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including Hipparcos/Tycho and the AC2000.2, as well as unpublished measures of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