UCAC

UCAC. The **U**nited **S**tates **N**aval **O**bservatory **C**CD **A**strometry **C**atalog is a high-precision, all-sky astrometric star catalog. It was created to fill the gap between the historic, lower-precision surveys like the AGK2 and SAO Catalog and the highly accurate but magnitude-limited Hipparcos mission. The project was conducted using a specialized astrograph at the United States Naval Observatory Flagstaff Station in Arizona.

Overview

The primary goal of the United States Naval Observatory project was to produce a dense, high-accuracy reference catalog of stars in the 8 to 16 magnitude range. This magnitude band was critically underserved prior to its completion, lying between the bright stars of the Tycho-2 Catalogue and the faint limits of photographic surveys like the Hubble Space Telescope guide star catalog. The observational work was carried out with a unique 8-inch aperture Schmidt telescope equipped with a large-format CCD camera, an instrument specifically designed for this precise task. The resulting catalog provides precise positions, proper motions, and photometric data for millions of stars, serving as a fundamental link for celestial reference frames.

Data Releases and Versions

The catalog was released in several sequential versions, each representing an improvement in data reduction and sky coverage. The initial public data release, known as UCAC1, covered the southern hemisphere and demonstrated the feasibility of the project. This was followed by the more complete UCAC2, which included the first full-sky coverage and improved proper motions derived from a combination with other catalogs like the AC2000.2 and the Tycho-2 Catalogue. The final and definitive release, UCAC3, incorporated refined reduction techniques and additional data sources, offering the best precision. Subsequently, the project evolved into the URAT series, continuing the legacy of high-precision ground-based astrometry.

Observational Characteristics

Observations for the catalog were taken in a single bandpass, a special wide-bandpass Cousins R-band filter, which optimized the detection of stars for astrometric purposes. The survey's astrometric precision is exceptionally high for a ground-based project, with positional accuracies for a single observation around 20 milliarcseconds for stars brighter than 14th magnitude. The catalog's limiting magnitude of approximately R=16.5 ensured a high density of reference stars across the entire celestial sphere. To derive proper motions, the positions were combined with those from earlier epoch catalogs such as the Astrographic Catalogue and scans from the Palomar Observatory Sky Survey.

Scientific Applications

The catalog has been extensively used as the primary astrometric reference frame for a wide array of astronomical research and operations. It played a crucial role in the reduction of data from missions like the Spitzer Space Telescope and the Wide-field Infrared Survey Explorer. Its dense network of stars is ideal for identifying and characterizing minor planets, comets, and performing follow-up observations for objects discovered by surveys like the Lincoln Near-Earth Asteroid Research project. Furthermore, its proper motion data has been vital for studies of stellar kinematics within the Milky Way and for identifying high-velocity stars and members of moving groups like the Hyades cluster.

Comparison with Other Catalogs

Compared to the pioneering space-based Hipparcos catalog, the UCAC contains many more stars but with slightly lower astrometric precision per star. It significantly outperforms earlier photographic catalogs such as the SAO Catalog and the Henry Draper Catalogue in both accuracy and star density. The subsequent Gaia (spacecraft) mission has now superseded it in virtually all parameters, providing microarcsecond-level astrometry for over a billion stars. However, the catalog served as the state-of-the-art ground-based reference system for over a decade, bridging the critical period between the Hipparcos and Gaia (spacecraft) eras. Its design philosophy and techniques directly informed later projects including the Pan-STARRS and the Dark Energy Survey.

Category:Astrometric catalogs Category:United States Naval Observatory