The Importance of Schmidt Astrometry for the Optical Identification of Sources detected in other Wavebands

Abstract

AbstractThe early optical identifications of radio sources were restricted to bright galaxies and to quasars with ultraviolet excesses, since such objects were rare and unlikely to land by chance within the large radio position error boxes. Other types of objects, such as very high redshift quasars and distant galaxies, were discriminated against because they did not look unusual and so were not readily recognised. Eventually much more accurate interferometric radio positions became available, enabling unambiguous identification of many optically faint objects. Now the internal accuracy of the radio positions can exceed that of the optical positions, requiring optical astrometry to be done to higher precision than ever before. A related problem is the correct registration of the two reference frames, which should be solved soon using a combination of data from the Hipparcos satellite and the Hubble Space Telescope.The digitised data now available from the Schmidt sky surveys make it possible to automate the optical identification of sources, such as those in the IRAS infrared and Rosat X-ray surveys. Very accurate digitised optical surveys are also needed to prepare lists of targets for multi-object spectroscopy using fibre optic systems.