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)
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Note: 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