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

We report new accurate radio position measurements for 30 sources, pre liminary positions for two sources, improved radio positions for nine additional sources which had limited previous observations, and optica l 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 so urces, which are evenly distributed across the sky. The observations w ere made using Mark III data format in four separate sessions in 1988- 89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Ka shima, Japan. We observed a total of 54 sources, including ten calibra tors and three which were undetected. The 32 new source positions brin g the total number in the radio reference frame catalog to 319 (172 no rthern and 147 southern) and fill in the zone -25-degrees > delta > -4 5-degrees which, prior to this list, had the lowest source density. Th e 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 r eference frame sources and to map accurately the optical on to the rad io 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 preci sion of 0.07'', mostly due to the zero point error when adjusted to th e FK5 optical frame using the IRS catalog. To date we have measured op tical positions for 46 sources.