The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)
Norbert Zacharias, Charlie Finch, Terry Girard, Arne Henden, Jennifer, Bartlett, Dave Monet, Marion Zacharias

TL;DR
The UCAC4 catalog provides a comprehensive, precise, all-sky star catalog with over 113 million objects, combining multiple data sources to improve positional and proper motion accuracy for astronomical research.
Contribution
This paper presents the final UCAC4 catalog, integrating diverse observational data and correcting previous issues to deliver an improved, extensive star catalog with enhanced accuracy and coverage.
Findings
Over 113 million objects cataloged with proper motions.
Positional accuracy of 15-100 mas per coordinate.
Proper motion errors estimated at 1-4 mas/yr.
Abstract
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-…
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