Star Catalogs for Astrometrica

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

Gaia DR2
Gaia Data Release 2 (Gaia DR2) is the second intermediate release from ESA's Gaia astrometry mission. It includes data for about 1.7 billion sources down to magnitude 21. For about 1.3 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 2 milli arc seconds (mas) for stars of 20mag. Uncertainties in proper motion is expected to be less than 0.1 mas/year for stars brighter than about 15mag and around 1,2 mas/year for very faint stars of 20mag.
Photometry Gaia DR2 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 0.02mag for stars of 20mag in the red and blue band. Astrometrica will use the G band magnitude as a substitute to the V band, and GRP and blue GBP as a subsitute for the R and B bands, respectively.
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 will query VizieR to download reference star data from Gaia DR2.

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.

URAT-1
After finishing the UCAC 4 catalog, the USNO started obervations for a new, improved star catalog with the USNO Astrograph and a larger, improved CCD camera. After only three years, the URAT-1 (USNO's Robotic Astrometric Catalog) was published. It inlcudes positions, proper motions and magnitudes for 228,276,482 objects bewteen R-magnitudes 3 and 18.5. It covers most of the northern sky, extends down to Declination -15° in some areas, and also includes the "Pluto field" around RA = 18h 30m Dec=-20°.
Reference star positions in URAT are precise to about 0.005" to 0.04", depending on observing history and brightness of the individual star. Native magnitudes have been measured in one single, non-standard color (between R and I). URAT also includes five-band photometry (B,V,g,r,i) from the APASS (AAVSO Photometric All-Sky Survey) for over 37 million stars (about 16% of the stars in URAT).
Astrometrica can access a local copy of the URAT-1 or will 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.

Obsolete Star Catalogs
The USNO-A2.0 and USNO-SA2.0 (a subset of the USNO-A 2.0) have effectively been replaced by the USNO-B1.0 catalog. Since then, the USNO-B1.0 has been replaced by the NOMAD and PPMXL catalogs. Use of the USNO-(S)A2.0 and USNO-B1.0 catalogs is no longer recommended, however, Astrometrica can still access local copies of these catalogs, which have originally been distributed on CD-ROM.
Similarly, the UCAC 2 and UCAC 3 have been replaced by the UCAC 4, and the CMC-14 has been replaced by the CMC-15. Access to UCAC 2, UCAC 3 and CMC-14 data in Astrometrica, both trough VizieR or a local copy of the respective data set, has not been disabled, so users may continue to use it if they choose to do so.
NOMAD is merged catalog compiled by the USNO, with positions and magnitudes for 1.1 billion stars from several source catalogs, including Hipparcos, Tycho-2, UCAC 2, and USNO-B 1.0. For each star the presumably best astrometric and photometric data were chosen from onw of the source catalogs. As most of these star catalogs are now outdated, usage of the NOMAD is no longer recommended, though Astrometrica can still access the NOMAD data on VizieR.