Frequently asked FOS questions (last updated: 19 January 1998) ============================== This is a list of frequently asked questions (FAQ's) about the FOS in the post-COSTAR era, compiled from notes taken by the FOS team up through the spring of 1997. The questions are grouped into the following 12 sections: 1. Target acquisition a) different acquisition modes b) pointing accuracy 2. Side switching 3. Aperture throughputs, flux calibration and accuracy a) aperture throughputs b) flux calibration and accuracy 4. Aperture wheel, aperture location and orientation 5. Wavelength calibration and accuracy 6. Polarimetry 7. Re-calibration of spectra a) CALFOS b) other IRAF/STSDAS software 8. Observations with paired apertures 9. Scattered light 10. Rejection limit (RE]LIM) 11. Exposure times 12. Miscellany - currently includes Y-bases, dead diodes, and Modified Julian Dates. 1111111111111111111111111111111111111111111111111111111111111111111111111111111 1. Target acquisition using the binary search (ACQ/BIN) option. l.a) different acquisition modes Q: How do I tell what type of acquisition was performed? A: Check the OPMODE keyword in your .shh header. Q: How do I interpret the acquisition image? A: The FOS paper products provide useful diagnostics of acquisition inages, dwell scans, and exposure sequences. Please refer to Chapter 30 of version 3 of the HST Data Handbook for a thorough discussion of the interpretation of FOS acquisitions. Q: What are the limitations of the binary acquisition technique? A: Binary acquisition is recommended for targets that are point-like (in the wavelength range covered by the detector being used), not highly variable ( < +/- 1.0 mag) with reasonably well known spectral energy distributions, and not brighter than the MIRROR limits. Active galaxies with nuclear regions dominated by the galaxy starlight cannot use ACQ/BIN; these observations require ACQ/PEAK. Crowded fields or regions with many knots as in Seyfert galaxies can use ACQ/BIN to acquire the brightest target ONLY IF no more than 5 knots/stars are brighter than the faint limit of the acquisition. ACQ/BIN can also be used to acquire the nth brightest object in a field, but this is a seldom used mode . Q: Can the gratings be used during binary acquisition and what are the brightness limits? A: The binary target acquisition uses ONLY the MIRROR, gratings cannot be used. The brightness limits for the MIRROR and hence for a binary acquisition are given in Tables 1.11 and 1.12. of the FOS Instrument Handbook version 6. Q: What is the wavelength range and sensitivity of the FOS when using the MIRROR? A: The passband of the MIRRORs is effectively the same as for the detectors. Hence, for the BLUE detector the wavelength coverage will be from 1150-5500A, and the RED detector coverage will be from 1500 - 8000A. As a working procedure, assume that the efficiency of the mirrors is ~nearly wavelength independent, such that Fig. 1-1 of the version 6 FOS Instrument Handbook approximates the effective sensitivity of the FOS when using the MIRRORs. Roughly, the pivot wavelength for flat spectra is ~2430A, Blueside, and ~3240A, Redside. Q: How accurately should we know the spectral energy distribution of our target? A: Since binary acquisition expects the brightness of the target to be within a given range of counts, the success of the acquisition depends on how accurately one can predict the counts. Thus, knowledge of the spectral distribution over the wavelength range of the MIRROR is necessary. For example, for quasars Fn ~ n-1 is a reasonable approximation. If the spectral distribution is unknown or if there is an uncertainty of more than 1 magnitude in target brightness, then ACQ/BIN is not recommended. Q: What is the overhead time required if an offset target acquisition is used? A: The time required for physical movement of the telescope from the offset star to the target is very small. Therefore, only the overheads associated with the acquisition mode being used need be included in orbit-packing calculations. Q: Do I need to do a peak-up after slewing to the target? A: You must add in quadrature the pointing uncertainty of your offset slew and the uncertainty of your target acquisition method. If the observation is in the 4.3" or the 1.0" aperture and the accuracy of the acquisition of the offset star is 0.08" or better, then the target typically can be acquired directly into the aperture and observations started. There is no need for an extra ACQ/PEAK in such a case. However, if an aperture smaller than 0.5" is being used or the S/N of the observation has to be >30, then an ACQ/PEAK after the offset slew is essential. (Also, see section 1.b, Pointing Accuracy.) Q: Do I need to put the WFPC2 early acquisitions in the observation summary forms? A: Yes, the WFPC2 early acquisitions have to be given in the observation summary forms. The early acquisitions are treated as separate WFPC2 observations. Requiring Guide Star selection with the same GS Plate ID is essential! l.b) pointing accuracy Q: What is the pointing accuracy for ACQ/BIN? A: The 1 sigma pointing accuracy of an ACQ/BIN is 0.08" for the BLUE detector and 0.12 n for the RED detector. These pointing accuracies imply that observations with the 4.3" and 1.0" that do not require high photometric accuracy (S/N < 30) can be performed without the need for a peakup. If a pointing accuracy better than 0.1" is required, then a small peakup is recommended. Q: Are offsets determined from the Guide Star Catalog (GSC) sufficient? A: The guide star catalog positional accuracy is approximately 0.3", hence, GSC-determined offsets are not sufficiently accurate and an extra ACQ/PEAK will be required after the slew. Offsets determined from the WFPC or the WFPC2 are preferable. Q: What is the accuracy WFPC or WFPC2 early acquisition (EARLY ACQ) astrometry? A: An offset slew of 1 arcmin between targets whose coordinates are based upon WFPC early acquisition astrometry typically yields a 1 sigma pointing uncertainty of 0.08 arcsec. The accuracy of WFPC2 astrometry was not determined at the time this was written but should be comparable. It is vital that the same GS Plate ID be used for the FOS and the WFPC/WFPC2 observations as coordinate frame offsets can exist for targets located near the edges of overlapping GS plates. Several early acq derived acquisitions failed for this reason. Q: How do I determine the uncertainty in the pointing of the FOS? A: In the Instrument Handbook v6, section 2, acquisition modes and their pointing accuracies are described. One should use these rules and the pointing information in the 'Paper Products' provided with the data (or generated with iraf.stsdas.hst_calib.paperprod software). Q: How do I confirm that the pointing of an FOS observation was correct? A: To be absolutely sure, you'd have to take an ACQ image after the slew of the telescope (the slew is AFTER the ACQ/PEAK). If all ACQ/PEAK images are ok, and data are good, then you may assume that the FOS was pointed properly. There is also a very costly procedure that uses engineering data to confirm pointing. This option is only available if it is deemed necessary by an Instrument Scientist. 222222222222222222222222222222222222222222222222222222222222222222222222222222 2. Side-switching Q: What was the overhead for a side-switch? A: 50 minutes, which often was hidden in an occultation if the orbit was properly packed. Q: Is a peak-up required after a side-switch? A: A peak-up is required after a side switch for all observations with aperture 0.5" or smaller OR if pointing accuracy better than 0.15" is desired. The peak-up strategy should be the same as the last stage of peak-up used before the side-switch. Q: Was 6 orbits the limit for side switches? A: There was no limit on the number of orbits imposed directly by the side switch process. This statement supersedes any statements in the version 5 FOS Instrument Handbook. All FOS observations were limited by the number of SAA-free orbits available. The number of orbits commonly available were a typical minimum of 6 and a typical maximum of 9, depending on the declination of the target and the time of the year. FOS could not observe during SAA passage. Q: When observing the same object with redside and then with blueside, does the telescope slew? A: Yes. There is no change in guidestars, but the telescope must slew the distance between the apertures, ~57". The Phase II Proposal Instructions, figure 7.1, page 7.8, as well figure 1.3 of the Instrument Hanbook, illustrate the aperture layout. 333333333333333333333333333333333333333333333333333333333333333333333333333333 3. Aperture throughputs, Extended Source Flux Calibration 3.a) Aperture throughputs Q: How do I calculate FOS aperture throughput for extended sources? A: Please refer to the HST Data Handbook V2, chapter 17, and ISRs CAL/FOS-106,136 and 140. There is also a "cookbook" for this procedure available through help@stsci.edu. CALFOS calibrates the data as if the object is a point source well centered in the aperture. For extended sources which overfill the aperture, the GO can redo the calculation to get a more appropriate flux. Please refer to chapter 17 of the HST Data Handbook V2 and ISRs CAL/FOS-106, 136 and 140. A cookbook for this procedure is also available through helpOstsci.edu. For extended sources smaller than the aperture Field of View (FOV): one needs to produce a model of the target; convolve it with an FOS PSF (see ISR CAL/FOS-148); normalize the total counts in this image to 1; then sum the counts within the FOV of the aperture. This is then the throughput factor to use in scaling the pipeline's flux value after first factoring out the pipeline's point-source throughput factor.(see the Data Handbook, ISR 106 or the extended source flux calibration cookbook from help@stsci.edu). Q: Are FOS calibrated fluxes calculated relative to the 4.3 aperture? Do I have to multiply the fluxes by a relative throughput to get the correct flux for other apertures? A: For point sources, no action by the user is necessary. Flux calibration observations were made with the 4.3" and 1.0. aperture. Wavelength dependent ratios of the throughput(also measured in observations of stellar standards) of the other apertures wrt the 4.3 were used to produce flux calibrations for those apertures. The pipeline's conversion from counts to flux takes into account whichever aperture was used. 3.b) Flux calibration and accuracy Q: Why doesn't my post-costar FOS data appear to be flux calibrated even though the header indicates that this step was performed? A: For post-costar FOS data before about March 29th, 1994, there were no Inverse Sensitivity Files to use for flux calibration, so dummy files were used in the pipeline. Hence, although the header keyword switch was set to COMPLETE, this calibration step was not correctly implemented. The pixels in the dummy file are all set to 1.0, so the clh file resulting from application of this 'flux correction' is actually still in units of corrected counts per second. If your data is affected by the use of such a "dummy" file, the proper IVS files can be retrieved from the archive and used to re-calibrate your data. Also, 0.1"-Pair and Bar aperture data receive a dummy pipeline flux calibration. Recalibration using a 4.3" aperture IVS for Bar data and the 4.3" IVS * throughput correction of 1/(~0.4-0.5) for 0.1"-Pair data is recommended. Q: For the FOS 1" aperture, what is the percentage of flux measured by the FOS if the source is centered. What about if its not centered? A: Instrument Science report 97 provides an analysis of flux variations due to pointing offsets from the target. For a well- centered source the 1" aperture throughput is ~90-95% of the 4.3" aperture. 444444444444444444444444444444444444444444444444444444444444444444444444444444 4. Aperture wheel, aperture orientation, location Q: How does one find the exact center position of the FOS aperture on the sky? A: Check your Paper Products(iraf.stsdas.hst_calib.paperprod). There is also a cookbook for this available through help@stsci.edu. Also check page 15 of the FOS Instrument Handbook, version 6.0. The information is in the keywords RA_APER1 and DEC_APER1, however these keywords should not be relied upon for moving targets. (If you have a moving target--contact help@stsci.edu for assistance.) Q: How do I find out where the aperture is pointed using an ACQ IMAGE? A: The center pixel of an ACQ image is at the aperture center. The pixel centers are diode width/4 apart in X and diode height/16 apart in Y. The target offset from center of the ACQ IMAGE will be the nominal offset from aperture center in any associated spectra. There are cookbook descriptions of ACQ IMAGE data available from help@stsci.edu. Q: Which way is the FOS 0.25 x 2.0 arcsec slit oriented? Is the Y-axis the short axis, or the long axis? A: The y-axis is the long axis, which defines the aperture angle. For PA_APER=0, the y-axis is aligned with North, as shown: ----- | | | | | | N^ | | y-axis | | | E<-- | | | | _____ x-axis Q: Is there more than one aperture wheel, or are the same apertures used for both redside and blueside? A: There is one aperture wheel, but there are two sets of apertures. When the 4.3" aperture is positioned over the redside detector, another, separate 4.3" aperture is positioned over the blueside detector. So, there are two physically different apertures, but they are the same size and are positioned simultaneously over the red and blue detectors. For further information see: CAL/FOS ISR-131 and ISRs related to aperture sizes at http://www.stsci.edu/ftp/instrument_news/FOS/fos_bib.html 5555555555555555555555555555555555555555555555555555555555555555555555555555555 5. Wavelength calibration and accuracy Q: What is the GIM correction? A: It is a correction to account for variations in the earth's magnetic field made necessary by problems with the magnetic shielding of the FOS detectors. The deflection of the photo-electrons in the FOS Digicon tubes can be adjusted to compensate, in both the X and Y directions, for the Geomagnetic induced Image Motion(GIM). Q: Was the GIM correction made on-board, or was it part of the pipeline calibration? A: For data acquired after April 5, 1993, the GIM correction was done on-board. For data prior to that date, the CALFOS software could be used to perform a correction for the offset ONLY along the X, wavelength dispersion axis. The Y shift is calculated and reported by the software but no correction to the recorded number of counts can be performed. Q: Why do I see wavelength offsets between different FOS spectra of the same object? A: The offset may be due to the non-repeatability in the positioning of the filter-grating wheel. Please refer to ISRs CAL/FOS-49 and 60 and 145 for a description of this positioning problem. Q: There is an unexplainable loss of signal at one end of the spectrum. The tilt or slope of the continuum of my spectrum is odd. A: Imperfect target centering in the aperture may combine with the curvature of the FOS spectra (caused by imperfections in the magnetic focusing of the photoelectrons onto the diode array) can lead to effects such as these. If the observed object spectrum is significantly offset on the diode array relative to the standard star observations used to create the sensitivity calibration, the flux calibration may not adequately match your observation. Please see ISRs CAL/FOS-96 and 110. Q: What is the dispersion of the prisms? A: The dispersion in Angstrom/pixel is as follows: wavelength RED BLUE detector detector 1500 0.24 0.31 3000 6.6 6.6 6000 56.9 57.7 9000 114.8 140.7 Q: What is the wavelength accuracy of the FOS gratings? A: The wavelength accuracy of the FOS observations depends on the accuracy with which the target has been acquired and the accuracy with which the FOS Filter-Grating-Wheel (FGW) has been positioned. Mis-centering parallel to dispersion in the aperture will produce a wavelength zero-point offset. For precision wavelength observations, we recommend a pointing accuracy of one quarter-stepped pixel (0.07") or better in order to remove the acquisition-related centering uncertainty. The FGW-related uncertainty can introduce a random offset of up to approximately 0.3" diodes (1 diode corresponds to 250 km/see with all FOS high-dispersion gratings). For precision wavelength analyses an FOS WAVE exposure must be acquired immediately before or after the science exposure with no motion of the FGW between the exposures. Typical WAVE exposures are of the order of a few seconds so that the proposer is charged only the standard 6 minute ACCUM overhead. Q: Could FOS WAVE observations be hidden in Earth occultation? A: No, HST flight software did not allow this. Q: What type of velocity correction is made to FOS data? What is the maximum amount of shift to heliocentric wavelengths at 2000A? A: There is no on-board velocity correction made to FOS data. The FOS is unable to resolve wavelength shifts of the order of HST's orbital motion. The GHRS does have high enough resolution to be affected by these velocities and its detectors are operated accordingly. However, if wavelength shifts of +/- 30 km/s are significant to your measurement then the Earth's orbital motion should be removed post-facto. Q: Is the FOS wavelength scale converted to air wavelengths? A: No, unlike IUE, all HST wavelengths are vacuum wavelengths. 6666666666666666666666666666666666666666666666666666666666666666666666666666666 6. Polarimetry Q: Would I be wasting my time to try to detect 0.5% polarization in the UV ? A: The non-repeatability in positioning the filter grating wheel implies that the intrinsic measurement uncertainties are larger than 0.5%. The errors are further affected by the instrumental polarization from the COSTAR mirrors. All these various errors indicate that targets with intrinsic polarization of <=2% in the UV will be very difficult to detect with the FOS. Further, due to the extra reflections and the increased instrumental polarization the number of photons required to produce a definitive detection has increased such that objects with V magnitude brighter than ~14.5 are preferable. Q: What are the polarimetry overhead times? A: The overhead times, in addition to the 6 minutes of ACCUM overhead, are as follows: POLSCAN 4: 5 minutes POLSCAN 8: 8 minutes POLSCAN 16: 12 minutes Q: Can a polarimetry observation be split over two orbits? A: No, polarimetry observations cannot be split over two orbits, a complete POLSCAN cycle has to be done in a given orbit. Naturally, S/N can be improved by combining POLSCAN cycles obtained in different orbits. Q: Does CALFOS handle multiple read polarimetry data? A: CALFOS versions earlier that 1.3.2 do not properly handle polarimetry data that have nread > 1. You won't get an error from CALFOS, but the answers will be wrong. CALFOS V.1.3.2 (and greater) correctly handles multiple read polarimetry data. Also, note that the 'stsdas.hst_calib. fos.spec_polar' package can be used to analyze polarimetry data. **All** polarimetry data obtained via the HST archive should be reprocessed with the current CALFOS. A: Version 1.3.2 properly handles this data. 7777777777777777777777777777777777777777777777777777777777777777777777777777777 7. Spectral (re)calibration with CALFOS and other IRAF/STSDAS software tools 7.a) CALFOS Q: How can I find out more about the uncertainties in my calibration? A: See the Data Handbook, Chapter 15. It explains sources of photometric error and some possible problems with data. Q: In what order are the calibration steps performed in the pipeline? A: See the Data Handbook, Chapter 14. It explains the calibration procedure in detail. Q: How is the scattered light correction implemented in CALFOS? A: See ISR CAL/FOS-103. Q: Is there a list of changes made to CALFOS and dates they were implemented? A: Here's a summary of changes. While running iraf and with the FOS package loaded you should be able to cd to fos then cd to calfos and page ycalfos.f to see a complete listing of CALFOS versions and change notes. (pipeline implementation dates may lag the dates given there for the software edits, however the .trl, trailer, file of a given data set will list which CALFOS version was used to perform the calibrations.) CALFOS Changes Delivered 1.3.0 Fix GIM round-off error; 7-Oct-93 check YFGIMPEN keyword before applying GIM corr; fix missing error msg; fix memory allocation bug in BAC_CORR 1.3.1 Added scattered light correction; 21-Mar-94 Fix polarimetry nread>1 bug. 1.3.2 Added PEDIGREE checking; 19-Apr-94 Fix background scaling for polarimetry nread>1 data. 1.3.2.2 Retain negative Stokes I values 30-Aug-94 in polarimetry routines. 1.3.2.31 Fix bug causing uncalibrated 16-Sep-94 polar data to crash. 2.0 White dwarf flux scale implemented 21-Oct-94 Proper units|in .clh file for all data > April 19,1994. Focus correction and sensitivity degradation accounted for. 2.2 updated so/that post-costar data would 16-May-95 run properly. 2.3.2 Scattered light algorithm updated end-Dec-95 adding median filter w/4sigma reject. 2.4.1 AIS correction available for mid-Mar-96 pre- and post-COSTAR. Q: What is the CCS9 file? CALFOS 'asks' for it, but it isn't listed in my raw data header. A: The CCS9 file is a new reference table related to the implementation of a scattered light correction for some of the FOS gratings . It is a new addition to CALFOS in V.1.3.2. The task, ADDNEWKEYS can be used to update older data headers with the proper keywords and switches before re-calibrating. For a detailed explanation of this process please refer to CAL/FOS-103 and page 245 of the Data Handbook. The scattered light correction will be performed on spectra from those gratings which expose only a subset of the diodes so that counts in the unexposed region can be used as a measure of the scattered light. The correction used is the mean value in this region, after the model background is subtracted and pixels with values 4 sigma from the median are rejected. Any data previously calibrated without this correction should be reprocessed. Q: CALFOS gives me a warning that the GIM correction is not being applied. Is this something to worry about? A: No, since April 5th, 1993 the GIM correction is done on-board, so it is skipped in the pipeline. The correction is still an option in CALFOS so that data taken before the on-board GIM correction was installed can be re-calibrated with the GIM correction. Be carefull, the software does not protect you from applying the correction a second time to data that was already 'de-GIMed' on-board. Q: How does CALFOS handle Rapid-Read mode differently than ACCUM mode? How do I produce a final spectrum from Rapid-Read data? A: CALFOS creates a c3h/c3d file for Rapid-Read spectra. This file is the total flux and total propagated error for each data group of the flux file (clh). RCOMBINE will produce an average of the fluxes from all the readouts. Q: When I run CALFOS on my data I get a warning that reads: " WARNING: Fill data in object, frame 1" etc. What does this mean? A: This may mean that you lost some data due to tape recorder tape turnaround or somewhere else in the transmission of the data. Look for diagnostic messages in the ocx, pdq and trl files. An example: "Due to a tape recorder track change groups of data were lost.'' In that case, to find out which groups you've lost use HEDIT to look at the keyword FPKTTIME for EACH GROUP and look for time gaps. Comments found in the Paper Products may also help identify the cause of the FILL. 7.b) other IRAF/STSDAS Tasks Q: How do I stitch spectra of different wavelengths together? A: There is a "cookbook" for this procedure available through help@stsci.edu. Although, unimplemented at this writing, an STSDAS task that will perform this function is in preparation and may be available by the time you are reading this. Q: Is there an IRAF task I can use to combine the groups of my RAPID READ images? A: Yes, RCOMBINE will combine groups of an image. Q: Is there a way to combine my polarimetry images in IRAF? A: Yes, PCOMBINE will do the job. Check the SPEC_POLAR package in IRAF for other useful polarimetry analysis tasks. Q: Can I combine spectra and weight it by error or variance? A: Yes. Use the PCOMBINE task in SPEC_POLAR (in IRAF). It does not work on multigroup images so you will have to copy the last group of your ACCUM data to an image of its own, or use RCOMBINE to combine your RAPID READ/groups into a single group image. You will have do this for the c1h, c2h and cqh before you run PCOMBINE. Also, you must use HEDIT to update the headers of your new clh, c2h and cqh file so that NREAD = 1. Q: Can I deconvolve my spectra? A: Yes. If you feel that deconvolution is necessary, there is a cookbook available through help@stsci.edu. Q: Can I convolve a spectrum by a Gaussian to match the resolution of a lower resolution spectrum? A: Yes. This can be done with the IRAF task, GAUSS. You will need to calculate your kernel size in PIXELS. There is a cookbook available through help@stsci.edu. Q: Do the tasks LINEFIND and DISPFITY need the positions of the lines in diodes or in pixels? A: LINEFIND and DISPFITY need the positions in diodes. The documentation on these tasks is currently poor because the tasks were not intended for public use. Assistance is available through help@stsci.edu. 8888888888888888888888888888888888888888888888888888888888888888888888888888888 8. Observations with paired apertures Q: Can the exposure time spent on the SKY aperture be less than that on the OBJ aperture? A: No, the exposure time on the SKY observation is equal to that on the OBJ observations. It cannot be changed. The OBJ and SKY observations cycle in alternating samples, of ~10s duration, while building up the specified exposure time. The proposal Logsheet exposure time is therefore divided equally between OBJ and SKY. Q: Will I get only the sky subtracted spectrum? A: The calibration pipeline produces both the calibrated OBJ and SKY spectra and stores the SKY in the c8h file. It also produces a sky subtracted spectrum and provides it in the clh file. Q: Can I use a paired aperture but avoid sky subtraction? A: Yes. You must specify STEP-PATT=SINGLE as an optional parameter (in Cycle 6 this became the default). Then, observations will be obtained through only the specified upper or lower of the aperture pair. For Cycle 5 the default became the upper (or 'A') of the paired apertures. Pre-Cycle 5 the default was the lower (nearer OTA V1 axis), B, aperture. 999999999999999999999999999999999999999999999999999999999999999999999999999999 9. Scattered light Q: What is the amount of scattered light in the G130H grating? A: The scattered light is due to the gratings down-scattering light originating from >2500A. The amount of scattered light in the G130H grating spectral region depends on the spectral energy distribution of the target. Hence, very red objects like red stars and elliptical galaxies could have their UV spectrum totally dominated by the scattered light. Q: I have pre-costar data. Is there a scattered light routine I can use on my data? A: For pre-costar data in some detector/grating combinations you can run the task ADDNEWKEYS to update your data headers, and then run CALFOS in STSDAS version > 2.4.1. This will perform a simple scattered light calculation (described in CAL/FOS-103 and on page 245 of the Data Handbook). At the same time one should set the AIS correction switch to PERFORM so that the white dwarf flux standard will be applied along with the time dependent throughput and instrumental sensitivity corrections. 101010101010101010101010101010101010101010101010101010101010101010101010101010 10. Rejection limit (REJLIM) Q: Is REJLIM possible after COSTAR A: Yes, REJLIM can be used after COSTAR. However, it was presented as an OPTIONAL PARAMETER only in the Engineering version of Phase II proposal instructions, not the General Observer version. There were extremely few FOS science observations taken with REJLIM turned on. Mostly, it was used in calibration and DARK measurements. A search of the FOS team's database of HST proposals yields the following list of proposals with possible use of REJLIM; 1050, 1053, 1316, 260, 28, 3198, 3897, 4132, 4135, 4698 and 4856. The keyword YNOISELM in the .ulh header indicates the value of REJLIM. YNOISELM=65535 is equivelent to REJLIM = 0 (turned off). There is a datatype mismatch between the true value of that keyword and the one used by the pipeline software that populates the value entered into the header. ISR-76 indicates that a value of -1 had to be used in the commanding to force the onboard software to turn REJLIM off. This appears consistent with the ulh value which would be the result of having treated a signed integer(-1) as unsigned, yielding 65535. Q: How does REJLIM work? A: REJLIM is used with FOS ACCUM or IMAGE mode. The detector read-out STEP-TIME is chosen such that only a specified number of counts (typically 1) is expected to be detected, in total, in all of the diodes, in the detector live-time for a particular STEP-TIME. (For a more complete discussion of the difference between LIVE-TIME and STEP-TIME, see the FOS Data Handbook, but for simplicity's sake, assume they are the same.) For example, with REJLIM=1, if more than 1 count is detected by the entire diode array in the specified read-time, the on-board software assumes that the signal contains contamination by a burst of signal from cosmic ray induced particle background, the affected data are discarded with no backup, and data taking continues for the remainder of the specified exposure time. (Note: the discarded readout is NOT repeated!) If only 1 count is detected, then it is assumed that the count must be from the source and the data are saved. If it were expected that cosmic rays produced stronger bursts of background counts, then a higher value of REJLIM could be used to exclude only those STEP-times in which such bursts were detected. Our best understanding of the nature of the background currently suggests most background events do NOT occur in bursts, so that REJLIM > 1 is not recommended. ----> Please note that REJLIM is an "all-or-nothing" operator on each detector read-time. If an imprudent choice is made for the read-time, a large portion or ALL of an observer's data could be lost! Naturally, REJLIM should be considered ONLY for observations of very faint sources! See also, CAL/FOS ISR-76. Q: How was REJLIM specified in the proposal? A: This was specified as a parameter ONLY during the phase II part of the proposal submission process. However, it was presented as an OPTIONAL PARAMETER only in the Engineering version of Phase II proposal instructions, not the General Observer version. You MUST refer to the RPSS or RPS2 input file to see if REJLIM was specified. It was specified via the comment field. 1111111111111111111111111111111111111111111111111111111111111111111111111111111 11. Exposure times Q: How can I get my exposure time from the FOS data header? A: Header Keyword EXPOSURE gives the live integration time in seconds that went into gathering the counts in any given pixel. Note that this is not the same as your requested total exposure time (elapsed time for recording the data) (unless NXSTEPS and 0VERSCAN = 1) because only 512 pixels (diodes) are exposed at a time while building up the typical 2064 wavelength pixels. See the FOS Instrument Handbook, v.6 pages 58-59). Q: How do I get the Total elapsed time of an integration for an FOS observation? A: Header keyword EXPTIME. The FOS Instrument Handbook, v.6 pages 58-59, gives the following formula for calculating total elapsed time of an integration (in seconds) Delta T = (LIVETIME + DEADTIME) * INTS * NXSTEPS * OVERSCAN * YSTEP * NPAT * NREAD * 7.8125E-6 Livetime and Deadtime are in units of the on-board FOS microprocessor clock ticks, 7.8125 microseconds apiece. All words in capitals above are header keywords. The start time of EACH GROUP is FPKTTIME from the header of that group. Q: What is the difference between diodes and pixels and how does this affect the exposure times? A: In normal ACCUM, RAPID, or PERIOD mode observations the spectrum is shifted in the X (dispersion direction, parallel to the diode array) by quarter-diode steps 20 times (NXSTEPS=4 times OVERSCAN=5). This leads to separate integrations having been performed at 2064 different wavelength PIXEL positions by the 512 DIODES. Your requested exposure time is the time necessary for the 512 diodes to sample the 2064 pixels. Since the 2064 wavelength positions are not integrated simultaneously, the actual exposure time received by each pixel EXPOSURE is not the same as the allocated duration of the exposure, EXPTIME. Except for some pixels at each the end of the the array, EXPOSURE is 1/NXSTEPS of EXPTIME. 1212121212121212121212121212121212121212121212121212121212121212121212121212 12. Miscellany Q: What is Modified Julian Date? A: HST uses this form of dating. Modified Julian Date (MJD) = Julian Date - 2400000.5 MJD starts at midnight UT unlike the more common JD that starts at noon UT. Q: How can I recognize a dead or noisy diode in my spectrum? A: This is explained in Chapter 32 of the Data Handbook version 3. Your "flakey" diode will typically be 20, OVERSCAN * NXSTEPS, pixels wide. First pixel affected = (N - 1) * NXSTEPS + 1 Last pixel affected = (N - 1) * NXSTEPS + (NXSTEPS * OVERSCAN) N = diode #, 1 to 512. OVERSCAN and NXSTEPS values can be found in the data header. Beware: in IRAF diodes are numbered 1-512, but the FOS documentation often labels them 0-511. Q: What is a Y-base? A: The unit of measure of Y position on the FOS photocathode. The strength of the magnetic deflection of the Digicon tube is adjusted so that 256 ybase steps will equal the height of the FOS diodes.