Hubble Space Telescope
Guide Star Catalog

WCSTools Catalogs

GSC 1.0

The original version of this catalogue 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 catalogue 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.

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 catalogue 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 catalogue. This is the version currently used by HST operations and published to the community.

To enable rapid access of specific stars in the catalog, WCSTools software numbers each star using its region number (0001-9537) and a four-digit star number within each region, separated by a decimal point. sgsc lists GSC stars by number or sky region. imgsc lists the ACT stars within an IRAF or FITS image with a world coordinate system defined in its header. The catalog can also be plotted by skymap.


The HST GSC contains nearly 19 million objects brighter than sixteenth magnitude, of which more than 15 million are classified as stars. They are organized into 9537 regions on the sky. The parts of the sky covered by the regions are tabulated in the tables/regions.tbl file which is found on each CDROM. The sources in each region are in separate FITS ASCII tables files which contain source number within the region, right ascension and declination in degrees to five decimal places, position error in arcseconds, magnitude to .01 magnitude, error to .01 magnitude, class, bandpass, and a flag to tell how many entries there are for that object.

GSC Classification Codes

   0 Star
   1 Galaxy
   2 Blend
   3 Non-star (can be galaxies, blended objects, defects, etc)
   4 Unclassified
   5 Artifact
The multiple flag is used to flag the same object appearing on more than one plate.

GSC Bandpass Codes

   0 : J       photographic  IIIaJ + GG495  
   1 : V       photographic  IIaD  + W12
   2 : FGS
   3 : B       photoelectric
   4 : V       photoelectric
   5 : R       photographic  IIIaF + RG630
   6 : V495    photographic  IIaD  + GG495
   7 : O       photographic  103a0 + no filter


The purpose of the GSC-ACT project is to recalibrate the Hubble Guide Star Catalog (GSC), version 1.1, using the ACT (Astrographic Catalog/Tycho) data from the US Naval Observatory. The recalibrated catalog might be available on CDROM, and a GSCACT option which does the recalibration on the fly may be added to WCSTools.


The Guide Star Catalog I. Astronomical Foundations and Image Processing
B.Lasker, C.Sturch, B.McLean, J.Russell, H.Jenkner, M.Shara
The Astronomical Journal, 1990, Volume 99, pg.2019.
The Guide Star Catalog II. Photometric and Astrometric Models and Solutions.
J.Russell, B.Lasker, B.McLean, C.Sturch, H.Jenkner
The Astronomical Journal, 1990, Volume 99, pg.2059.
The Guide Star Catalog III. Production, Database Organization, and Population Statistics.
H.Jenkner, B.Lasker, C.Sturch, B.McLean, M.Shara, J.Russell
The Astronomical Journal, 1990, Volume 99, pg.2081.
The Guide Star Catalog IV. User Interfaces, Utilities, and Astronomical Applications. IN PREPARATION
The Tycho Input Catalogue - Cross-matching the Guide Star Catalog with the Hipparcos INCA Data Base.
D.Egret, P.Didelon, B.McLean, J.Russell, C.Turon
Astronomy and Astrophysics, Vol. 258, no. 1, p. 217-222.

[GSC at HST] [skymap] [WCSTools sgsc] [WCSTools imgsc]