UCAC4 is a compiled, all-sky star catalog covering mainly the 8 to 16 magnitude range in a single
bandpass between V and R. Positional errors are about 15 to 20 mas for stars in the 10 to 14 mag
range. Proper motions have been derived for most of the about 113 million stars utilizing about 140
other star catalogs with significant epoch difference to the UCAC CCD observations. These data are
supplemented by 2MASS photometric data for about 110 million stars and 5-band (B,V,g,r,i) photometry
from the APASS (AAVSO Photometric All-Sky Survey) for over 50 million stars. UCAC4 also contains
error estimates and various flags. All bright stars not observed with the astrograph have been added
to UCAC4 from a set of Hipparcos and Tycho-2 stars, so UCAC4 should be complete from the brightest
stars to about R=16, with the source of data indicated in flags. UCAC4 also provides a link to the
original Hipparcos star number with additional data such as parallax found on a separate data file
included in this release.
The proper motions of bright stars are based on about 140 catalogs, including Hipparcos and Tycho,
as well as all catalogs used for the Tycho-2 proper motion construction. Proper motions of faint
stars are based on re-reductions of early epoch SPM data (-90 to about -20 deg Dec) and NPM (PMM
scans of early epoch blue plates) for the remainder of the sky. These early epoch SPM data have also
been combined with late epoch SPM data to arrive at proper motions partly independent from UCAC4
(Girard et al. 2011). The NPM data used in UCAC4 are not published. No Schmidt plate data are used
in UCAC4.
The unpublished plate measure data obtained by StarScan from the AGK2, the Hamburg Zone Astrograph,
the USNO Black Birch Astrograph, and the Lick Astrograph have contributed to considerable
improvement in proper motions for stars mainly in the 10 to 14 mag range (down to the UCAC limit for
Lick data); however, these data do not cover all sky.
If you use UCAC4, please cite both Zacharias et al. (2013)
and the dataset Digital Object Identifier (DOI): 10.26131/IRSA17.
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UCAC4 Overview
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