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 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 refraction is shown to contain components correlated over 2° or more on the sky.