CCD astrometry of faint compact extragalactic radio sources I. First results and description of the program

Optical positions relative to the Hipparcos Catalogue (ESA 1997) have been obtained for the optical counterparts of 15 faint (B similar to 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-field 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 five of the above objects, namely 0936-853, 1221-829, 1604-333, 1647-296 and 1706+006, we are proposing a new optical identification. 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 identification as astrophysically interesting astrometric outliers). Twelve of the CERS observed are either defining or are candidate fiducial 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.