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9.3 FGS Control

Two different kinds of computers can control the three FGSs. One is HST's housekeeping computer, the DF 224. The other is the Fine Guidance Electronics (FGE) microprocessor associated with each FGS. Both the FGEs and the DF 224 control the FGSs while they are guiding HST. However, when one of the FGSs is being used as an astrometer in POSITION mode, its FGE controls it, and when it is scanning in TRANSFER more, the DF 244 controls it.

The fundamental time interval for an FGS is 25 milliseconds, the shortest time over which the FGSs can compute a fine error signal or respond to commands. Independent of instrument mode or activity, the DF 224 gathers all FGS data at 25 millisecond intervals, assigns them specific locations in the engineering telemetry stream, and downlinks them to the ground. The engineering telemetry format at the time of transmission determines what FGS data are included in the downlink and the rates at which they are reported.

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