SCHMIDT PLATE ASTROMETRIC REDUCTIONS USING PRELIMINARY HIPPARCOS AND TYCHO DATA

New reductions of Schmidt plate measurements have been performed, usin g preliminary Hipparcos and Tycho data for the astrometric reference s tars. The measurements reported here have been obtained with the MAMA( 1) automatic measuring machine of the Centre d'Analyse des Images in P aris. The Hipparcos data considered, the 'combined 30-month solution', or 'H30', is a preliminary catalogue derived from the analysis of the first 30 months of data (out of a total of 37 months), and represent the merging of the intermediate FAST and NDAC Consortia's sphere solut ions. The Tycho data considered here, referred to as 'T30d', are deriv ed from the preliminary analysis of the first 30 months of Hipparcos s tar mapper data. The preliminary Hipparcos and Tycho data are of unpre cedent accuracy, and have been used to show that the ultimate accuracy which can be obtained on positions derived from Schmidt plates measur ements is about 0.07 arcsec, compared with errors of at best 0.15 arcs ec but more typically about 0.3 arcsec obtained on the basis of previo usly-available catalogues. The best accuracy is obtained using Tycho s tars which, although of lower individual accuracy than the Hipparcos p ositions, are very numerous (the Tycho Catalogue will include about on e million stars) and thus allow a very good modelling of Schmidt plate field distortions. This accuracy is attainable for plates taken at ep ochs close to the Hipparcos mid-mission epoch. Plans to determine accu rate proper motions for Tycho Catalogue stars using Tycho positions an d, for example, Astrographic Catalogue data, will allow an extension o f this possibility to a much wider range of plate epochs.