Just how good are the positions?
The absolute positions appear to be good to about 5". The boresight corrections appear to be the leading cause of aspect problems. There are residuals boresight errors that mimic roll-angle-dependent offsets at the level of a few arcsec. The relative positions appear to be good at nearly the 1" level, once the ROSAT field has been registered with known positions. An attitude reconstruction error of unknown origin produces slightly egg-shaped images.
Systematic discrepancies were noted between the optical and X-ray positions of ROSAT targets. In an MPE study, the 1 sigma scatter of the distribution of optical versus X -ray positions was 6.1 arcsec for the PSPC (B) and 6.4 arcsec for the HRI. The PSPC data showed a systematic offset of 6.9 arcsec, while the HRI showed negligible systematic offset (the HRI always had a boresight correction of 10 arcsec in the Y-direction). For a long time, no correction was made to SASS to account for this systematic effect in the PSPC data because there was a large scatter in the offsets noted (up to 20 arcsec), and the effect was not understood. However, it finally was decided to correct the SASS for the mean offset of six arcsec in the PSPC data. The correction was implemented in December 1992 at MPE (SASS 6-2), and affects data processed since then (or processed since Jan 1993 in the USA, which is equivalent to observations made since about September 1992). Also, all REPROCESSED (Rev1 or later) data will have this (or an updated) boresight correction folded in.
However, there were some inaccuracies in the original MPE study, notably that the nearest SIMBAD source was always taken as the optical counterpart, when, in fact, that was not always the ID with the best position. This innaccuracy meant, of course, that the boresight error was originally underestimated. A new study then showed the deviations from expected (SIMBAD) positions still lay clustered around 3-4 arcsec on the positive detector y axis. Another 3-4 arcsec correction is required, in the same sense, as the original corrections do not get rid of the scatter in the difference between optical and X-ray positions. This additional correction will be implemented in SASS 7_0 onwards. The reason for the discrepancy is not yet understood.
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