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ROSAT Status Report #174: ROSAT NEWS No.66

Date: Thu, 15 Oct 1998 15:07:31 +0200
Subject: ROSAT NEWS No.66

=                                                                       =
=              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              =

          WFC STATUS

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.

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.

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 ===================================