GSPC-II in the Southern Hemisphere: comparison of photometric data from CTIO and ESO.

Construction of the Guide Star Photometric Catalog II (GSPC-II) is a long term project to extend the all-sky BV photoelectric photometry of the GSPC-I with modern CCD BVR data to approximately V=21 with an accuracy of 0.05 magnitudes. The observing program for the 894 southern hemisphere sequences, centered on the SERC UK Schmidt plate survey fields, is a collaborative effort between STScI and the Astronomical Observatory at Torino (OATo). The data are being acquired with 1m class telescopes at CTIO and La Silla in Chile. Because the CCDs and filter sets used at the two observatories are not identical, we periodically observe a sequence of fields in common to monitor any zeropoint shifts. We present here an analysis of photometric data for 4 fields observed in common: S029-G, S344-H, S876-I and M15. These provide the some of the material needed for a reliable evaluation of the external photometric errors and the overall accuracy. The error analysis shows that our 0.05 magnitude error requirement is met down to approximately V=20. Residual systematic errors are at the 0.02 magnitude level. The southern survey is currently 25% complete. This work is partially supported by NASA grant NAGW-3644 from the UV-Optical-Gravitational Astrophysics Research Program.