CCD astrometry of faint compact extragalactic radio sources

Optical positions relative to the Hipparcos Catalogue (ESA 1997) have been obtained for the optical counterparts of 15 faint (B 20 − 23) southern compact extragalactic radio sources (CERS). Most of these sources are not adequately observable (or simply not visible) by means of direct photography with a conventional wide- eld telescope, so the positions were determined using a multi-step procedure involving CCD and photographic observations, a method which responded both to the need to attain small rms errors and the need to refer the positions to a standard astrometric system. Estimated precisions as good as 60 mas were achieved. For ve of the above objects, namely 0936−853, 1221−829, 1604−333, 1647−296 and 1706+006, we are proposing a new optical identi cation. A comparison with VLBI radio positions available for these sources is presented. The residuals obtained are in most cases consistent with the precision of our optical data, and provide an evaluation of these faint objects as possible radio/optical link sources (or their identi cation as astrophysically interesting astrometric outliers). Twelve of the CERS observed are either de ning or are candidate ducial objects of the present realization of the International Celestial Reference System (ICRS, Ma et al. 1998), so the optical data obtained for them is potentially useful to help maintain and possibly improve the current link of the Hipparcos reference frame to the ICRS