The Second US Naval Observatory CCD Astrograph Catalog (UCAC2)

TLDR

UCAC2, released July 2003, provides all‑sky coverage from –90° to +52° declination and supersedes the earlier UCAC1 catalog. UCAC2 derives positions and proper motions for 48 million sources using an 8‑inch Twin Astrograph with a 4K CCD, supplements photometry from 2MASS, and combines data from over 140 ground‑ and space‑based catalogs to compute epoch positions and proper motions. UCAC2 achieves positional precision of 15–70 mas with systematic errors ≤10 mas, and proper‑motion uncertainties of 1–3 mas yr⁻¹ for stars up to 12th magnitude and 4–7 mas yr⁻¹ to 16th magnitude.

Abstract

The second US Naval Observatory (USNO) CCD Astrograph Catalog, UCAC2 was released in 2003 July. Positions and proper motions for 48,330,571 sources (mostly stars) are available on 3 CDs, supplemented with Two Micron All Sky Survey photometry for 99.5% of the sources. The catalog covers the sky area from -90° to +40° declination, going up to +52° in some areas; this completely supersedes the UCAC1 released in 2001. Current epoch positions are obtained from observations with the USNO 8 inch (0.2 m) Twin Astrograph equipped with a 4K CCD camera. The precision of the positions are 15–70 mas, depending on magnitude, with estimated systematic errors of 10 mas or below. Proper motions are derived by using over 140 ground- and space-based catalogs, including Hipparcos/Tycho and the AC2000.2, as well as yet unpublished remeasures of the AGK2 plates and scans from the NPM and SPM plates. Proper-motion errors are about 1–3 mas yr-1 for stars to 12th magnitude, and about 4–7 mas yr-1 for fainter stars to 16th magnitude. The observational data, astrometric reductions, results, and important information for the users of this catalog are presented.