Accurate optical positions of extragalactic radio reference frame sources

Optical positions on the 50 mas level in the Hipparcos system have been obt ained for 327 extragalactic, radio reference frame sources for both hemisph eres from a 2-step procedure. Positions of secondary reference stars in the 10 less than or equal to V less than or equal to 14 magnitude range were o btained from photographic plates taken at the Hamburg astrograph (Northern Hemisphere) and the US Naval Observatory astrograph at Black Birch (Souther n Hemisphere) using Hipparcos stars for astrometric plate solutions. Positi ons of the optical counterparts of compact, extragalactic, radio reference frame sources were then obtained from CCD direct imaging at the KPNO and CT IO 0.9 m telescopes, using the secondary reference star positions and corre cting for field distortions. Several previously unknown counterparts could be identified, and several previously suspected identifications turned out to be empty fields. CCD images are available on the World Wide Web to serve as finding charts. A comparison between the optical and radio positions re veals systematic offsets of 10 to 40 mas per coordinate as a function of th e observing run. Corrections for Galactic rotation and solar motion have be en attempted because of a lack of individual proper motions for the seconda ry reference stars. Neither 0.9 m telescope is an astrometric instrument, a nd limitations are clearly seen in remaining systematic errors. Orientation angles between the radio system and our optical data are zero within an ac curacy of 4 mas (standard error). A significant reduction of the systematic errors will be achieved with new wide-held CCD imaging currently been take n in parallel to new 0.9 m observations, to provide more secondary referenc e star positions with higher precision at a common epoch, which will allow extended modeling of the mapping properties of 0.9 m observations.