A RADIO OPTICAL REFERENCE FRAME .5. ADDITIONAL SOURCE POSITIONS IN THE MIDLATITUDE SOUTHERN-HEMISPHERE

We report new accurate radio position measurements for 30 sources, preliminary positions for two sources, improved radio positions for nine additional sources which had limited previous observations, and optical positions and optical-radio differences for six of the radio sources. The VLBI observations are part of the continuing effort to establish a global radio reference frame of about 400 compact, flat spectrum sources, which are evenly distributed across the sky. The observations were made using Mark III data format in four separate sessions in 1988-89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Kashima, Japan. We observed a total of 54 sources, including ten calibrators and three which were undetected. The 32 new source positions bring the total number in the radio reference frame catalog to 319 (172 northern and 147 southern) and fill in the zone -25-degrees > delta > -45-degrees which, prior to this list, had the lowest source density. The VLBI positions have an average formal precision of less than 1 mas, although unknown radio structure effects of approximately 1-2 mas may be present. The six new optical position measurements are part of the program to obtain positions of the optical counterparts of the radio reference frame sources and to map accurately the optical on to the radio reference frames. The optical measurements were obtained from USNO Black Birch astrograph plates and source plates from the AAT, and KPNO 4 m, and the ESO Schmidt. The optical positions have an average precision of 0.07'', mostly due to the zero point error when adjusted to the FK5 optical frame using the IRS catalog. To date we have measured optical positions for 46 sources.