A.1 FGS Control

Two different computers interface with and control an FGS at various times. The first is the 486, which controls the HST spacecraft and its pointing control system. The second is the Fine Guidance Electronics (FGE) microprocessor associated with each FGS. Both the 486 and the FGEs control the FGSs while they are guiding HST. During astrometric science observations, control is shared by the two computers: the FGE operates the FGS during Position mode, while the 486 operates the FGS during Transfer mode scans.