ROSAT Status Report #67:
Oct 12, 1993
This is to report on a roll angle problem. This information was found on the UK bulletin board.
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Note 2.22 PSPC 22 of 24
LTVAD::JULO 58 lines 14-SEP-1993 17:47
-< error in roll angle for PSPC & HRI >-
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It has recently been realized that there is a systematic error in the roll angle with respect to the astronomical coordinate frame of the PSPC and HRI. The effect of this roll error is not included in the current SASS processing. (It will be included in the "Rev1" reprocessing of the entire dataset planned for the near future.)
The error is small, being 0.185 degrees for the PSPC. This corresponds to a position error of 6 arcseconds at the radius of the support ring.
In principle users can correct for the roll angle error by rotating an image about the centre of the field of view. However, there is also a known attitude solution error which causes X-ray position offsets of order 6 arcseconds (see note 15.1). The origin of this attitude solution error is not known. Before rotating an image to correct for the roll angle error, the image should be shifted to correct for the attitude solution error. As this cannot be done without accurate positions for a number of the X-ray sources in the field of view, I expect that users will make the roll correction only rarely.
Below I have included a note from MPE on the subject of the roll angle error.
------------------------------------------------------------------------------- From: MPE::MPECR1::MAK 14-SEP-1993 11:57:47.75 To: MPE::LTVAD::JULO CC: Subj: rot. angles different
Dear Julian,
the values of the rotation angles I determined for a rotation around detector center are different for PSPC and HRI. The direction of rotation, however, is the same.
1) PSPC: 0.185 deg +/- 0.099 deg (derived error).
In the usual sky image where north is up and east is to the left (the way you usually have it on your screen) the sources are rotated in the counterclockwise sense, i.e. you must turn your RA-DEC coordinate system in the counterclockwise direction to make the X-ray positions of sources match up with the catalog positions (or --- equally --- you must turn the X-ray image back in the clockwise direction to keep the coordinate system).
2) HRI: 0.4 deg +/- 0.1 deg (estimated error).
The rotation is counterclockwise again.
It is planned to consider these values (rounded to 0.2 deg for PSPC) in future SASS (re-)processings. Note that (in addition to the rotation effect) your image can have an unpredict- able translatory offset of order 10 arcsec in any direction. If you can reliably identify a few sources in the image you can apply the corresponding shift.
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