Photometric Calibrators for all-Sky Surveys: The GSPC-II Program

Accurate astrometric and photometric calibrations of all-sky photographic surveys, such as the Palomar Quick-V survey and the UK SERC-J survey, digitised for the construction of the Hubble Space Telescope Guide Star Catalog, are of extreme astrophysical value. Their wide range of applications includes, for example, galactic structure, stellar populations, extragalactic studies, and deep sky catalogs for mission planning and telescope operations. The Guide Star Photometric Catalog (GSPC-I, Lasker et al. 1988) provides stellar calibration sequences, placed approximately in the center of each survey plate, with a limiting magnitude of V = 15 mag. The goal of our program is to extend these calibrators to V = 20 mag in the Johnson-Kron-Cousins B, V and R passbands, with an accuracy of 0.05 mag. This is accomplished by taking CCD frames centered on the faintest star of each GSPC-I sequence. Typically, both long and short exposures are acquired in each field in order to link the bright and faint ends of the sequence.