radio-source catalogA numbered list of radio sources detected in a survey at a given frequency down to a limiting flux density. A well-known example is the third Cambridge catalog, 3C. The catalog usually lists the source positions and flux densities together with any other information that may be available about angular sizes, etc. Early nomenclatures persist, such as ‘Cassiopeia A’ for the brightest source in Cassiopeia and 3C461 for the 461st source in the Cambridge 3C catalog, but most sources are classified by their position in the sky. Thus a source at right ascension 23h21m07s and declination +58.5° is designated 2321+585. Positions are usually quoted at the standard epochs of 1950.0 or 2000.0, and would need to be corrected for precession to find their actual position. Most of the sources in radio catalogs are radio galaxies or quasars.
From one or more catalogs source counts can be compiled, in which the total number of sources per unit solid angle (steradian) above a given flux density is plotted as a function of the flux density. These counts are important in cosmology since different mathematical models of the Universe predict different source counts: they provide one method of comparing theory with the real world.