Optical positions of ICRF sources using UCAC reference stars

New optical positions on the 30 mas precision level have been obtained for 172 extragalactic International Celestial Reference Frame (ICRF) sources mainly in the range -30degrees less than or equal to delta less than or equal to +25degrees. Results are presented from a pilot investigation including four Cerro Tololo (CTIO) 0.9 m runs (1999-2001). Reference stars in the R approximate to 10-16.5 mag range from a preliminary US Naval Observatory CCD Astrograph Catalog (UCAC) are used. Systematic errors have been investigated, and a field distortion pattern based on the residuals has been removed. The errors of the fainter stars in the CTIO data were assessed by evaluating an auxiliary set of CCD observations of common ICRF sources, taken at the 1.60 m Cassegrain telescope of the Laboratorio Nacional de Astrofisica, Brazil. A significant improvement in the optical positions was achieved over a previous determination of source positions. The mean optical positions are compared with the ICRF radio positions. The overall optical minus radio offsets are -6 and -15 mas for right ascension and declination, respectively. The formal internal error of these mean offsets is approximate to2.3 mas. This indicates a possible systematic error in the UCAC declinations of approximate to10 to 15 mas. Both the optical counterpart observations and the optical reference stars are observed about 9 yr after the Hipparcos mean epoch, and our results set an upper limit for a possible Hipparcos system rotation with respect to the International Celestial Reference System for the z-axis of about 0.7 mas yr(-1).