HST Guide Star Catalog v2.2:
The GSC II (currently GSC 2.2) is a catalog of 435,457,355 stars measured from Schmidt plates at the Space Telescope Science Institute. It is magnitude limited to 18.5 in photographic F or 19.5 in photographic J., with bright objects filled in from the Tycho 2 Catalog. The catalogue data are currently stored on a server at STScI: BLASTA queries this server remotely.
If you use GSC II information, please add the following acknowledgement:
The Guide Star Catalogue-II is a joint project of the Space Telescope Science Institute and the Osservatorio Astronomico di Torino. Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, for the National Aeronautics and Space Administration under contract NAS5-26555. The participation of the Osservatorio Astronomico di Torino is supported by the Italian Council for Research in Astronomy. Additional support is provided by European Southern Observatory, Space Telescope European Coordinating Facility, the International GEMINI project and the European Space Agency Astrophysics Division.