TOWARD THE HIPPARCOS LINK - ASTROMETRY OF STARS AND EXTRAGALACTIC SOURCES IN THE FIELD OF THE SPIRAL GALAXY-M-51

We present absolute proper motions of 14 Hipparcos stars from ground b ased photographic astrometry in a 2 degrees x 2 degrees field around t he spiral galaxy Messier 51. These are part of a catalogue of 340 star s with B less than or equal to 16, for which positions, proper motions , and B magnitudes have been derived from a series of 16 photographic plates. Due to large epoch differences of up to 88 years between the o bservations, the proper motion accuracy is typically less than or equa l to 1 mas/a. An extragalactic calibration of the proper motion zero-p oint is achieved by measuring the fictitious proper motions of bright extragalactic objects in the field. For this purpose, a cross-correlat ion technique was developed. The resulting precision of the proper mot ion link to an extragalactic frame is estimated as 1.0 mas/a. Measurem ents of double stars and high proper motion stars are also discussed.