SIMBAD references

The optical positions of 63 extragalactic nebulae measured in the frame defined by the annual series of Carlsberg Meridian Catalogues Nos 1-8 are compared with their VLBI radio positions in the International Celestial Reference Frame. The differences between these radio and optical positions are interpreted as showing the global distortion of the Carlsberg optical frame, which is linked to that of the FK5. North of the equator the Carlsberg optical frame is within 0.05" of the ICRF; but south of the equator it deviates by 0.07" in right ascension and 0.10" in declination. The general form of the deviations follow those of the FK5, which are revealed in a recent comparison of FK5 with a preliminary version of the Hipparcos catalogue (H30).