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Scanning of the POSS R--band plates of the northern hemisphere, completed recently at the STScI, together with the scans of the ``Quick-V'' plates taken for the HST Guide Star Catalogue, make possible the determination of proper motions of large numbers of stars in selected--target or survey modes. In the first mode, we have been obtaining motions for stars of kinematic and astrophysical interest as requested by several collaborators (H. Jahrei\ss - ARI--Heidelberg: candidate nearby stars; J. Liebert - U. of Arizona: hot DAs from the Palomar-Green survey; M. Parthasarathy - Indian Inst. of Astroph.: low-mass, post-AGB stars; P. Green - CfA: high-latitude carbon stars; Rex Saffer: sdO stars). We report on a test of the derived motions for a set of the Naval Observatory parallax program stars and discuss the completeness of the Luyten Two--Tenths Survey.
We have also searched for proper motions over entire POSS regions using an overlapping subplate technique. This method is very flexible in that it computes individual relative proper motions against several sets of reference stars, giving a thorough analysis of the errors and providing a check against spurious measuremts due to statistical fluctuations. The method is vulnerable to cosmetic and astrometric defects of the plates, and also fails for large proper motions. Using the existing plate archive it is possible to measure about one million previously unknown proper motions down to 0\farcs035 and V=17.5. We present a comparison of our results for the region of the NGP with those of other work, and with another selected region near the galactic equator.
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