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In order to calibrate the proper motions of the Hipparcos astrometry satellite, our group has mea- sured accurate absolute proper motions of Hipparcos stars in small elds around optically bright extragalactic radio sources or bright galaxies with star-like features. In addi- tion, we also use elds where relative proper motions are calibrated by measurements of large numbers of stars and galaxies on wide-eld plates. The median internal accu- racy of our relative proper motions, based on photographic plates with epoch dierences up to 100 years (typically 70 years), is 1.0 milliarcsec/year (mas/a), while the calibra- tion to an inertial system in each of the 13 elds has a median uncertainty of 1.3 mas/a. We compute the rotation from the Hipparcos proper motions (median internal errors 0.9 mas/a) to the ex- tragalactic reference frame represented by our absolute proper motions, using 88 stars in common. The three com- ponents of the angular velocity vector have internal er- rors of 0.3 mas/a. Our rotation solution has been used together with those of independent groups for the extra- galactic calibration of the Hipparcos proper motion system (Kovalevsky et al. 1996). It compares favourably with the adopted mean solution.

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