SIMBAD references

Optical astrometry of 12 fields containing quasi-stellar objects (QSOs) is presented. The targets are radio sources in the International Celestial Reference Frame with accurate radio positions that also have optical counterparts. The data are used to test several quantities: the internal precision of the relative optical astrometry, the relative parallaxes and proper motions, the procedures to correct from relative to absolute parallax and proper motion, the accuracy of the absolute parallaxes and proper motions, and the stability of the optical photocenters for these optically variable QSOs. For these 12 fields, the mean error in absolute parallax is 0.38 mas and the mean error in each coordinate of absolute proper motion is 1.1 mas yr^–1. The results yield a mean absolute parallax of -0.03 ± 0.11 mas. For 11 targets, we find no significant systematic motions of the photocenters at the level of 1-2 mas over the 10 years of this study; for one BL Lac object, we find a possible motion of 4 mas correlated with its brightness.