========================================================================= = = = ROSAT NEWS No. 66 --- 15-Oct-1998 = = = =-----------------------------------------------------------------------= = ROSAT Scientific Data Center at the = = Max-Planck-Institut fuer Extraterrestrische Physik (MPE) = = Postfach 1603, D-85740 Garching, Germany = =-----------------------------------------------------------------------= = e-mail address: rosat_svc@mpe.mpg.de = = ROSAT Service Area (including ROSAT Data Archive): = = www address: http://wave.xray.mpe.mpg.de/rosat = = ftp rosat_svc.mpe-garching.mpg.de user: anonymous = = interactive account (including ROSAT Result Archive): = = telnet xray.mpe-garching.mpg.de user: xray no password = =-----------------------------------------------------------------------= = XUV Center: GXUVDC@AIT.PHYSIK.UNI-TUEBINGEN.DE = = WFC Archive access via telnet/ftp ait.physik.uni-tuebingen.de = = user: xuv (password: xuv_archive) = = www address: http://astro.uni-tuebingen.de/rosat.html = ========================================================================= Contents: SEVERE DAMAGE TO THE ROSAT HRI WFC STATUS ROSAT AO-9 CALL FOR PROPOSALS POSTPONED SEVERE DAMAGE TO THE ROSAT HRI ============================== On Sept. 20, 1998, at 0:47 UT the ROSAT HRI was apparently damaged by an accidental slew during which the telescope axis pointed near the sun. The extent of the damage is still being evaluated. After the complete failure of the last XRT startracker on Apr 28, 1998 every attempt was made to continue ROSAT operations using a new configuration of the Attitude Measuring and Control System (AMCS), which made use of the startracker of the Wide Field Camera. At the end of August there was hope that the new configuration was working sufficiently reliably to resume observations. The nominal timeline was resumed on August 31, with observing efficiencies better than 50 % of that attained with the original AMCS configuration. The new AMCS configuration had never been foreseen before the launch of ROSAT. Certain limitations of it were therefore unavoidable (only three reference stars per target; long time delays between star measurement and evaluation of the stars by the AMCS; no derivation of angular velocities from unidentified, and thus unreliable, stars). An unfortunate chain of events in combination with these limitations finally led to the event on Sept 20, 1998. One of the reaction wheels came close to its revolution limit during a ROSAT night; the torque demands during a subsequent slew then caused the wheel to exceed its limit. The AMCS was no longer able to completely control the slew and the telesope axis pointed close to the sun. A sharp increase of the HRI countrate to 1000 cts/s, in combination with a sudden change (softening) of the HRI spectrum at 0:47 UT, indicated an anomalous event in the HRI detector. Shortly afterwards the HRI high voltage was switched off automatically. To investigate the status of the HRI several tests have been performed during the last two weeks, including operations out of the focus position, pointings at the (UV) dark Earth, at Vega, and at a calibration source (WFC Meaty). One quarter of the HRI shows extremly low count rates. When not looking at the sky, countrates of 1000 cts/s or more are observed, showing orbital variations not yet fully understood. To date, we have not been able to detect a celestial source although the UV calibration lamp functions, showing the expected spot pattern. These tests led to the conclusion that the HRI UV filter was destroyed during the event on September 20. The channel plates, however, may still be in order. Varying the HRI parameter settings (high voltage, threshold limits) up to now have not improved the operation of the detector. Investigations are continuing. WFC STATUS ========== At the time of the Sun pointing on 20 September, the WFC's protective sun-shutter was closed, preventing solar radiation from reaching the filters and detector. To date, the sun-shutter has remained closed. It is planned to perform a calibration measurement on an astrophysical source when the spacecraft operations allow. ROSAT AO-9 CALL FOR PROPOSALS POSTPONED ======================================= As a consequence of the ROSAT HRI damage the Call for Proposals (AO9), with a deadline of November 30, 1998, has been postponed until a final decision on the HRI status has been reached. ========================= end of NEWS ===================================