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Published online by Cambridge University Press: 14 August 2015
The ability to identify the many tens of thousands of extragalactic radio sources now known depends upon the precision with which the radio positions and structures are known as well as the availability of high quality plate material. For the brightest radio sources, positions with accuracy better than 1 arcsec are now routinely available and radio structures with angular resolution 1–5 arcsec can be readily measured with instruments such as the Westerbork Synthesis Radio Telescope, the Cambridge 5-km Telescope and the VLA. The sensitivity of the VLA is such that these observations can be extended to sources in the flux density range ~ 1–10 mJy.