For a quick summary of the properties of these two catalogs see:
http://www-gsss.stsci.edu/gsc/gsc2/CatalogProperites/GSCIIProperties.htm
The GSC I catalog (version 1.0) was constructed to support
pointing and target aquistion for the HST. The catalog contains
approximately 19 million stars and other objects in the sixth to
fifteenth magnitude range, primarily determined from an all-sky,
single epoch collection of 6.4 degree by 6.4 degree Schmidt plates.
The Schmidt plates for north of +6 degrees consists of a 1982-1984
epoch "Quick V" survey (IIaD, 20 minute exposure) obtained from
the Palomar Observatory. The southern fields consist 0f 50-75 minute
exposure IIIa-J Schmidt plates, from the UK SERC J/EJ survey (epoch
1975-1988) and a supplemental 4 minute IIa-D southern Galactic
plane extension (epoch approximately 1988).
For more information on the Schmidt plates used to construct
the GSC I catalog see:
http://www-gsss.stsci.edu/PlateMaterial/plateMaterial.htm
GSC 1.1 is the catalog used for control and target acquistion for the HST.
The improved catalog corrected a number of known problems in the GSC
1.0. The corrected defects included spurious entries (principally due
to false detections on the diffraction spikes of bright objects), grossly
incorrect entries for the brighter stars (V<7) that were produced
from heavily over-exposed Schmidt images, and different entries for
the same object having more than one name because of blend-resolution
difficulties, as well as astrometric errors at the plate edges.
GSC 1.2
An astrometric re-calibration of the GSC 1.1 reducing the systematic
errors present in the GSC 1.1 positions. This catalog has not been been
installed on the HST Guide Star Selection system, so it must not be used
for HST observation planning.
The GSC II will be an all-sky catalog based Schmidt plates at two epochs
and three bandpasses, from the Palomar (POSS I and POSS II) and UK Schmidt
telescopes (SERC/UK surveys). This catalog will contain postions,
magnitudes, colors and proper motions for all objects to at least 18.5 in
photographic F. The construction of the GSC II is in progress. A
preliminary catalog, GSC 2.2 is now available.
The GSC2.2 is an all-sky, magnitude-selected subset of this data that
has been extracted to support telescope operations at the GEMINI and
VLT telescopes. This Telescope Operations version contains positions,
classifications, and magnitudes for 435,457,355 objects.
The magnitude limits (18.5 in photographic F and 19.5 in photographic J)
were implemented to ensure the photometric quality of the released data.
Since bright objects are extremely overexposed on the Schmidt plates,
positions from the Tycho-2 catalog were used.
The final version (GSC 2.3), expected to be released in 2002,
will also contain proper motions.
For more information on the Schmidt plates used to construction
the GSC I catalog see:
http://www-gsss.stsci.edu/PlateMaterial/plateMaterial.htm