THE FOURTH US NAVAL OBSERVATORY CCD ASTROGRAPH CATALOG (UCAC4)

Abstract

The fourth United States Naval Observatory (USNO) CCD Astrograph Catalog,\nUCAC4 was released in August 2012 (double-sided DVD and CDS data center Vizier\ncatalog I/322). It is the final release in this series and contains over 113\nmillion objects; over 105 million of them with proper motions. UCAC4 is an\nupdated version of UCAC3 with about the same number of stars also covering\nall-sky. Bugs were fixed, Schmidt plate survey data were avoided, and precise\n5-band photometry were added. Astrograph observations have been supplemented\nfor bright stars by FK6, Hipparcos and Tycho-2 data to compile a UCAC4 star\ncatalog complete to about magnitude R = 16. Epoch 1998 to 2004 positions are\nobtained from observations with the 20 cm aperture USNO Astrograph's red lens,\nequipped with a 4k by 4k CCD. Mean positions and proper motions are derived by\ncombining these observations with over 140 ground- and space-based catalogs,\nincluding Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of\nover 5000 plates from other astrographs. For most of the faint stars the first\nepoch plates from the Southern Proper Motion (SPM) and the Northern Proper\nMotion (NPM) programs form the basis for proper motions. These data are\nsupplemented by 2MASS near-IR photometry for about 110 million stars and 5-band\n(B,V,g,r,i) APASS data for over 51 million stars. Thus the published UCAC4, as\nwere UCAC3 and UCAC2, is a compiled catalog with the UCAC observational program\nbeing a major component. The positional accuracy of stars in UCAC4 at mean\nepoch is about 15 to 100 mas per coordinate, depending on magnitude, while the\nformal errors in proper motions range from about 1 to 10 mas/yr depending on\nmagnitude and observing history. Systematic errors in proper motions are\nestimated to be about 1 to 4 mas/yr.\n