Accurate Optical Positions for Radio Source Identifications

Abstract

The optical identification of strong radio sources was a key step in establishing a grid of southern hemisphere calibrators, both for Parkes and for Molonglo. However, the measurement of precise positions for these optical counterparts presented some special astrometric problems and led to some novel solutions. This paper summarises the progress of optical position measurement from a radio astronomer's viewpoint and examines the role of the COSMOS database against this background. The source 0007-44, from the first Parkes catalogue, is used as a case study to illustrate the present-day approach to optical identifications.