Up: Optical positions of compact
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,
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 (
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).
Up: Optical positions of compact
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