How accurate are the coordinates produced by the PSPC processing?
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 data 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 was decided to correct the SASS for the mean offset of 6 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) data will have this (or an updated) boresight correction folded in.
However, some inaccuracies existed in the original study--notably that the nearest SIMBAD source always was taken as the optical counterpart, when in fact that was not always the ID with the best position. This problem meant, of course, that the boresight error was underestimated originally. 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. It is thought that this dilemma may be due to star tracker problems. It seems that the star tracker has some variations in quantum efficiency between the pixels. This situation means that the centroiding to get the position of the guide stars can be skewed. It seems unlikely that this problem can be corrected, as not all of the star tracker pixels are calibrated. The star tracker problem seems like the most likely origin of the large scatter in the discrepancies in X-ray positions. The additional correction will be implemented in SASS 7_0 onwards.
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