2008PASP..120..644Z

The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring photographic astrograph plates to allow determination of proper motions for the USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates, about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan comprises a CCD camera, a telecentric lens, an air-bearing granite table, stepper motor screws, and Heidenhain scales to operate in a step-stare mode. The repeatability of StarScan measures is about 0.2 µm. The CCD mapping as well as the global table coordinate system has been calibrated using a special dot calibration plate and the overall accuracy of StarScan , data is derived to be 0.5 µm. Application to real photographic plate data shows that position information of at least 0.65 µm accuracy can be extracted from coarse-grain 103a-type emulsion astrometric plates. Transformations between ``direct'' and ``reverse'' measures of fine-grain emulsion plate measures are obtained on the 0.3 µm level per well-exposed stellar image and coordinate, a level that is at the limit of the StarScan machine.