-------------+--------------------------------------------+--------------------- | FFFFFF OOOOOO SSS | STScI | FF OO OO SS | STAN - FOS Analysis | FFFF OO OO SS | Number 12 News | FF OO OO SS | July 1996 | FF OOOOOO SSS | -------------+--------------------------------------------+--------------------- CONTENTS: - FOS NEWS + FOS Internal Wavelength Calibration - RECENT HST SAFING EVENT - RECENT PREPRINTS - APPENDIX: FOS CONTACTS ================================================================================ FOS Internal Wavelength Calibration ----------------------------------- by M. Dahlem Using a model by M. Rosa (ST-ECF; ESA) based on first optical principles (FOPs) characterizing the FOS, we cross-checked the FOS internal wavelength calibration. M. Rosa has shown that the S-distortion of FOS spectra is due to the distortion of the image tubes. This effect can be modelled very precisely by a tangent. Once this analytical description of the spectral distortion is known, it can be used to check the currently implemented wavelength calibration which is based on polynomial fits to on-orbit calibration lamp spectra. In the pipeline processing wavelength templates are used that are stored in the FOS calibration database. If the positions of all reference lines used for the polynomial fits were known with perfect accuracy (and the FOP model also correct), then the (third-order) polyn- omial fits to FOS spectra taken with high-resolution gratings should match exactly the tangent form of the FOP model. However, this is not the case. Many lines (possibly blends or inaccurate laboratory measure- ments) are shifted with respect to the diode positions where they should be. We investigated how this affects the quality of the FOS wavelength calib- ration by marking the "bad" lines and taking them out of the reference lists on which the polynomial fits are based. Then spectra for which fits had been performed using all reference lines where re-fitted, with the new, reduced sets of lines. For FOS/BL the result is that all wavelengths > 1300 Angstrom are accurate to within 0.1 diodes (in most areas much better - <0.02 diodes). Only shortward of 1300 Angstrom a systematic offset exists of up to 0.15 diodes. These numbers can be compared with the internal non-linearity of the FOS diode arrays of 0.02 diodes and with HST's orbital motion of 8 km/s (0.025 diodes) which is not corrected for. Therefore an update of the current FOS wavelength calibration, at least longward of 1300 Angstrom, is not warranted. ================================================================================ Recent HST Safemode Event ------------------------------- by K. Rudloff and R. Doxsey HST entered inertial hold safemode at 17:31 UT on Saturday, July 13th. For reasons that are yet to be understood, one of the four elements of the on-board quaternion (which represents HST's idea of where it is pointed) jumped by the equivalent of 7 degrees. The sudden jump caused a loss of guide star lock just before an earth occultation. As the occultation ended, a safemode check was performed to determine whether HST was pointed at the earth. Because the quaternion was in error by 7 degrees, the check failed and safemode was entered (even though HST was sufficiently clear of the Earth and there was no risk to the FGSs). The safemode entry proceeded normally and all Instrument responses were as expected. An investigation of the problem by flight software staff is in progress. The discontinuous jump is consistent with a single bit flip in the words holding the data, but the cause of the bit flip is still unknown. There is no simple diagnostic which can be run on the computer system without additional risk, so, for the foreseeable future, one of the on-board tape recorders will be devoted to recording engineering data. This will provide a means for diagnosing the problem in the event that it is repeated. There have been no signs of additional problems since the event on July 13th, and HST returned to normal science operations on Wednesday evening, July 17th. STScI staff are actively modifying the schedules to fit within the constraints of using a single science tape recorder. ================================================================================ 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. DEUTSCH, E.W.; MARGON, B.; WACHTER, S.; ANDERSON, S.F. "HST imaging of bright galactic X-ray binaries in crowded fields" ApJ 11/10/96 GRINDLAY, J.E.; COOL, A.M. "HST/FOS discovery of probable CVs in NGC 6397" IAU Symp. 174 NOLL, K.S.; JOHNSON, R.E.; LANE, A.L.; DOMINGUE, D.L.; WEAVER, H.A. "Detection of ozone on Ganymede" Science accepted SILBER, A.D.; ANDERSON, S.F.; MARGON, B.; DOWNES, R.A. "Time resolved ultraviolet spectroscopy of DQ herculis: eclipses and pulsations" AJ 9/96 BALDWIN, J.A.; CROTTS, A.; DUFOUR, R.J.; FERLAND, G.J.; HEATHCOTE, S.; HESTER, J.J.; KORISTA, K.T.; MARTIN, P.G.; O'DELL, C.R.; RUBIN, R.H.; TIELENS, A.G.G.M.; VERNER, D.A.; VERNER, E.M.; WALTER, D.K.; WEN, Z. "Physical conditions in low ionization regions of the Orion nebula" ApJ accepted ================================================================================ 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. ================================================================================