SIMBAD references

Based on 28 individual catalogues, in which the positions of optical counterparts of extragalactic radio sources are listed, we compiled a combined catalogue by means of a weighted least squares adjustment. The catalogue is in the system of FK5/J2000.0. It consists of about five hundred sources, among which there are 56 primary sources with position uncertainties as 0.09". Comparisons show that the orientation differences between optical and radio frames are A₁=-0.013"±0.012", A₂=0.077"±0.012", A₃=0.005" ±0.009". Local relative deformations are not obvious within the precision of ground-based optical observations.