Under what conditions do poor aspect errors usually appear? and how good are the attitude solutions?
The SAO verification team has found that aspect errors greater than 20" appear, usually when the total observation has been obtained in at least 2 pieces with 6-12 months between the observations. The usual conditions are that one or more of the following occur: i) when a bright source is in the field, it will look doubled (tripled, quadrupled, ...) or at least elongated; ii) when the image is filtered on the observation interval, and the resulting images are blinked, a spatial displacement appears; iii) signatures in the K-timing test that coincide with the gap in the data.
Systematic discrepancies have been 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 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 ~ September 1992). Also, all REPROCESSED (Rev1) data will have this (or an updated) boresight correction folded in.
There were some inaccuracies in the original study however, 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 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. It is thought that this problem may be due to star tracker problems. It seems that the star tracker has some variations in quantum efficiency between the pixels. These variations mean that the centroiding to get the position of the guide stars can be skewed. It seems unlikely that trouble can be corrected for, as not all of the star tracker pixels are calibrated. It seems like the star tracker problem is 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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