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| UCAC4 | |
|---|---|
| Name | UCAC4 |
| Type | Astrometric star catalog |
| Epoch | J2000.0 |
| Entries | ~113,780,093 |
| Release | 2012 |
| Maintainer | United States Naval Observatory |
| Instruments | CCD astrograph |
| Wavelength | Optical |
UCAC4 USNO CCD Astrograph Catalog, Fourth Edition, is a comprehensive astrometric and photometric star catalog covering most of the sky for stars roughly between magnitude 8 and 16. Compiled and released by the United States Naval Observatory and collaborators, it provides positions, proper motions, and photometric estimates used by observatories, space missions, and researchers. The catalog bridges earlier photographic surveys and modern digital surveys, serving as a reference for projects involving the Hipparcos frame, the Two Micron All-Sky Survey, and follow-on astrometric efforts.
UCAC4 continues a sequence begun with earlier USNO CCD Astrograph Catalog releases, aiming to supply precise positions and proper motions tied to the International Celestial Reference Frame realized by the International Astronomical Union. The project involved instrumental work at the Naval Observatory Flagstaff Station and data reduction pipelines influenced by methodologies from the Hipparcos mission and the Tycho-2 catalog. It complements infrared and deep optical surveys such as 2MASS, Sloan Digital Sky Survey, and catalogs from the European Space Agency.
The compilation combined CCD observations from the USNO astrograph with older photographic plate data and external catalogs. Primary imaging epochs derive from the USNO CCD instrument, while historical positions were drawn from digitized sky surveys including those at the Palomar Observatory and the UK Schmidt Telescope. Proper motions were calculated by linking CCD positions to epochal measurements from catalogs like Tycho-2, Hipparcos, and ground-based photometric catalogs. Cross-matching incorporated infrared astrometry from 2MASS and positional anchoring to the International Celestial Reference Frame via catalog tie-ins to space-based missions.
UCAC4 provides J2000.0 positions and two-component proper motions for over one hundred million stars, with mean magnitudes in a band approximating a red-optical passband. Photometric entries include estimated magnitudes derived from CCD fluxes and supplemented by cross identifications with 2MASS for near-infrared J, H, K magnitudes. The catalog format records positional uncertainties, magnitude errors, and flags for multiplicity or crowded-field conditions relevant to observations at facilities such as the Cerro Tololo Inter-American Observatory or the Kitt Peak National Observatory.
Reported positional precisions vary with magnitude and field crowding, typically at the 15–100 milliarcsecond level per coordinate for well-measured stars. Proper motion accuracies depend on time baselines and the quality of historical plates; typical systematic errors can arise from plate-scale distortions, charge-transfer inefficiency in CCDs, and zonal errors linked to telescope flexure. The USNO team characterized systematics by comparisons with Hipparcos and Tycho-2 and by evaluating residuals against 2MASS positions, documenting color-dependent offsets that can affect studies using data from the Hubble Space Telescope or ground-based interferometers.
Extensive cross-identifications connect UCAC4 entries to external catalogs to facilitate multiwavelength science. Major links include identifiers from 2MASS, matches to entries in Tycho-2, and overlaps with subsets of the Hipparcos catalog for bright-star anchoring. The catalog also provides cross-reference fields useful when comparing to survey catalogs such as the Sloan Digital Sky Survey and when validating against space mission catalogs from the European Space Agency or instruments on the Hubble Space Telescope.
Data distribution used standard fixed-width and binary table formats compatible with common astronomical software tools like those maintained by the Space Telescope Science Institute and the International Virtual Observatory Alliance. UCAC4 data were accessible via bulk downloads and through query services hosted by the United States Naval Observatory and partner data centers, enabling integration into pipelines for facilities like the Large Binocular Telescope or the Very Large Telescope.
UCAC4 has been widely used for astrometric reference frames in observations from small telescopes to large facilities, for proper-motion studies of stellar kinematics in the solar neighborhood, and for target selection in exoplanet and variable-star programs. Its ties to 2MASS and to bright-star catalogs made it useful for calibration in photometric surveys and for epoch-to-epoch proper-motion searches for nearby high-proper-motion stars and brown dwarfs identified in follow-up studies with the Spitzer Space Telescope and ground-based spectrographs. UCAC4’s role as an intermediate astrometric resource proved particularly valuable prior to releases from the Gaia mission, informing planning and cross-calibration for space-based and ground-based observational campaigns.
Category:Star catalogs Category:United States Naval Observatory