COMPARISON OF GROUND-BASED PROPER MOTIONS FROM MERIDIAN OBSERVATIONS AND ASTROGRAPHIC CATALOG PLATES WITH HIPPARCOS PROPER MOTIONS

During the preparation of the Hipparcos Input Catalogue the Bordeaux a utomatic meridian circle has measured the positions of about 2800 star s in the declination zone -2 degrees to +31 degrees. These stars conce ntrate mainly in the visual magnitude range 10(m) to 11(m). In this zo ne the (x,y) measurements of all Astrographic Catalogue (AC) plates ha ve been newly reduced, and at least two independent positions with epo chs around 1905 are available for the great majority of the 2800 stars . The proper motions obtained by combining the AC positions with the B ordeaux meridian circle observations (BXAC) have a formal mean error o f +/-3 mas/yr. A comparison with Luyten's NLTT catalogue allowed the e limination of spurious high proper motions. After the application of a dditional verifications altogether 2509 new proper motions can be assu red with very high probability. As a by-product it turned out that in the corresponding magnitude range the mean errors of the mainly photog raphically determined NLTT proper motions are in the order of 23 mas/y r. Comparison of the BXAC proper motions with the 30-months solution ( H30) of the Hipparcos catalogue for 1029 stars in common provides for the first time a reliable and independent assessment of the precision to be expected by combining a new reduction of the fainter stars in th e Astrographic Catalogue with modern ground-based observations. The H3 0 data proved that the accuracy of the BXAC proper motions is on avera ge about 6 mas/yr, i.e. a factor of two less accurate than internally determined. This is obviously due to the fact that the adopted errors of the AC positions of fainter objects were underestimated. In additio n, it turned out that the systematic deviations of the BXAC proper mot ions were throughout smaller than +/-4 mas/yr.