The USNO Twin Astrograph is a 20 cm telescope constructed in 1970 for the purpose of extensive astrometric surveys. It was originally deployed the USNO's Washington DC site, was temporarily relocated to a New Zealand observatory to observe southern hemisphere stars, and then to Cerro Tololo Inter-American Observatory, then to the USNO Flagstaff Observatory. Over time it has been was improved, including conversion from photographic plates to a CCD, then called the USNO CCD Astrograph (USAC). Resulting catalogs:
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.