Can the Fundamental System be Replaced by a Reference System of Galaxies

Extract

Quasistellar objects with and without radio emission have recently given unforeseen and vigorous impetus to astrometry. Great effort has been made in the search for the optical counterparts of radio sources. For the purpose of identifications accurate positions were demanded, optical as well as those by radio techniques. Optical positions of radio sources are also needed to serve as calibrators for radio positions. In all cases the objects under consideration are optically much fainter than those ever used previously in high precision astrometry. Finally, methods have been developed for the absolute measurement of radio positions to an accuracy which competes with that known from fundamental observations of bright stars.