Proper-motion catalog from SDSS ∧ USNO-B

We present a catalog of 390,000 stars with proper motions mu greater than or equal to 20 mas yr(-1) and magnitudes r' less than or equal to 20, drawn from the intersection of the SDSS Data Release One and USNO-B. We first use SDSS quasars to correct the proper motions in each of the two source catalogs, finding that these corrections for systematics are comparable to the statistical errors. The combined proper motions have errors of sigma(mu) = 3.9 mas yr(-1), significantly better than any previous catalog at these magnitudes. While cross-correlating SDSS and USNO-B removes the vast majority of the very large number of spurious proper-motion stars in each, some contamination remains, especially at faint magnitudes, r' greater than or similar to 19.5, and high proper motions. We present a diagrammatic method to estimate the level of contamination in selected subsets of the catalog, which should be useful for choosing appropriate selection criteria for various applications.