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[ROSAT AO-21991, NRA 91-OSSA-31991],
[Hasinger1990a, Hasinger1992a]
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 
with
respect to the optical axis of the XRT\
(i.e., to the S/C 
-axis;
Sect. 2.5.3).
The field of view of each sensor is a rectangle measuring
with an overlap of about 
.
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 
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 
,
as compared to the previous limit of three stars in a 
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.