An all-sky set of (B)-V-R photometric calibrators for Schmidt surveys. GSPC2.1: First release

We publish 622 CCD photometric sequences in the Johnson-Kron-Cousins standard system, distributed both in the northern and southern hemispheres, useful for the calibration of photographic photometry of Schmidt survey plates. The collection and reduction of the CCD data presented here are part of a long-term program devoted to the construction of the Second Guide Star Photometric Catalog (GSPC-II). The GSPC-II is an all-sky catalog of photometric stellar sequences with a limiting magnitude of V = 19 or fainter, in the (B), V ,a nd R passbands of the Johnson-Kron-Cousins system. Standard photometric errors are at the level of0.07 for a V 19 magnitude star. These sequences are being used by teams of the Space Telescope Science Institute (STScI) and the Osservatorio Astronomico of Torino (OATo) for the photometric calibration of the Second Guide Star Catalog.