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We are establishing a reference system of at least 400 extragalactic sources over the entire sky which are compact and flat spectrum in the radio and display optical emission. To date we have published and/or have in preparation VLBI radio positions for 398 sources as well as optical positions from a combination of astrograph and large telescope photographs for 51 of these.
This paper is an overview of the progress to date in determining the differences in the reference frames. The zero point for the VLBI right ascensions was determined using optical positions for 28 quasars measured in the system of the FK5. The radio positions are adopted as the true positions of the sources and the optical$-$radio differences show some sytematic offsets in the optical reference frame. Our results are compared with the preliminary results from the Hipparcos satellite, which also measures the offsets in the optical reference frame.
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