MERIDIAN CIRCLE REDUCTIONS USING PRELIMINARY HIPPARCOS POSITIONS

New reductions of differential observations made by the Bordeaux and L a Palma automatic meridian circles have been undertaken, replacing the positions of the FK5 reference stars by those in the Hipparcos 18-mon th intermediate solution. A remarkable improvement in the post-fit res iduals of the classical least-squares adjustment is obtained for both instruments, with data covering several years. The residuals show smal l systematic waves corresponding to (as yet unexplained) instrumental errors. After correction of these errors, and after incorporating an i mproved modelling of the instrumental parameters, a new reduction of t he Bordeaux observations has been made. We present a comparison of the resulting positions for 173 radio stars with the intermediate Hipparc os positions, indicating the ultimate accuracy that can be attained wi th automatic meridian circles. In the future these instruments, equipp ed with properly calibrated CCDs, should make an effective contributio n in extending the Hipparcos reference frame to the range 11-16 mag. T his reference frame is represented by the 120 000 Hipparcos stars and the one million Tycho stars, containing all stars on the sky brighter than 11.5 mag.