Astrometric CCD positions for extragalactic radio sources

Extragalactic radio source positions referred to Hipparcos or Tycho catalogues are useful for the determination and checking of the orientation between es and the reference frames defined by those catalogues and the International Celestial Reference Frame (ICRF), which represents the new astrometric fundamental system. We have employed a method that uses secondary reference frames, formed by Guide Star Catalog and fainter stars, corrected towards more precise reference systems, to derive astrometric positions from CCD observations. The primary reference catalogue used is a Tycho-like catalogue, the Twin Astrographic Catalogue, TAC (Zacharias et al. 1996). Here we present the optical position determinations for 18 quasars, all belonging to the ICRF, referred to TAG. The final precision on the quasar positions was 0.05 arcsec on average. The comparison with the radio positions shows good agreement with previous determinations for those sources.