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 "Sky Surveys" 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.
IMPORTANT - HST observers should now use GSC 2 for observation planning
The original version of this catalog was created to support the
identification and use of Guide Stars for the pointing of the HST.
It is based on the Palomar Quick-V survey in the northern hemisphere
and the SERC-J survey in the south. This catalog contains objects
in the magnitude range 7-16 and the classification was biased to
prevent the use of a non-stellar object as a guide star. The photometry
is based on a photoelectric sequence (9-15th mag) near the center
of each Schmidt plate. Only stellar photometry was performed so
magnitudes of galaxies in this catalog are systematically in error.
Astrometry was determined using the AGK3, SAOC or CPC stars depending
on plate declination. Although the relative astrometry (required
for HST) is about 0.3 arcsec, there are known systematic errors
near the plate edges of about 1-2 arcsec.
revised version of the GSC was created with the cooperation of
consortia. As part of the preparation for
this mission, an input catalog containing the best available data
for all stars to at least 11th magnitude was created. By merging
these data with the GSC, we were able to add the missing bright
star data to provide a complete all-sky catalogue down to the GSC
limiting magnitude. In addition, many of the false objects due
to artifacts around the halos and diffraction spikes of the bright
stars were identified and corrected as well as a number of reported
errors. A number of astrographic plates centered on southern hemisphere
bright stars (m<3) were also processed and added to the catalog.
In collaboration with the Astronomisches Rechen-Institut in Heidelberg,
a new astrometric reduction of the GSC has been completed. This
new version, GSC 1.2 has dramatically reduced the plate-based position-dependant
and magnitude-dependant systematics present in GSC 1.1
Guide Star Catalog I - Astronomical Foundations and Image Processing