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Chapter 9
FGS Overview
In This Chapter...
FGS Capabilities / 9-2
FGS Design / 9-3
FGS Control / 9-15
Target Acquisition and Tracking / 9-16
Transfer Scans / 9-21
FGS Guiding / 9-21
FGS Astrometry / 9-22
Astrometry with FGS 3 / 9-23
The primary purpose of the three Fine Guidance Sensors (FGS) on HST is to maintain the pointing stability of the telescope at the milliarcsecond level. The FGSs, designed and built by Hughes Danbury Optical Systems (HDOS), routinely point the spacecraft with a precision of 5 mas or less. As part of the second servicing mission, HDOS upgraded the spare FGS, which replaced the original in Bay #1, with a commandable mechanism to enhance the instrument's performance. HDOS is currently upgrading the returned FGS1 for an expected return to the telescope during the third servicing mission.
The high precision of the FGSs makes them excellent astrometers. The narrow point spread function of HST and the dynamic range of the FGS make it an unparalleled science instrument for many important astronomical investigations. Detecting and resolving multiple star systems and planetary companions, delineation of objects in crowded fields, measuring stellar angular diameters, parallax, proper motion, positional surveys, occultation studies and photometry are among its many uses. This chapter describes how each Fine Guidance Sensor operates, summarizing its capabilities, its design, and its modes of operation.
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