The Catalogs and Surveys Branch of the Space Telescope Science Institute constructed the Guide Star Catalog I (GSC I) to support the pointing and target acquisition for the Hubble Space Telescope. For the last decade the GSC I has been used for numerous other purposes, for example, the observation planning for fiber optic spectrographs, the preparation of finding charts, and the operation of ground-based telescopes. 

The GSC I catalog is an all-sky catalog of positions and magnitudes for approximately 19 million stars and other objects in the sixth to fifteenth magnitude range. The GSC is primarily based on an all-sky, single-epoch collection of Schmidt plates. For centers at +6° and north, a 1982 epoch "Quick V" survey was obtained from the Palomar Observatory, while for southern fields, materials from the UK SERC J survey (epoch = 1975) and its equatorial extension (epoch = 1982) were used. See plate material for more information on the plates.

All the plates were digitized at the Space Telescope Science Institute into 14000 X 14000 rasters at a 25 micron sampling interval using two modified PDS micro-densitometers. For more information on the plate processing consult Lasker et al., 1990, AJ, Vol 99, 6.

The GSC 1.1 is used for control and target acquistion of the Hubble Space Telesope.

The GSC 1.2 is an astrometric recalibration of the GSC 1.1, and should not be used for HST operations.