ROSAT Mission Status
HRI (SAO) instrument in full-time use
High Voltage Change
Over the first four years of the mission, the gain of the HRI gradually dropped by a channel or so. On 21 June 1994, the high voltage setting was increased, resulting in a recovery of the gain (as measured by PHA distributions) to approximately the initial, post-launch level. See the February 1995 version of the HRI Calibration Report (anon ftp at SAO: sao-ftp.harvard.edu) for details.p>
We are currently re-investigating the UV leak to address concerns raised by H. Arp (Astron and Astrophys, 294, L45, 1995) that celestial UV sources may appear in PHA channels 1-3. Our initial conclusion, based on observations of NGC1097, is that the signal seen in the lowest 3 channels is noise due to the variation in gain over the detector (see Fig 18 of the HRI Calibration Report).
Source position determination has remained stable over the course of the mission, based on monitoring of UV calibration pinhole mask data, and comparison of M31 data taken early in the mission with data taken in 1994.