Astrometric Calibration of the Sloan Digital Sky Survey

The astrometric calibration of the Sloan Digital Sky Survey is described. For point sources brighter than r ~ 20 the astrometric accuracy is 45 milliarcseconds (mas) rms per coordinate when reduced against the USNO CCD Astrograph Catalog, and 75 mas rms when reduced against Tycho-2, with an additional 20 - 30 mas systematic error in both cases. The rms errors are dominated by anomalous refraction and random errors in the primary reference catalogs. The relative astrometric accuracy between the r filter and each of the other filters (u g i z) is 25 - 35 mas rms. At the survey limit (r ~ 22), the astrometric accuracy is limited by photon statistics to approximately 100 mas rms for typical seeing. Anomalous…