WHAT GAIA WILL SEE: ALL-SKY SOURCE COUNTS FROM THE GSC2

摘要

The Guide Star Catalogue Ⅱ (GSC2) is an all-sky catalogue based on several photographic Schmidt plate surveys completed over the last century. The GSC2 has a magnitude limit of 20.5 in the photographic red passband (R_F) and nearly a billion classified objects ('stars' and 'nonstars'), making it a valuable data set for Gaia mission studies. Using the known passbands of the GSC2 and Gaia, synthetic photometry is used to determine the colour transformation from the GSC2 (B_J, R_F) photometry to the Gaia G band: we find that within the GSC2 photometric errors, R_F ≈ G. We therefore adopt the GSC2 counts in R_F to estimate the number of sources that Gaia will detect, after correcting for blending in the GSC2, an important effect in regions of high object field density.