7 Conclusions

The level of accuracy and homogeneity attained by the Hipparcos Catalogue provides the necessary workframe to "fine-tune" the selection of Benchmark objects to link the radio/optical extragalactic reference frames, and subsequent testing of their equivalence. However, for this to be fully realized, it is necessary to improve the accuracy of the optical positions, in particular the precision of the measurement of the optical counterparts. As shown by the relations given in Sect. 4.1 of Paper I, in the case of optically faint CERS, $\epsilon_{\rm r}$ completely dominates the total internal error. One way to achieve this is by means of CCD observations in combination with conventional photographic astrometry. First results from a pilot program being carried out by us (Costa & Loyola, in preparation) show that it is possible to obtain positions for the optical counterparts of very faint ($V \sim$ 20-22) CERS, relative to stars in the field of the CCD, with an average precision of 30 mas, yielding total internal errors of the order of 70 mas.

Acknowledgements

We are indebted to Cerro Tololo Interamerican Observatory for donating the plate material that made possible the continuation of our survey. We are also indebted to Profs. C. Anguita and F. Noël for their interesting comments, and to M. Wishnjewski and L.E. González for measuring the plates. This work was partially financed by the Fondo Nacional de Investigación Científica y Tecnológica (proyecto 1970767 Fondecyt).