Note: The Guide Star Catalog has been constructed to support the
operational need of the Hubble Space Telescope.
It contains over 25 million positions for about 19 million
objects, of which more than 15 million are classified as stars.
The version 1.2 is a new astrometric reduction which results in
absolute position errors about 0.3-0.4 arcsec.
(see GSC pages at URL http://www-gsss.stsci.edu/gsc/gsc.html)
The Naval Observatory Merged Astrometric Dataset (NOMAD) contains
astrometric and photometric data for over 1 billion stars derived from
the Hipparcos (I/239), Tycho-2 (I/259), UCAC2 (I/289), and USNO-B1.0
(I/284) catalogs for astrometry and optical photometry, supplemented
by 2MASS (II/246) near-infrared photometry.
An efficient remote query program findnomad1 is
available in the cdsclient
package, for Unix/Linux platforms
Note: The limiting magnitude is about 15.2 in the GSC photometric system.