g High R_Special _Bessel 100 90 80 70 60 50 40 30 20 10 U TS IM 500 400 500 600 700 800 3900 1000 1100 Wavelength nm Transmission Figure 2 3 The FORS2 filters which can either be used in the ubvRIz sequence or the ugRIz sequence to cover the full wavelength range with broad band filters
81. never possible for instance to benefit from the calibration data taken in the context of the FORS instrument calibration plan Window Read out window read out is not supported with either mosaic 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 8 Please note that low gain denotes high charge conversion factors K in e adu and 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 Linearity Full Well Capacity Dark Current etc some more characteristic data of the CCDs are given in Table 2 8 1 None of the CCDs will saturate before reaching the numerical truncation limits 65535 adu Read out Speed and Times the detectors readout speed are 200 kHz and 100 kHz for the imaging and spectroscopic modes with both E2V and MIT mosaic detectors Approximate readout times for various modes are given in Table 2 10 FORS User Manual VLT MAN ESO 13100 1543 25 FORS2 CCD Quantum Efficiency 300 400 500 600 700 800 900 1000 1100 Wavelength nm Figure 2 6 Quantum efficiency of the MIT red and E2V blue CCDs The individual curves show the slight difference in QE of the 2 detectors in each mosaic The dashed line shows where the fringing will limit the S N achievable with the E2V detector E2V CCDs mosaic
82. of proper adjustments of the calibration exposure times for delivered service mode OBs unless otherwise stated in the readme file of the program Lamp U SPECIAL B V R I exp time s 200 4 4 6 10 Table 4 3 Approximate exposure times seconds for FOR S2 imaging screen flat calibrations for the Bessell and special broadband filters SR collimator high gain readout The Blue 1 and Blue 2 lamps should be used 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 only since they are subject to changes due for instance lamp replacements The observatory 40 FORS User Manual VLT MAN ESO 13100 1543 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 Please note that red internal flat field lamps FlatRed 1 and FlatRed 2 can t be used anymore after the installation of the external calibrat
83. order approximation e the improper flat field correction crr e the color dependent offset eg to the nominal retarder plate zero angle e the incomplete and color dependent retardation of 90 es A degree of the quarter wave plate Observations at only one retarder plate angle would cause hardly correctable Stokes parameter cross talks in the case of objects with non negligible linear polarization The color dependence of the retarder angle eg would cause an additional polarization of AV 2e9U and the incomplete retardation ca 0 4 90 degree quarter wave would cause the additional polarization of AV es es amp eg in radians UVQ being the Stokes parameters One would get V err 2eU eaQ 4 2 PER 0 45 POE oc fe V erp 260U e50 4 3 qe gua cas 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 eg amp eg 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 0 the normalized flux differences F 0 must be calculated _ 8 FH 81 fe Gi where 0 i 22 5 is the angle of the retarder plate 0 lt i lt 15 F 0 4 4 Tf the polarimetry is obtained from the normalized flux differences no absolute flux calibration of the data is requir
84. ortant points to be verified now 1 don t mix observing modes in one OB 2 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 3 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 manual FORS User Manual VLT MAN ESO 13100 1543 31 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 Select the 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 FORS2 img acq target acquisition FORS2 img obs crsplit science exposure or similar for imaging polarimetry FORS2 ipol acq fast target acquisition FORS2 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 SPECPHOT HIT I or PMOS the OB would typically consist of the following three templates FORS2 lss acq f
85. ose to the center of the field of view Appendix F World Coordinate System Information The header of the FITS file used for preparing a FORS target mask with FIMS should contain the following keywords for a linear scale CTYPE1 CRVAL1 CRPIX1 CTYPE2 CRVAL2 CRPIX2 CDELT1 CROTA1 CDELT2 CROTA2 EQUINOX RA TAN 12 345678 512 0 DEC TAN 12 34567 525 5 3 234E 5 10 0 3 234E 5 10 0 2000 0 Te aa UMS ie OY TA a m gg 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 DEC in deg y coord of reference pixel Pixel x scale degrees per pixel rot in degrees from N to E y scale degrees per pixel rot in degrees from N to E equinox for ESO instruments where PC keywords are the rotation matrix CTYPE1 CRVAL1 CRPIX1 CTYPE2 CRVAL2 CRPIX2 CDELT1 CDELT2 PC001001 PC001002 PC002001 PC002002 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 RA 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 X scale degrees per pixel y scale degrees per pixel cos CROTA sin CROTA sin CROTA cos CROTA
86. persion AAA filter A A A mm A pixel at Acentral XGRIS 600B 92 4452 3300 6012 50 0 75 780 XGRIS 3001191 8575 6000 11000 108 1 62 660 0G5904 32 XGRIS 3001191 8575 5032 6600 108 1 62 660 The central wavelength is defined as the wavelength Acentral 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 Acentral 267 pxx dispersion 2 6 4 High Time Resolution Mode Multiple Shift Mode HIT MS A multiple shift MS mode is also available This mode is predominantly implemented for fast spectroscopy and allows a block of rows to be shifted together rather than a single row as is the case with the previously implemented one shift OS mode In the MS mode two user defined slits can be used with a mask manufac tured for the MXU which place the spectra of the target and a comparison star for slit loss determination onto a small region of the CCD After a pre defined wait time the rows of the CCD are rapidly in 50 microsec shifted causing the exposed region to be moved into the storage area the unexposed region of the CCD and a new region to be illuminated This shift and wait scheme continues until the first pair of spectra taken reach the limit of the storage region and the CCD is subsequently read out in the normal way This HIT MS mode is offered in visitor mode with either the MIT or E2V mosaics It is offered
87. r 7 90r 2 e 80r J 80r 7 rob 1 70b 9 60r E S 60r 2 sob E 50r 1 5 401 1 5 40r y o 50 F 1 o 50 F 7 20r 1 20r 7 10 L 1 10 1 0 1 1 1 0 1 1 1 300 400 500 600 700 800 400 500 600 700 800 Wavelength nm Wavelength nm R_Bessel 100 m 90r 1 E 80r 1 HON 1 5 605 2 Got 1 5 40r 1 5 30r 1 b ZO 7 10F 1 O Luh 1 1 u a 500 600 700 800 90010001 10012001300 Wavelength nm Figure B 2 Additional Bessell filter transmission curves 52 Transmission Transmission FORS User Manual VLT MAN ESO 13100 1543 u Gunn v Gunn 100 100 90 L 90 1 80 lt 80 1 70 L 70 60 L 1 S 60 1 50r J 2 50 401 E 40 1 301 S 30 J 20 L E 20 J 10 L 10 J 0 0 1 300 400 300 400 500 Wavelength nm Wavelength nm g Gunn r Gunn 100 100 r 7 90 L J 90 1 801 g 80 J 70 L 4 A 70 eor o 60 7 50r 50 1 40r 7 E 40 7 30 L 5 30 J 20 L E 20 1 10 10 1 0 1 0 1 1 L f 400 200 600 200 600 700 800 900 1000 1100 Wavelength nm Wavelength nm 100 90r e 80r New 70 Le S eor 50 E 40 S zob 20F 10r 0 1 f 1 700 800 900 1000 1100 Wavelength nm Figure B 3 Gunn filter transmission curves FORS User Manual VLT MAN ESO 13100 1543 53 B 2 Interference Filters Table B 2 lists all presently available interference filters used with for FORS2 Their characteristics are giv
88. r I 4046 557 Hg 7281 349 He I 4077 831 Hg 7346 200 Cd 4347 500 Hg I 7383 900 Cd 4358 343 Hg 7383 981 Ar I 4471 479 He I 7385 300 Cd 4678 160 Cd 7438 900 Ne I 4713 200 He I 7488 870 Ne I 4799 920 Cd 7503 868 Ar I 4916 070 Hg 7514 652 Ar I 4921 929 He I 7535 800 Ne I 5015 675 He I 7635 106 Ar I 5085 824 Cd 7724 210 Ar I 5341 100 Ne I 7948 176 Ar I 5400 562 Ne I 8006 157 Ar I 5460 742 Hg 8014 786 Ar I 5764 419 Ne I 8103 693 Ar I 5769 598 Hg 8115 311 Ar I 5790 656 Hg 8264 523 Ar I 5852 488 Ne I 8300 326 Ne I 5875 620 He I 8377 367 Ne I 5881 900 Ne I 8408 210 Ar I 5944 830 Ne I 8424 648 Ar I 5975 534 Ne I 8495 360 Ne I 6029 977 Ne I 8521 442 Ar I 6074 338 Ne I 8591 259 Ne I 6096 160 Ne I 8634 648 Ne I 6143 063 Ne I 8654 384 Ne I 6163 594 Ne I 8667 944 Ar I 6217 281 Ne I 8681 900 Ne I 6266 495 Ne I 8704 150 Ne I 6304 790 Ne I 8853 867 Ne I 6334 428 Ne I 8919 500 Ne I 6382 991 Ne I 9122 968 Ar I 6402 246 Ne I 9201 800 Ne I 6438 470 Cd 9224 499 Ar I 6506 528 Ne I 9300 850 Ne I 6532 880 Ne I 9354 218 Ar I 6598 953 Ne I 9425 380 Ne I 6678 149 He I 9657 784 Ar I 6678 300 Ne I 9784 501 Ar I 6717 040 Ne I 10140 000 Hg 6907 160 Hg 10394 600 Cd 6929 468 Ne I 10830 171 He I 6965 431 Ar I 7032 413 Ne I Table D 1 Wavelengths of the arc lamp lines with the corresponding element FORS User Manual VLT MAN ESO 13100 1543 10000 Image F2_PRLL_
89. r radial velocity studies line width determinations abundance analyses it is recommended to take a through slit image before and after each science observation 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 in SR mode and fixed slit widths The approximate offsets of the slits with respect to the central 0 28 slit are given in Table 2 5 They are shown in terms of offsets on the sky as well as on the CCD collimator dependent The requested slit for an observation is selected by a decker mask See appendix E 2 for the orientation The actual LSS slit positions 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 instrument after maintenance typically a few times per year and following Visitor Mode runs with the auxiliary E2V detector However the centering accuracy of the objects on the slits is not affected by these variations in the projected 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 involves the operator selecting the target on a direct image of the target field 2 for faint sources the acquisition can be done with blind offsets the offsets w
90. rallel 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 observing modes IMG LSS and IPOL are supported for both collimators some restrictions apply in the modes MXU MOS HIT and PMOS direct imaging IMG imaging with occulting bars OCC imaging polarimetry IPOL multi object spectro polarimetry PMOS SR collimator only multi object spectroscopy with masks MXU SR collimator only multi object spectroscopy with movable slitlets MOS SR collimator only longslit spectroscopy LSS high time resolution imaging and spectroscopy HIT SR collimator only FORS User Manual VLT MAN ESO 13100 1543 EE co TEN L A o o Collimator Y Collimator Drive Unit Section Filterwheels u aa ao Filter CTI EC ka LM E Camera Section Interference A Filterwheels B SS Figure 2 1 Schematic view of the FORS instruments Standard Resolution High Resolution 250 e Figure 2 2 Light paths for the standard and high resolution collimators FORS User Manual VLT MAN ESO 13100 1543 5 2 2 Standard Instrument Configuration FORS is operated with a standard configuration with certain opto mechanical components permanently mounted in fixed positions This instrument configu
91. ration 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 configu ration changes The current standard configuration is listed below in Table 2 1 The interference filters given in Table 2 3 and up to 10 MXU masks will be mounted on user request Please note that the instrument standard configuration will be only modified in exceptional cases upon request and with the approval of ESO Such requests should be submitted to usd help eso org before the beginning of the observing period with a justification for the changes Instrument Location P2PP Entry Name Component Name Focal area MOS 19 slitlet multi object spectroscopy unit LSS 9 slit longslit mask unit MXU mask exchange unit for multi object spec troscopy with up to 10 masks polarimmask Mask unit for imaging polarimetry with HR collimator Collimator unit COLL SR 6 Standard resolution collimator COLL HR 47 High resolution collimator Retarder swing arm RETA4 4 Quarter wave plate mosaic RETA2 5 Half wave plate mosaic Wheel 1 Wollaston wheel WOLL 34 13 Wollaston prism g_HIGH 115 Standard g band filter GRIS 1501 27 Grism 1501 GRIS 600R1 19 Grism 600RI GRIS 600z 23 Grism 600z z GUNN 78 Standard z band filter Wheel 2 grism wheel GRIS 10287429 Grism 1028z GRIS 1400V 18 Grism 1400V GRIS 600B 22 Grism 600B GRIS 1200B 97 Grism 1200B GRIS 1200R 93 Grism 1200R GRIS 3001111 Grism 300
92. rection is along the X direction of the CCD in all spectroscopic modes with the exception of the HIT mode where a dedicated grism that has been rotated through 90 degrees is used 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 4 1 Grisms and Order Sorting Filters Normal Grisms a set of normal grisms is available 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 4 All grisms are mounted in the grism or Wollaston wheels of the parallel beam section Please note that some grisms are only available in visitor mode see Table 2 4 Holographic Grisms in addition to the normal standard grisms some medium resolution high throughput grisms are available with FORS2 GRIS 1400V 18 GRIS 1200B 97 GRIS 1200R4 93 GRIS 1200g4 96 GRIS 102872 29 GRIS 600R11 19 and GRIS 600z4 23 These grisms are based on volume phased holo graphic gratings which are cemented between two glass prisms see Figures C 1 for the 1st order throughput measurements A special note about grisms 6
93. requency 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 All other values scale with the exposure level sigma in masters goes down by a factor sqrt N where N 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 2002 ADU raw pattern 20000 0 696 0 596 18 2596 0 7 Table 4 2 Large scale structure and small scale noise in sky flats high gain CCD readout Table 4 3 gives typical exposure times for screen flats for the SR collimator and Bessell filters The numbers are indicative only since they are subject to changes due for instance to lamp replacements The observatory staff has updated values at hand and takes also care
94. se files will be saved by FIMS in directory fims SET 4 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 5 Prepare the observing blocks a typical OB in imaging mode with occulting bars OCC mode will consist of two templates FORS2 img acq align target acquisition FORS2 img occ crsplit science exposure or similar for imaging polarimetry FORS2 ipol acq target acquisition FORS2 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 HIT MS or PMOS the OB would typically consist of the following three templates FORS2 mos acq target acquisition FORS2 mos obs slit through slit image FORS2 mos obs off science exposures here the MOS mode as an example but with an identical sequence of observing templates for the other spectroscopic 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 FORS2 ima cal coll collimator selection FORS2 mos cal daycalib screen flats amp arcs where the first template is only used to select the collimator There are a few imp
95. sing 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 10 FORS User Manual VLT MAN ESO 13100 1543 2 3 4 HR Collimator Field Stop 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 po larimetry 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 FORS2 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 FORS User Manual VLT MAN ESO 13100 1543 11 2 4 Spectroscopy Spectroscopy Modes FORS2 offers four spectroscopic observation modes LSS P MOS MXU and HIT A variety of grisms with different wavelength ranges and dispersions is available see Table 2 4 The grisms can be combined with filters for order separation or more specialized settings The dispersion di
96. sition 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 M1 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 than 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 in G Avila G Rupprecht J Beckers Atmospheric Dispersion Correction for the FORS Focal Reducers at the ESO VLT Optical Telescopes of Today and Tomorrow A Ardeberg ed Proc SPIE 2871 1135 1997 Chapter 4 Calibrating and Reducing F RS Data 4 1 Calibration Plan The VLT observatory aims at providing calibrations of the FORS instruments with an accuracy as listed in Table 4 1 Applicants have to request additional observation time including overheads if much h
97. ssion filter z_SPECIAL 43 Special z band filter width 20nm FILT 917 64 88 Special z band filter width 6nm FILT 530 25484 Munich intermediate band filter FILT 500 5 85 Munich O III filter FILT 503 5 86 Munich O III filter redshifted by 1800 km s Table 2 3 Exchangeable filter set 2 3 3 User Provided Filters The installation of user provided filters in FORS2 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 ESO via the Instrument Operation Team email fors2 eso org The diameter of user provided filters shall not be smaller than 138mm parallel beam to avoid vignetting which would be eguivalent 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 to be sent to the users on reguest 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 focu
98. test data is prepared by the data handling administrators on DVD Reduced data no matter if pipeline or interactive reduction are not on the package and the visitor is required to back up any of these files that they wish to keep Please send us your end of mission reports with evaluations and suggestions available from WEB page http www eso org sci facilities 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 greater than 20Hz The guide stars are automatically found from the GSC 2 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 the HR collimator Depending on 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 same guide star otherwise it will automatically try to find a new one Whether or not such te
99. the Ne lamp at higher resolution Note that the switch on times can be defined individually for each lamp in the corresponding calibration template For grism GOOD 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 a slit width of 1 Calibration spectra taken with the different grisms are plotted in figures D D 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 FORS User Manual VLT MAN ESO 13100 1543 41 exposure times for delivered service mode OBs unless otherwise stated in the README file Wavelength calibration exposures are done during the day only with the telescope in zenith and the instrument in calibration position 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 9 45 by the equation fo fe JU Pe R 555 a 1 f f being the ordinary and extraordinary beam of the object measured for a given retarder plate angle 0 1 as 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
100. tion plan as described in Section 4 1 The combination of two filters at the same time are generally not supported in normal operation since these setups would require testing and software reconfiguration 6 FORS User Manual VLT MAN ESO 13100 1543 Components and modes that are no longer offered please see Appendix H FORS User Manual 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 25 pixel with the standard resolution collimator and 0 125 pixel with the high resolution collimator and the binned 2x2 15 um pixels of the MIT CCD mosaic as well as for the binned 2x2 15 um pixels of the E2V 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 Standard Resolution High Resolution Image Quality 80 in 072 80 in 071 within 4 0 within 2 0 collimator focal length 1233 mm 616 mm camera focal length 280 mm 280 mm final f ratio 3 13 6 25 MIT mosaic Pixel Scale 2x2 0725 pixel 0125 pixel Pixel Scale 1x1 7 07125 pixel 7 070632 pixel Field of View 6 8x 6 8 42x42 E2V mosaic Pixel Scale 2x2 0725 pixel 0125 pixel Pixel Scale 1x1 0125 pixel 7 070632
101. trument performance 17 restrictions 16 LADC 34 LSS mode 13 slit orientation 70 x offsets 15 manuals 1 MOS mode 13 fims only 13 movable slits 13 slit lengths 13 slit orientation 69 slitless spectroscopy 15 MXU mode 13 restrictions 15 SR collimator only 15 target acquisition 15 visitor mode arrival time 15 observing 29 fast or fims 29 OB preparation 30 OCC mode 10 overhead times 31 32 example 31 P2PP WEB page 1 Paranal Science Operations contact information 2 WEB page 1 pipeline data reduction 42 PMOS mode 17 instrument performance 17 restrictions 17 polarimetry 16 chromatism of the half wave plate 42 43 circular polarization 41 imaging polarimetry 16 instrument performance 17 linear polarization 41 slitless spectro polarimetry 17 78 FORS User Manual VLT MAN ESO 13100 1543 spectro polarimetry 17 Rapid Response Mode 23 replaced components 75 RRM 23 service mode observations contact information 2 observing with FORS 29 WEB page 29 slitless spectroscopy 15 SPECPHOT mode 13 spectroscopy 11 astrometric requirements 11 38 catalog of the HgCd He Ne and Ar lines 58 data reduction for pre imaging data 38 field of view 11 flat fields 39 grisms holographic 11 grisms standard 11 grisms response 55 instrument flexures 12 lamp exposure times 40 longslits 13 order separation filters 11 other filters 11 overview table of all grisms
102. ts in the field of 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 See section 2 4 8 for general comments on slitless spectroscopy Grisms and Filters for PMOS all grisms but GRIS 6002123 GRIS 1501 27 and GRIS 600RI 19 together with the recommended order separation filters can be used in PMOS mode The above mentioned grisms are 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 1 Retarder Plate Angles the retarder plate angles can be selected from a set of fixed predefined angles see Table 2 6 Collimator Constraints spectropolarimetry PMOS is possible only with the SR collimator 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 slit length is the standard 20 The other MOS slits are set up to the same position and slit width as slit 10 and can serve for sky background measurements Blind offset acquisitions are supported 2 5 3 Performance of the Polarimetric Modes The polarization optics allow the deter
103. tting 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 Photometric zero points are routinely calculated for the standard Bessell or special broadband filters of the instrument They are derived from the flux of the stars in e7 not in ADU They are provided to the users as part of the service mode package However they are meant for monitoring purposes and should not be used for scientific data without careful checks In particular they use default values for extinction and colour coefficients that may not be appropriate for a given night For details see http www eso org observing dfo quality FORSI1 gc zeropoints zeropoints html Or http www eso org observing dfo quality FORS2 gc zeropoints zeropoints html Science Data Science data are pipeline processed if they are obtained in a pipeline supported mode both for Visitor Mode and Service Mode data Any standard mode IMG observation can expect a reduced file if appropriate calibra tion data bias twilight flat are available For the spectroscopic modes all LSS MOS and MXU observations can expect reduced data i e wavelength calibrated and flat fielded 2 dimensional spectra and 1 dimensional extracted spectra corrected for sky background see however
104. us interval 1 0 pixel s Solid circle Zero Flexure Rodius interval 1 0 pixel s Solid line a 50 Su zat Solid line amp 50 180 180 180 188 Figure 2 4 Results of flexure measurements as a function of the rotator position for the SR collimator at zenith distance of 40 The panels show the flexure in un binned pixels across X and along Y the slit The solid green circle represents zero flexure zenith distance COLL SR COLL HR 0 15 lt 0 06 lt 0 03 30 lt 0 10 0 05 45 lt 0 14 lt 0 07 60 lt 0 18 lt 0 09 In all standard configurations telluric emission lines will fall into the wavelength range of FORS which allow to correct for any shifts rotation between science and calibration observations However when using the 1200B 97 grism there may be no telluric lines in the observed spectra The same is true for the 600B 22 or 1400V 18 grisms with MOS blades located towards the right red side of the instrument In the case of the MOS mode observations adding empty slitlets at the top and bottom of the mask which contain a wavelength range including 5577 can be helpful Shifts and or rotations between science observations of the same mask during the night cannot be corrected using sky lines In this case the through slit image can allow to look for potential movements of targets If FORS User Manual VLT MAN ESO 13100 1543 13 high precision is required e g fo
105. vations for target acquisitions through slit images and the science observation Additionally flat field templates for HIT I HIT S and HIT MS mode and an arc line spectral template for HIT S and HTT MS modes For the HIT I imaging mode the OB would consist of three templates in the following order FORS2 hiti acg fast target acguisition FORS2 hiti obs slit fast through slit image FORS2 hiti obs exp fast science exposures Very similarly in case of the HIT S spectroscopic mode FORS2 hits acg fast target acguisition FORS2 hits obs slit fast through slit image FORS2 hits obs exp fast Science exposures And finally in case of the HIT MS spectroscopic mode FORS2 hitms acq target acquisition FORS2 hitms obs slit through slit image FORS2 hitms obs exp Science exposures 2 6 6 Calibration plan The bias frames of the normal spectroscopic modes can be also used for modes HIT I HIT S and HIT MS This is not the case for flats fields and arcs of course 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 standard stars are to be selected by the HIT mode users and the respective observation blocks are to be prepared by the users In the case of the HIT MS mode a separate MXU mask will nee
106. vatory staff for calibration exposures In most cases 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 CCD read out modes 200kHz 2x2 low imaging and 100kHz 2x2 high spectroscopy will be supported 30 36 FORS User Manual VLT MAN ESO 13100 1543 for FORS2 2 any standard star observations to correct for telluric absorption lines radial velocity standards gt 0 any day or night calibrations for slitless spectroscopy 5 any day or night calibrations for spectroscopy with filters other than the recommended order separation filters GG375 GG435 OG590 and FILT 465 250 6 any day or night time polarimetric calibrations for retarder plate angles different from 0 22 5 45 67 5 degree linear and 45 45 degree circular polarimetry 7 any PMOS screen flats at retarder plate angles different from 45 0 degrees 8 any IPOL screen flats 9 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
107. w 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 SKY FLAT IMG high and low spatial frequency flat taken in twilight e photometric zero points from standard star observations e MASTER NORM FLAT ISS high spatial frequency flat slit function e DISP COEFF LSS wavelength calibration e WAVELENGTH MAP LSS image in which each pixel has its wavelength as pixel value 44 FORS User Manual VLT MAN ESO 13100 1543 e MASTER NORM FLAT MOS MXU high spatial frequency flat slit function e SLIT LOCATION MOS MXU table with the slit positions and edges e DISP COEFF MOS MXU dispersion coefficients from the wavelength calibration e CURV COEFF MOS MXU coefficients to describe and correct the spatial curvature of the spectra e WAVELENGTH MAP MOS MXU image in which each pixel has its wavelength as pixel value e SPATIAL MAP MOS MXU image in which each pixel has its position along the slit as pixel value Usually MASTER SKY FLAT IMG are used for flat fielding IMG data This removes all multiplicative artifacts in the image 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 fi
108. were 0 05 pixels in SR and 0 06 pixels in HR mode FORS1 SR Ar 2 091 10 9 r 1 228 1076 r2 0 360 10 r FORS1 HR Ar 9 515 10 9 r 3 605 1076 r2 1 001 1073 r The radial offset derived from the eguations above has to be subtracted from the measured position on the CCD The radius r 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 1 in pixels measured on the MIT CCDs FORS2 SR Ar 2 113 1079 r 2 158 1076 r2 0 537 10 r FORS2 HR Ar 7 133 10 9 r 3 782 1076 r2 0 160 107 r with r rx 15 24 binning Ar Ar 15 24 binning I 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 FORS1 to Kueyen In this case three fields of standard UCAC2 FORS User Manual VLT MAN ESO 13100 1543 37 Calibration Mode Collimator Frequency 4 Number Time Results Accuracy Bias every 5d 5 Day bias l
109. without filters atmospheric dispersion corrector or with a broad band filter to reduce the sky brightness in case of the TR 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 images taken with the targets on the slits typically have to be repeated in case of corrections of the order of 1 pixel Two loops are required to verify safely that the targets are on the slits 32 FORS User Manual VLT MAN ESO 13100 1543 Telescope telescope preset guide star acquisition active optics LADC resetting 3 min 0 75 min 2 min 1 min Interactive Acquisition excluding exposure time one loop IMG occulting TPOL one loop MOS MXU PMOS one loop LSS HIT two loops through slit exposure 1 5 min per loop 2 0 min per loop 1 5 min per loop 2 0 min per loop Instrument instrument setup collimator exchange retarder plate setup 0 5 min 4 5 min 1 0 min Exposure integration time E2V read out 100kHz binned E2V read out 200kHz binned E2V read out 200kHz unbinned MIT read out 100kHz binned MIT read out 200kHz binned MIT read out 200kHz unbinned user defined 39s 288 788 Als 31s 62s Table 3 1 Operational overheads with FORS2 on the VLT The through slit exposure is typically e
110. xecuted twice It is important to include the overhead times while preparing proposals and service mode observations packages FORS2 mos acq telescope preset guide star acquisition active optics 2 loops acq image integration time acquisition procedure FORS2 mos obs slit 2 loops instrument setup through slit integration time 2 60s through slit image 2 120s FORS2 mos obs off NEXP 1 amp NOFF 1 instrument setup science integration 1 3000s 100kHz 2x2 CCD readout 1 41s all OB execution time 180s 45s 120s 5s FORS2 img acq 120s telescope preset 180s guide star acquisition 45s 30s active optics 2 loops 120s 120s FORS img obs crsplit NEXP 1 amp NOFF 5 240s instrument setup 30s science integration 5 600s 3000s 30s 200kHz 2x2 CCD readout 5 31s 155s 3000s all OB execution time 3530s Als 3931s 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 template exist for the PMOS and IPOL science templates to setup the retarder plates This is now the 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 FORS User Manual VLT MAN ESO 13100 1543 33 3 7 Visitor Mode 3 7 1 The final pac
111. y FORS instrument The image scale of FORSI images taken before March 22 2003 would need to be corrected in the FTTS headers before usage This can be checked with a few 2MASS or UCAC2 stars to verify the image scale in the center of the field of view This should be discussed with the observatory staff usg help eso org before submitting the respective masks pre imaging source alignment quality FORS2 optimum FORS1 after March 22 2003 optimum other images amp catalogs optimum 2 4 3 Instrument Flexures The image motion due to instrument flexure under gravity is less than 0 2 un binned pixels over a 1 hour exposure with the standard and a 2 hour exposure with the high resolution collimator for zenith distances less than 60 Arcs and flats 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 given zenith distances FORS2 UT1 Flexure in x coordinate as function of Rotator Angle 9 amp Altitude a FORS2 UT1 Flexures in y coordinate as function of Rotator Angle 8 amp Altitude a Solid circle Zero Flexure Radi
112. y standard stars at center and end of night are observed whenever possible to assess the stability of the night Programs requiring photometric conditions might however want to submit a standard star OB to be executed close in time to the science OB to confirm the extinction stability on short time scales Note that a time dependent illumination pattern was revealed in FORS images Moehler et al 2010 PASP 122 93 which prevents photometric accuracies better than 1 to be reached for this reason standard star fields are being observed each time with different spatial offsets and rotation angles with the aim of eventually finding an analytic correction to the illumination pattern 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 night to be used by obser

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