Catalogs of discrete sources have been prepared from extensive surveys using instruments especially designed for this purpose. Initially these surveys were made at relatively long wavelengths near one meter, but as techniques have improved at the shorter wavelengths, the surveys have been extended to wavelengths as short as a few centimeters.
In order to isolate the discrete sources from the intense emission observed from the galactic background, most of the earlier surveys were made with interferometer systems, which are relatively insensitive to the distributed background emission (see Chapter 10).
Today, catalogs of sources based on surveys made between 10 MHz and 5 GHz are available. Some of the surveys, particularly those made at the longer wavelengths, cover essentially the entire observable sky down to source densities of about 103 ster-1. Other instruments, intended mainly for cosmological studies, have reached source densities of about 105 ster-1 over very restricted parts of the sky. New instruments just coming into operation will reach densities of 106 ster-1.
Generally the surveys have produced approximate values for the position and flux density for the cataloged sources. These catalogs have then been used as the basis for subsequent more accurate measurements over a wide range of wavelengths of properties such as
A useful summary of radio source surveys has been compiled by Dixon (1970), and contains the results of many separate surveys. The two most widely used catalogs are based on the Cambridge 3C and 3CR surveys and the Parkes survey, which contain between them the great majority of sources which have been studied in detail and for which optical data are available.