-------------+--------------------------------------------+--------------------- | FFFFFF OOOOOO SSS | STScI | FF OO OO SS | STAN - FOS Analysis | FFFF OO OO SS | Number 15 News | FF OO OO SS | January 1997 | FF OOOOOO SSS | -------------+--------------------------------------------+--------------------- CONTENTS: - FOS NEWS + Important FOS/BL G130H Sensitivity Changes + Observed FOS PSFs and LSFs, ISR-148, is Undergoing Revision - New Version of the Data Handbook is Planned - RECENT PREPRINTS - APPENDIX: FOS CONTACTS ================================================================================ Important FOS/BL G130H Sensitivity Changes -------------------------------------- by Tony Keyes Following are two important items pertaining to FOS/BL G130H sensitivity changes and methods of analysis. 1) FOS/BL G130H sensitivity measured with the 4.3 aperture appears to have increased in the wavelength range 1250-1600 by approximately 5% between July 1995 and late December, 1996. A possible decrease in sensitivity of several percent also seems to have occurred for the region 1150-1180 Angstroms. The 25 April 1996 sensitivity measurements for FOS/BL G130H indicated that instrumental sensitivity remained consistent with the mean sensitivity derived from all prior pre-COSTAR measures within 3% though at certain wavelengths these measures were near one 3% extremum. No changes to the pipeline FOS sensitivity reference file were deemed necessary after the April 1996 observations. No additional successful sensitivity measurements were obtained until 23 December 1996. These (and two more recent) sensitivity measures indicate a 5% increase in the wavelength region 1250-1600 Angstroms relative to the pipeline sensitivity, which is virtually identical with July, 1995 sensitivity. Naturally, these changes can be very important for analyses requiring precise absolute or relative spectrophotometry with G130H. Possible explanations for the observed changes include image motion on the photocathode and migration of the photosensitive bialkali materials in the photocathode. As part of FOS close-out activities we will provide new and more detailed time-dependent sensitivity corrections including a more precise discussion of color uncertainties in this spectral range. At the present time, we suggest an interim correction of 0.25% per month INCREASE in instrumental sensitivity SINCE 1 July 1995 be applied to the FOS/BL 1250-1600 Angstrom range. NOTE: the sense of this correction means that corrected FOS fluxes will be SMALLER than pipeline calibrated values in this region. Essentially no correction seems required at 1200 A. A linear ramp between 1200 and 1250 can be applied. 2) We wish to remind observers that FOS instrumental sensitivity is a very steep function of wavelength shortward of 1200 Anstroms. Although instrumental sensitivity is determined to 3% accuracy in this region, this steep sensitivity gradient requires accurate knowledge of observed wavelengths to insure accurate flux calibration. Common instrumental uncertainties can produce important systematic errors in pipeline processing. Both telescope pointing uncertainties and FOS filter-grating wheel (FGW) position uncertainties can affect observed FOS wavelengths. An error of 0.5 Angstroms, or 2 pixels, produces little effect longward of 1200 A., but yields systematic photometric errors of 2% at 1180 A. and 4% at 1150 A. Larger miscenterings will yield proportionately larger systematic errors in this region. Target centering inaccuracies of only 0.15 arcsec or filter-grating wheel misalignment of 0.5 diodes will yield such 2 pixel offsets. Note that ACQ/BINARY alone is not sufficient to avoid pointing errors of this magnitude. The target acquisition strategies recommended in the FOS Handbook version 6 for programs requiring accurate photometry and/or wavelengths yield pointing accuracies of 0.04 arcsec (~0.5 pixels). Three-sigma FGW uncertainties are of the order of 1.2 pixels. Observers with a need for accurate absolute or relative flux calibration in the 1150-1200 A. region should examine fiducials in the G130H spectral range, such as Lyman alpha absorption, to determine the degree of miscentering along dispersion. Since the pixel-positions of flat field features are not affected by target centering, any re-calibration should proceed normally through flat field correction (.c5 output products), next the spectrum should be shifted by the appropriate amount, and then flux calibrated. Observed FOS PSFs and LSFs, ISR-148, is Undergoing Revision. ------------------------------------------------------------ by Ed Smith The post-COSTAR, FOS/RED, observed PSF provided in CAL/FOS ISR-148 includes artifacts. The image was created by co-addition of 30 separate data sets. Several of these data sets have been found to contain possible reflection and diffraction effects due to the target's proximity to the edge of the aperture. Therefore the FOS/RED PSF, in particular, should not be used. It is being reproduced without the artifacts and all other PSFs and LSFs provided in ISR-148 are being reviewed. This review will result in a new ISR. The revision will include PSFs appropriate for data taken in standard ACCUM mode as well as reproducing ISR-148's IMAGE mode PSFs. ================================================================================ New Version of the Data Handbook is Planned -------------------------------------------- by M. Voit A revised version of the HST Data Handbook, including new chapters on NICMOS and STIS, is being planned for this fall. To aid in our planning, we would like your comments on the current version (Version 2, December 1995). How can we improve it? What features do you find helpful or unhelpful? Does the current structure quickly lead you to what you need to know? What issues should be covered in greater depth? Is there any material you have found particularly confusing or unclear? Please send all comments to Mark Voit at voit@stsci.edu. ================================================================================ RECENT 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. KORMENDY, J.; BENDER, R.; AJHAR, E.A.; DRESSLER, A.; FABER, S.M.; GEBHARDT, K.; GRILLMAIR, C.; LAUER, T.R.; RICHSTONE, D.; TREMAINE, S. "Hubble Space Telescope spectroscopic evidence for a 1 x 10 (9)-M black hole in NGC 4594" ApJ 12-20-96 www.ifa.hawaii.edu/~kormendy BIANCHI, L.; VASSILIADIS, E.; DOPITA, M. "Hot evolved objects in different parent galaxies: the stellar winds of three planetary nebula nuclei in the Large Magellanic Cloud" ApJ accepted SCHULTZ, A.B.; HART, H.M.; HAMILTON, F.C.; KOCHTE, M.; BRUHWEILER, F.C.; BENEDICT, G.F.; CALDWELL, J.; CUNNINGHAM, C.C.; FRANZ, O.G.; KEYES, C.D.; BRANDT, J.C. "Lessons learned from an HST faint companion search program" Planets Beyond the Solar System HAMILTON, A.J.S.; FESEN, R.A.; WU, C.-C.; CRENSHAW, D.M.; SARAZIN, C.L. "Interpretation of UV absorption lines in SN1006" ApJ accepted WU, C.-C.; CRENSHAW, D.M.; HAMILTON, A.J.S.; FESEN, R.A.; LEVENTHAL, M.; SARAZIN, C.L. "Far-UV absorption lines in the remnant of SN 1006" ApJ 3-1-97 ================================================================================ 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 on the WWW at http://www.stsci.edu/public/propinfo.html. Analysis, STSDAS or any other questions can also be addressed to help@stsci.edu ================================================================================ To subscribe or unsubscribe send a message to listserv@stsci.edu with the Subject: line blank and the following in the body: [un]subscribe fos_news YOUR NAME ================================================================================ Comments, questions, suggestions, 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. ================================================================================