Star Catalogs for Astrometrica

Distribution of stars in the USNO-A2.0 catalog (Source: USNO)

Gaia DR3
Gaia Data Release 2 (Gaia DR3) is the second intermediate release from ESA's Gaia astrometry mission. It includes data for about 1.8 billion sources down to magnitude 21. For about 1.7 billion stars, positions, proper motion and parallax data is available. For the remaining stars, only the postion is given.
The positions in the Gaia catalog are very accurate, with a positional uncertainty of about 1 milli arc seconds (mas) for stars of 20mag. Uncertainties in proper motion is expected to be less than 0.04 mas/year for stars brighter than about 15mag and around 0,6 mas/year for very faint stars of 20mag.
Gaia DR3 includes photometry in a three color bands: A broad "white light" color band, called G-band, as well as in red (GRP) and blue (GBP) light. The uncertainties in the magnitudes is estimated between 0.001mag for brighter stars in the G band to about 0.05mag for stars of 20mag in the red and 0.1mag in the blue band.
Astrometrica will query VizieR to download reference star data from Gaia DR3.

ATLAS REFCAT 2
The ATLAS All-Sky Stellar Reference Catalog, or ATLAS REFCAT 2, provides data for about 1 billion stars down to magnitude 19 over the whole sky. Astrometry in ATLAS REFCAT is based on Gaia DR2, so positional errors is expected to be 2 milli arc seconds or less.
ATLAS REFCAT provides high quality photometry in the Sloan g, r, i and z color bands, collected by the Pan-STARRS and ATLAS telescopes, with a precision of about 0.01mag. In addition, ATLAS REFCAT includes broadband G, red GRP and blue GBP magnitudes from Gaia DR2.
Astrometrica will query VizieR to download reference star data from ATLAS REFCAT.

PPMXL
PPMXL is a combination of the data from the USNO-B1.0 and the infrared 2MASS catalog. It aims to be complete from the brightest stars down to about magnitude V=20 full-sky. PPMXL contains about 910,468,710 objects, and therefore is the largest collection of ICRS positions and proper motions at present.
The catalog includes astrometrc positions with an accuracy (at epoch J2000.0) 0.08" to 0.12" where 2MASS astrometry was available (that is, for 410 million objects), and 0.15" to 0.30" where no data from 2MASS could be used. Proper motions are absolute in the ICRS reference frame (not relative, as in the USNO-B1.0). Magnitudes in the PPMXL were taken from USNO-B1.0, so there is no improvement in the photometric calibration compared to the USNO catalog.
Astrometrica can access a local copy of the PPMXL or will query VizieR to download reference star data.

UCAC 4
Observations for the USNO CCD Astrograph Catalog, or UCAC, started in early 1998 and were completed in 2004. With these observations, the UCAC is the first modern high-density, full-sky star catalog that is not based on photographic images of the sky, but on recent CCD observations. After two intermediate releases, which did not cover the whole sky, the first complete catalog (UCAC 3) was released in 2009, followed by the final release of UCAC 4 in 2012.
The UCAC 4 includes positions, proper motions and magnitudes for 113,780,093 objects. Reference star positions in UCAC 4 are accurate to about 0.02" for brighter stars (10^mag to 14^mag), and a precision better than 0.1" is expected at the limiting magnitude of 16^mag. Native magnitudes have been measured in one single, non-standard color, but the catalog includes five-band photometry (B,V,g,r,i) from the APASS (AAVSO Photometric All-Sky Survey) for over 50 million stars.
Astrometrica can access a local copy of the UCAC 4, or query VizieR to download reference star data.

CMC-15
The 15. Carlsberg Meridian Catalog is the result of the observations of nearly 123 million stars with the Carlsberg Meridian Telescope on La Palma. It covers the sky between +50° and -40° declination.
Astrometric positions in CMC-15 are precise to 0.05" for bright stars (13^mag or brighter) and to 0.1" for stars at the limiting magnitude of 16^mag. There are no proper motions, but as the catalog positions were derived from recent CCD observations (1999-2011), the positions are still very precise at current epochs. The catalog includes red magnitudes, with errors from only 0.025^mag for bright stars (13^mag or brighter) to 0.17^mag at the limiting magnitude of 16^mag. The photometric data is therefore much better then that of most other astrometric catalogs.
Astrometrica can access a local copy of the CMC-15, or query VizieR to download reference star data.