1 Introduction

The last decade has witnessed a multiplication of efforts towards proper exploitation of all-sky Schmidt surveys. The principal requirement to this end is the use of fast measuring machines for the storage of the full astronomical information present in each plate, equivalent to about half a billion pixels. Extraction of the final astronomical parameters, i.e., single object classification, astrometry, and photometry, entails the development of ad-hoc reduction algorithms, which must not prescind from the digitization process. Various programs have been undertaken in this direction, including the construction of the second generation Guide Star Catalog (GSC-II) initially promoted by the STScI and later by a multi-partner collaboration. The original motivations for such an endeavour were driven by the required enhancements of the first Guide Star Catalog (GSC, Lasker et al. 1990) to continue supporting the HST mission. Currently the GSC partially meets HST specifications to provide guide stars with relative positions to about 0.3'' and photometry to better than 0.4 mag for telescope pointing and observation planning. Because it is a single epoch catalog, however, the errors due to proper motions presently exceed 1'' in some parts of the sky. Additionally, it is important to have at least one color to $V\simeq 18$ for planning purposes, such as verifying the absence of bright objects in camera fields. Similarly, new ground- and space-based telescopes will require access to a catalog of faint guide stars with comparable astrometry and photometry to provide data for active optics systems, target acquisition, observation planning, and efficient scheduling. These needs will be fulfilled by the GSC-II, with proper motions and colors to at least V=18 based on multi-color and multi-epoch Schmidt surveys (Lasker et al. 1995; Bucciarelli 1999; McLean et al. 2000).

Besides its operational use, the very nature of GSC-II makes it a tremendous resource to address a large variety of astrophysical investigations. In particular, GSC-II data can be successfully exploited for studies of Galactic structure and kinematics, where accurate astrometry and photometry are essential requirements (see, e.g., Spagna et al. 1996; Spagna et al. 1998; Terranegra et al. 1999; Sciortino et al. 2000).

Ultimately, the availability of external reference objects is of utmost importance for the calibration of the intrinsically non-linear response of photographic data. In particular, to derive good stellar photometry out of the survey plates one needs reference stars over the whole sky, covering a large dynamic range in brightness in at least two passbands.

About ten years ago, common scientific interests in the field of Schmidt astronomy led the STScI and the OATo to the definition of a joint program for the collection of an all-sky set of deep, multi-color photometric sequences. This program has produced a database with a wealth of CCD stellar sequences currently employed to calibrate the STScI Schmidt plate archive. The purpose of these sequences, in their present form, is restricted to the calibration of photographic plates, as opposed to providing accurate CCD photometry of single objects, which will be the goal of future releases of GSPC-II.

The aim of this paper is twofold: (a) give a detailed account of the observing program carried out by STScI and OATo, as well as of the content and methods of contruction of the current CCD data archive; (b) make available to the astronomical community an all-sky set of CCD photometric calibrators. The photometric quality of the sequences presented here is assessed by global statistics based on the fits to the Landolt's standard stars and on direct comparison against the GSPC-I catalog (Lasker & Sturch et al. 1988), as well as by semi-external checks of single object photometry in selected fields.