Cleaning the USNO-B Catalog through automatic detection of optical artifacts

The USNO-B Catalog of astrometric standards contains spurious entries that are caused by diffraction spikes and circular reflection halos around bright stars in the original imaging data. These spurious entries appear in the Catalog as if they were real stars; they are confusing for some scientific tasks. The spurious entries can be identified by simple computer vision techniques because they produce repeatable patterns on the sky. Some techniques employed here are variants of the Hough transform, one of which is sensitive to (two-dimensional) overdensities of faint stars in thin right-angle cross patterns centered on bright (< 13mag) stars, and one of which is sensitive to thin annular overdensities centered on very bright (< 7mag) stars. After enforcing conservative statistical requirements on spurious-entry identifications, we find that of the 1,042,618,261 entries in the USNO-B Catalog, 24,148,382 of them (2.3 percent) are identified as spurious by diffraction-spike criteria and 196,133 (0.02 percent) are identified as spurious by reflection-halo criteria. Surprisingly, the spurious entries are often detected in more than 2 bands and are not overwhelmingly outliers in any photometric properties; they therefore cannot be rejected easily on other grounds, ie, without the use of computer vision techniques. We return to the community in electronic form a table of spurious entries in the Catalog. Subject headings: astrometry — catalogs — methods: statistical — standards — techniques: image processing 1 Department of Computer Science, University of Toronto, 6 King’s College Road, Toronto, Ontario, M5S 3G4 Canada 2 Center for Cosmology and Particle Physics, Department of Physics, New York University, 4 Washington Place, New York, NY 10003 3 To whom correspondence should be addressed: [email protected] 4 Google, Mountain View, CA