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L. Winter (USRA/USNO), E. Holdenried (USNO)
Stellar proper motions are of critical importance to many areas of astronomy, from galactic dynamics studies to observing asteroid occultations. To improve proper motions, one can re-analyze decades-old astrometric data, then combine them with modern-epoch observations. The U.S. Naval Observatory has teamed with the Hamburg Observatory (Germany) to re-measure the AGK2 plates for improving the proper motions of the USNO CCD Astrograph Catalog project (UCAC) (See Zacharias et al. this meeting).
The AGK2 project, which photographed the entire Northern Hemisphere in 1929 and 1930, provides a rich source of positions for such work. The 2000 plates comprising the AGK2 were measured in the 1930s and 1940s but only to about 2 micron accuracy and only for stars brighter than V=10.5. To make a remeasurement worthwhile, more stars and an improved accuracy are required. It is also desirable to complete the measurements quickly.
To accomplish these goals, the StarScan measuring machine of the USNO has been upgraded over the last couple of years. A Pulnix TM1300 CCD camera with Schneider Xenoplan telecentric lens has been added. Extensive control and reduction software has been written to enable all stars on one photogaphic plate to be measured in one hour in both direct and reverse orientations. The machine calibrations done to date yields a measuring error of 0.08 micrometer in the field of view of the camera. The straightness of the machine is known to better than 0.12 micrometer. The airbearings will add a random error of approximatly 0.15 micrometer.
If the telescope and atmospheric errors can be corrected, positional errors of about 70 mas are possible. This will lead to about 1 mas/yr proper motion accuracies for the UCAC stars, an error comparable to that obtained by the Hipparcos mission.