USNO Twin Astrograph

The USNO Twin Astrograph is a pair of 20 cm telescopes constructed in 1970 for the purpose of extensive astrometric surveys. The telescopes are mounted together, covering the same field of view, and as designed, each incorporated a filter (one blue and one yellow), offering some photometry data on each star observed, the reason for having two. The astrograph was originally deployed the USNO's Washington DC site, was temporarily relocated to a New Zealand observatory to observe southern hemisphere stars, then to the Cerro Tololo Inter-American Observatory, and then to the USNO Flagstaff Observatory. Over time the astrograph has been modernized, including conversion from photographic plates to CCDs, and is now called the USNO CCD Astrograph (UCA). Resulting catalogs:

• Twin Astrographic Catalogs 1 and 2 (TAC 1.0, TAC 2.0).
• USNO CCD Astrograph Catalogs (UCAC), (1-5, UCAC-1, etc.), the latter also using Gaia data.

The catalogs offer proper motion information, cover more stars than Hipparcos, and provide independent measurements, useful for cross checking and combined statistical analysis with space astrometry efforts.