1 Introduction

Considerable attention has been devoted in recent years to the complex structure of phase space in the Galaxy, as depicted by both narrow angle deep CCD multicolor star counts and large-angle surveys of colors and proper motions (Robin et al. 1992; Majewski 1992; Spagna et al. 1996). Large-area surveys covering several square degrees utilize large format photographic plates, usually taken at Schmidt-type telescopes. Noticeable examples of photographic surveys on Schmidt plates are the first Palomar sky survey, POSS I (Minkowski & Abell 1963), its present day continuation, POSS II (Reid et al. 1991), the Quick-V survey, taken specifically for the construction of the first guide star catalog for the Hubble Space Telescope (Lasker et al. 1990b), and, for the Southern hemisphere, the ESO/SERC survey (Tritton 1983), and the on-going Second Epoch Southern (SES) survey ($\delta \le -17\hbox{$.\!\!^\circ$}5$)with the UK Schmidt (Lasker & Cannon 1990a).

As discussed by many authors, the photometric calibration of such large format plates is challenging (see e.g. Bienaymé et al. 1992; Andruk et al. 1994; Ojha et al. 1994; Spagna et al. 1996). Precisions to 0.1 mag or better are possible, especially for Kodak IIIa emulsions, but photometric quality appears to be particularly vulnerable to position-dependent systematics. Faint photometric standards, distributed evenly over the area studied, are essential for the most accurate results. Although the recent release of the TYCHO catalog will certainly help (especially for the zero-point "flat fielding'' across plates) today, the situation is far from ideal! Faint photometric standards of suitable bandpasses are not always available; and, when so, they tend to be concentrated in small regions compared to the extent of the plates. These are the motivations for the extension of the first Guide Star Photometric Catalog (GSPC I; Lasker et al. 1988) into the GSPC II (Postman et al. 1992).

The GSPC II is a joint effort of the ST ScI and the Osservatorio Astronomico di Torino. This project aims at the creation of a catalog of photometric standard sequences throughout the celestial sphere down to $V \approx 19$ in the B, V, $R_{\rm c}$ bandpasses, with an accuracy of 5% (Ferrari et al. 1993). More details on the project can be found in the cited references. The data presented in this article are part of the GSPC II photometric database.