Distribution of stars in the USNO-A2.0 catalog (Source: USNO)
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.
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.
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 (10mag to 14mag), and a precision better than 0.1" is expected at the limiting magnitude of 16mag. 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.
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.
Astrometric positions in CMC-15 are precise to 0.05" for bright stars (13mag or brighter) and to 0.1" for stars at the limiting magnitude of 16mag. 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 0.025mag for bright stars (13mag or brighter) to 0.17mag at the limiting magnitude of 16mag. 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.
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.