Astrometry of Poss-II Plates Using Tycho

Extract

We present a first attempt to compute an astrometric solution of POSS-IIplates using TYCHO stars as the reference catalogue. Also, we have made use of a special catalogue of stars observed with the Carlsberg Meridian Circlein La Palma between 1994 and 1995, brought into the HIPPARCOS reference system. Such a catalogue consists of a double strip along the equator, fairly dense (approximately 10 stars per square degree), faint (average magnitude 12.5), with an average error at epoch of ~ 150 mas per coordinate. Since the limiting magnitude of TYCHO stars is about 11.5, almost exactly one magnitude less than the CAMC special catalogues’ average, the latter has served as a very useful external comparison, although a more interesting magnitude range would have been 14-15. In this respect, we note that the Schmidt plates under study cover a large area (6.4 x 6.4 degree) and have fairly long exposures (~ 60 min); therefore, a possible magnitude dependence of the model needs to be carefully investigated. The results shown in the table above, which were obtained using a 2nd order polynomial in conjunction with a mask method using 5 plates only are very promising. The average external error over the entire plate area is about 0.2 arsecper coordinate, after the CAMC error have been subtracted.