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XRT Star Trackers (STC)

     

[ROSAT AO-21991, NRA 91-OSSA-31991], [Hasinger1990a, Hasinger1992a]

   figure1197
Figure 1.2: The fields of view (FOV) of the ROSAT position-sensitive proportional counter (PSPC), high-resolution imager (HRI), and star trackers (ST).

Two CCD star trackers are provided on the XRT (designated STC-1 and STC-2) for position sensing on guide stars. The optical axes of the star trackers are tilted by tex2html_wrap_inline16405 with respect to the optical axis of the XRT\ (i.e., to the S/C tex2html_wrap_inline16407 -axis; Sect. 2.5.3). The field of view of each sensor is a rectangle measuring tex2html_wrap_inline16409 with an overlap of about tex2html_wrap_inline16411 . The overlap region of the two star trackers also overlaps with about half of the PSPC and HRI fields of view (Figure 1.2).  

During operation, bright optical sources must be avoided by the star trackers, particularly the Moon and Earth's sunlit limb. These prevent any position determination when in the FOV.

The star tracker alignment matrices (between the star tracker and S/C coordinate frames) were corrected for misalignments which occurred during launch and kept constant thereafter. Small thermal motions of the star trackers with respect to the S/C are corrected to an accuracy of tex2html_wrap_inline16413 within the GSOC\ attitude solution S/W using the fiducial light filter (described in [Porte1988]).    

Due to a conventional digital electronics failure, use of STC-2 was lost 1990 September 8 (see also App. A).  

The software commanding of the remaining star tracker (STC-1) was improved over several months during the fall of 1991 to allow the tracking of six stars in search windows of tex2html_wrap_inline16415 , as compared to the previous limit of three stars in a tex2html_wrap_inline16417 window. This new software was uplinked in stages culminating in the resumption of normal S/C operations (following the interruption due to the loss of the Y-gyro; Sect. 1.2.3) on 1991 November 4. Despite occasional hiccups, no significant degradation in the performance on STC-1 has been detected.


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Next: South Atlantic Anomaly Detectors Up: 1.2 Payload Description Previous: In-Orbit Performance

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