1997A&AS..122..433T

In order to calibrate the proper motions of the Hipparcos astrometry satellite, our group has measured accurate absolute proper motions of Hipparcos stars in small fields around optically bright extragalactic radio sources or bright galaxies with star-like features. In addition, we also use fields where relative proper motions are calibrated by measurements of large numbers of stars and galaxies on wide-field plates. The median internal accuracy of our relative proper motions, based on photographic plates with epoch differences up to 100-years (typically 70-years), is 1.0-milliarcsec/year (mas/a), while the calibration to an inertial system in each of the 13 fields has a median uncertainty of 1.3mas/a. We compute the rotation from the Hipparcos proper motions (median internal errors 0.9mas/a) to the extragalactic reference frame represented by our absolute proper motions, using 88 stars in common. The three components of the angular elocity vector have internal errors of 0.3mas/a. Our rotation solution has been used together with those of independent groups for the extragalactic calibration of the Hipparcos proper motion system (Kovalevsky et al. 1996, A&A, submitted). It compares favourably with the adopted mean solution.