Guide Star Catalog (GSC)
The Guide Star Catalog (GSC) is a catalog of stars, galaxies, and other celestial objects, based on a digitization of photographic sky survey plates from the Palomar and United Kingdom Schmidt telescopes. There are two versions. GSC I is used for the control and target acquisition of the Hubble Space Telescope and is an all-sky optical catalog of positions and magnitudes of approximately 19 million stars and other objects in the 6th to 15th magnitude range. GSC II contains almost a billion objects down to about twentieth magnitude and is used to support operations of the Very Large Telescope, the Gemini Observatory, and Hubble.