UCAC

So what is the UCAC? I wasn't even familiar with this catalog. It's the USNO CCD Astrograph Catalog. From their web site (and I've added some brief, explanatory comments [in italics]):

This is an astrometric, observational program, which started in February 1998 at CTIO [Cerro Tololo Inter-American Observatory]. All sky observations were completed in May 2004. The final catalog is expected not before mid 2007. The second data release (UCAC2) became public in 2003. Positions accurate to 20 mas [milli-arcseconds] for stars in the 10 to 14 magnitude range are obtained. At the limiting magnitude of R=16 [red magnitude = 16] the catalog positions have a standard error of 70 mas. Proper motions [apparent motion of a star across the sky] are provided using various earlier epoch data. Photometry is poor, with errors on the order 0.1 to 0.3 magnitudes in a single, non-standard color.