Description :

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

GSC 1.0

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.
[more info]

GSC 1.1

A revised version of the GSC was created with the cooperation of the HIPPARCOS/TYCHO 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.
This is the version used by HST operations prior to cycle 15 and is published to the community.
[more info]

GSC 1.2

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
The PPM and AC reference catalogues were used and absolute position errors have been reduced to about 0.3-0.4 arcsec.
[more info]

References :

The Guide Star Catalog I - Astronomical Foundations and Image Processing
Lasker et al 1990 AJ 99, 2019
The Guide Star Catalog II - Photometric and Astrometric Models and Solutions
Russell et al 1990 AJ 99, 2059
The Guide Star Catalog III - Production, Database Organization and Population Statistics
Jenkner et al 1990 AJ 99, 2081
Some comments on the astrometric properties of the guide star catalog
Taff et al 1990 ApJ 353, L45
GSC 1.2- An Astrometric Re-Reduction and other Refinements
Morrison et al 2001 AJ 121, 1752

Properties :

Summary of catalog properties and coded values

Data Access :
GSC 1.0 : not available
GSC 1.1 : on-line | download (from CDS)
GSC 1.2 : on-line | download (from CDS)