-------------+--------------------------------------------+--------------------- | FFFFFF OOOOOO SSS | STScI | FF OO OO SS | STAN - FOS Analysis | FFFF OO OO SS | Number 8 News | FF OO OO SS | September 1995 | FF OOOOOO SSS | -------------+--------------------------------------------+--------------------- CONTENTS: - FOS NEWS + FOS Advisory: Polarimetry + New ACQ/PEAK Strategy - INSTRUMENT SCIENCE REPORT + Cycle 4 Photometric Calibration of the FOS (CAL/FOS 144) - Observatory Logs - Calibrating Hubble Space Telescope: Post Servicing Mission - RECENT FOS PREPRINTS - APPENDIX: FOS CONTACTS ================================================================================ FOS NEWS -------- by Jeff Hayes Operationally, the past 2 months have been rather uneventful. The instrument continues to function nominally. Over the last month and a half, we have had 20 GO/GTO proposals execute, along with 2 calibration programs. A presentation on the activities of the FOS group and the status of the instrument was made to User's Committee. FOS Advisory: Polarimetry ------------------------- This announcement presents a substantial revision of the status of FOS polarimetric capabilities for Cycle 6 and is effective immediately. Any questions concerning the issues in this announcement should be directed to the SSD HelpDesk (help@stsci.edu). Analysis of Cycle 4 polarimetric calibration observations indicates an absolute uncertainty of only 1-1.5% for FOS/RD linear polarization measures with spectral elements G190H and G270H. We had expected and advertised an accuracy level of 0.7% and a goal of 0.2% for FOS/RD. Since the throughput of FOS/RD is better by about a factor of two than for FOS/BL, the Cycle 6 FOS Instrument Handbook advised use of FOS/RD and we intended only to further calibrate FOS/RD for polarimetry. Calibrations of FOS/BL have shown limiting accuracies of 0.2% for high S/N observations. As a result, FOS spectropolarimetric recommendations are changed as follows: 1. All Cycle 6 proposers are advised to use FOS/BL for spectropolarimetric observations. 2. No further FOS/RD spectropolarimetric calibrations will be performed by STScI. 3. FOS/BL will be calibrated for spectropolarimetry in cycle 5 instead. The following is a more detailed summary intended to replace polarimetry summaries in sections 1.6.1 and 3.3.6 on pages 11 and 51 of the Cycle 6 FOS Instrument Handbook. Please note that the general discussion of polarimetry, including the calculation of expected polarimetry count rates, provided in section 1.6 (page 10) of the Handbook remains valid. a.) FOS/BL will be calibrated in cycle 5 for G190H, G270H, and G400H. The 1.0 aperture will be used for these calibration observations. For bright spectropolarimetric calibration sources, limiting absolute polarization uncertainty is 0.2% based upon cycle 4 measures. This level of accuracy is obtainable only for very high S/N observations. Naturally, for fainter sources the accuracy is limited by photon statistics obtained and by the intrinsic polarization of the target object. b.) The "B" waveplate should be used for FOS/BL G190H and G270H observations. The "A" waveplate should be used for FOS/BL G400H observations. c.) The 1.0 is the recommended aperture. Smaller apertures than 1.0 may be used, but the photometric aperture throughputs of smaller apertures relative to the 1.0 in polarimetry mode will not be measured by STScI. This will affect only the photometric calibration of small aperture polarimetric observations. Smaller aperture observations will take longer than 1.0 aperture observations to reach the same S/N, of course, but the polarimetric calibration is valid for smaller apertures. d.) Usage of the 4.3 aperture is NOT recommended. e.) FOS/BL G130H polarimetric observations longward of 1300 A. are not absolutely precluded, but will not be calibrated by STScI. Linear polarization limiting accuracies are uncertain, but are expected to be no better than 1 part in 5 or 0.4%, whichever is larger. Observers would be responsible for the lengthy calibration procedures - details are available from the Helpdesk. No spectropolarimetric measurements can be made shortward of 1300 A. New ACQ/PEAK strategy --------------------- Since 20 June 1995, we have new commanding for the ACQ/PEAK. With it, acquisitions are more efficient, with up to 20% less overheads, and we can now perform ACQ/PEAKs on moving targets. In short, this new strategy is more efficient, robust, and precise. This strategy is totally transparent to the user and does not affect the way in which Phase I proposals are completed. ================================================================================ INSTRUMENT SCIENCE REPORT ------------------------- ``Cycle 4 Photometric Calibration of the FOS'' (CAL/FOS 144) by Bohlin, Lindler, and Keyes reports the status of the absolute flux calibration of the FOS using 4 pure-hydrogen White Dwarfs through the 4.3 and 1.0 aperture on both FOS/RD and FOS/BL. Except for Red G190H and Red G160L, there are no observed systematic changes in sensitivity from Feb 1994 to July 1995. From Feb 1994 to July 1994, there was a 2% drop in sensitivity in the Red G190H, but from July 1994 to July 1995, there was a 3 to 8% *increase* in sensitivity. The Red G160L shows the same behavior shortward of 2200A. The overall uncertainties in the FOS flux calibration for point sources is on the order of 3%. A PostScript version will be posted to the FOS Documentation page on the WWW. Paper copies can be requested at help@stsci.edu. ================================================================================ Major Change to the Observation Logs ------------------------------------ by Olivia Lupie, Bruce Toth The Observation Logs contain a specialized set of pointing and engineering data associated with each science exposure (including the jitter information). They have the same first 8 characters as the rootname of the science exposure to which they pertain, and at present, are composed of a header (with extension CMH) and two fits tables (extensions CMJ and CMI). These files are now also included in the tapes mailed to GOs. Currently, to accurately define the jitter function, the very large CMJ file to produce a jitter image is used. Due to the excessive size of it and because the primary information of interest to an observer is the jitter distribution, it was decided to replace it with an image of the jitter. This image is appreciably smaller (64pix x 64pix; 0.002"/pix) and contains all the information from the high time resolution file which the user needs to deconvolve their image. The coordinate system is the detector coordinate orientation for all Science Instruments. An empty file is produced for moving targets, internal exposures and Earth calibrations. Jitter images for moving targets will be produced in a future release. Since this is a major change to the Observation Logs, the file extensions were changed. The Observation Log header file now has an extension of JIH. This file contains all the original contents of the CMH file plus the keywords needed to describe the jitter image. The table, which averages each parameter over a 3 second interval, now has an extension of JIT. The contents of this table remain exactly the same as the old CMI file. We would like to emphasize the importance of understanding the origin of the parameters and their errors before interpreting the results. For example, the absolute pointing provided in the Log is reconstructed from the guide star data and the guide star absolute coordinates whose catalog errors are of order several tenths of arcseconds. When the tracking mode uses the gyros only, the absolute pointing is reconstructed from the gyro data which has time-dependent errors due to gyro drift. Documentation describing the details and accuracy of the Observation Logs is available on line under the Observer page in the Documentation section (http://marvel.stsci.edu/ftp/documents/html/documents.html). Please note that the documentation is still undergoing revision. Help retrieving these files from the HST archive can be requested from archive@stsci.edu. Please send all other questions to help@stsci.edu ================================================================================ Calibrating Hubble Space Telescope: Post Servicing Mission ---------------------------------------------------------- The proceedings of "Calibrating Hubble Space Telescope: Post Servicing Mission" workshop (Anuradha Koratkar and Claus Leitherer, editors) were published. All participants of the workshop, cycle 4 and cycle 5 GOs and GTOs were mailed a copy. A very limited number of extra copies are available. If you are interested in receiving one, please send a message to help@stsci.edu. An electronic version of the proceedings will be posted to our Documents page. ================================================================================ RECENT FOS PREPRINTS -------------------- We draw your attention to these papers, based on FOS data, that will appear in the next few months. This list includes all preprints received by the STScI Library not yet published in the journals. Please remember to include our Library in your preprint distribution list. "Ultraviolet spectroscopy" Dufour, R.J. "HST/FOS spectroscopy of VW Hydri in superoutburst" Huang, M.; Sion, E.M.; Hubeny, I.; Cheng, F.H.; Szkody, P., ApJ 2/10/96 "HST FOS spectroscopy of the dusty SMC H II region N88A" Kurt, C.M.; Dufour, R.J.; Garnett, D.R.; Skillman, E.D.; Mathis, J.S.; Peimbert, M.; Torres-Peimbert, S.; Walter, D.K. ================================================================================ APPENDIX: FOS contacts ----------------------- Any questions about the scheduling of your observations should be addressed to your Program coordinator. Post-Observation questions can be addressed to your Contact Scientist. If you do not know who these persons are, you can find the information in http://presto.stsci.edu/public/propinfo.html. Analysis, STSDAS or any other questions can also be addressed to help@stsci.edu ================================================================================ Comments, questions, requests for issues, additions or deletions to the mailing list, etc. can be e-mailed to help@stsci.edu ================================================================================ The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. ================================================================================