  The Asteroid Terrestrial-impact Last Alert System (ATLAS) observes most of
the sky every night in search of dangerous asteroids. Its data are also used to
search for photometric variability, where sensitivity to variability is limited
by photometric accuracy. Since each exposure spans 7.6 deg corner to corner,
variations in atmospheric transparency in excess of 0.01 mag are common, and
0.01 mag photometry cannot be achieved by using a constant flat field
calibration image. We therefore have assembled an all-sky reference catalog of
approximately one billion stars to m~19 from a variety of sources to calibrate
each exposure's astrometry and photometry. Gaia DR2 is the source of astrometry
for this ATLAS Refcat2. The sources of g, r, i, z photometry include Pan-STARRS
DR1, the ATLAS Pathfinder photometry project, ATLAS re-flattened APASS data,
SkyMapper DR1, APASS DR9, the Tycho-2 catalog, and the Yale Bright Star
Catalog. We have attempted to make this catalog at least 99% complete to m<19,
including the brightest stars in the sky. We believe that the systematic errors
are no larger than 5 millimag RMS, although errors are as large as 20 millimag
in small patches near the galactic plane.
