Astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center （XSTPS-GAC）

We present astrometric calibration of the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anti-center(XSTPS-GAC). XSTPS-GAC is the photometric part of the Digital Sky Survey of the Galactic Anti-center(DSS-GAC),which is a photometric and spectroscopic sky survey, in combination with LAMOST.In order to select an astrometric reference catalog, we made comparisons between the four widely used astrometric catalogs, GSC2.3, USNO-B1.0, UCAC3 and PPMXL.PPMXL shows relatively small systematic errors in positions and more homogeneous proper motion distributions toward the Galactic Anti-center(GAC), and was selected as the reference catalog. Based on the high quality and bright reference stars that were picked out from PPMXL, we performed a 4th-order polynomial fitting in image units,to construct the transformation relation between coordinates used by XSTPS-GAC and standard coordinates, and to simultaneously correct the image distortions in the CCD. Then we applied the derived relation to all sources to obtain their mean celestial coordinates based on the International Celestial Reference System. For bright point sources with r 17.0 mag, the accuracy of astrometric calibration could reach about80 mas for each of the g, r, i bands, with systematic errors being less than 10 mas. But for the faint sources at the brightness limit of the survey, which was r ～ 19.0 mag, the accuracy can still reach 200 mas. After combining all observations, the final weighted average coordinates could reach an accuracy of less than 70 mas for bright stars. For faint stars, the rms residuals of weighted coordinates decrease to ～ 110 mas. The final combined XSTPS-GAC coordinates show a good consistency with the Sloan Digital Sky Survey.