The Potsdam contribution to the extragalactic link of the Hipparcos proper motion system

Absolute proper motions of 360 Hipparcos stars in 24 fields well distributed over the northern sky were derived from measurements of photographic plates taken mainly with the Tautenburg Schmidt telescope (134/200/400 cm). In each field (about 10 square degrees) a large number of galaxies was used to link the proper motions to the extragalactic reference system. Because of systematic magnitude-dependent errors, which affect the proper motion determination of bright stars, only the 256 stars with $B\,\ge \,9.0$ were used to determine the spin parameters of the Hipparcos system. The accuracy of the final result is 0.5 mas/yr for all three spin parameters