Abstract
           An updated GSC as astrometric reference for minor
                         planet observations.

                           Siegfried Roeser
                    Astronomisches Rechen-Institut
                          Heidelberg, Germany



Minor planet observers generally use the Guide Star Catalog (GSC) as
astrometric reference for the reductions of their CCD frames.  Two
kinds of problems are connected with this. First, GSC has no proper
motions, and second, it has been noted ( see e.g.  Marsden (1994)) that
astrometric discontinuities occur, when a minor planet crosses the
edges of the plates underlying GSC.  It is found that the cause of this
second problem is a radial magnitude term detected on the Schmidt
plates used for GSC.

Work is going on to remove this effect in a new reduction of GSC.  We
report on the status of this new reduction, which includes the
following:
i) new positions for all GSC stars on the PPM system,
ii) determination of proper motions for 4.5 million GSC stars using the
Astrographic Catalogue (AC) as first epoch,
iii) identification of the galaxies in GSC as additional reference
points with known (zero) proper motions.

Reference:

Marsden, B.G. In: Galactic and Solar System Optical Astrometry.
        Eds. L.V. Morrison, G.F. Gilmore. Cambridge University Press,
        Cambridge, UK 1994, P. 263.