Review of radio astrometry II. Optical measurements

The establishment of an astronomical reference frame which is based on positions of extragalactic objects is an old problem of fundamental astronomy. Radio astrometry has now achieved a level of accuracy, which rivals the best optical work, thus openingexcitingprospects to reach this aim within a relatively short time scale. To take full advantage of these encouraging developments for optical fundamental astrometry, precise positions of the faint optical counterparts of compact extragalactic radio sources and galactic radio stars have to be determined in the fundamental system of the FK4.