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A.J. Bradley (Spacecraft System Engineering Services), E. Nelan (Space Telescope Science Institute)
Sub-diffraction limited HST observations are conducted using the replacement Fine Guidance Sensor 1r (FGS1r) in its high angular resolution TRANS mode. This instrument was installed in HST during Servicing Mission SM2. Because of its superior performance, FGS1r was commissioned as a science instrument in June 1999, replacing FGS3 in this capacity. One of the performance parameters trended for the FGS1r is the so called the Coarse Track/Fine Lock (CT/FL) bias. It represents the difference in the position of a star's photocenter, as measured in Coarse Track, versus its interferometric position as measured in Fine Lock.
In TRANS mode the FGS1r instantaneous field of view is scanned across a target so that the detailed fringe morphology and visibility can be reconstructed for scientific analysis. The starting position for the scan is offset from the Coarse Track centroid in such a way that ideally (when the bias is zero) the star is centered with respect to the scan. With a current on-orbit CT/FL bias of 0.5" in FGS1r, scans must be sufficiently long, on the order of 1.6", just to assure that the target's fringes are fully sampled. Given a constant scan rate (40 mas/sec), it is desirable that short scans can be used, so that more scans can be made during the observing window. This yields the highest possible S/N ratio (individual scans are co-added), an important objective when observing stars of faint magnitudes (V>14.5).
To remove the CT/FL bias, and thereby enable shorter scan lengths, changes were made to the flight software and to the TRANS mode commands. This paper reviews the origin of the CT/FL bias error, the mechanics of the TRANS mode observing, and the software and commanding modifications implemented to remove the bias error. Results from an on-orbit test will be shown.
The author(s) of this abstract have provided an email address for comments about the abstract: Spacecraft-SES@home.com