The flight hardware and ground system for Hubble Space Telescope astrometry

An account is presented of the Fine Guidance Sensor (FGS) which furnishes guidance and astrometry in the HST, giving attention to its interferometric design and its astrometric performance capabilities, the astrometric observing modes and software commands used in their implementation, and expected HST states with regard to line-of-sight jigger, drift, and guidance mode during a given observation. FGS operational modes discussed encompass support-system module control, search, coarse track, fine lock, line-of-sight scan, and stop. In astrometric measurements, relative positional accuracies of the order of 0.003 arcsec are achievable; precise parallaxes, detection thresholds for planetary companions of nearby stars, and kinematic characterizations of star clusters will be facilitated thereby.