Fourth Cambridge Catalog of Radio Sources

The Fourth Cambridge Catalog of Radio Sources (4C), published in the 1960s, includes northern hemisphere radio sources detected at 178 MHz. Data was collected by the 4C Array at Mullard Radio Astronomy Observatory (MRAO). It made strides in angular resolution, using aperture synthesis.

A follow-up survey of the sources using a higher-resolution telescope distinguished more sources, which are cited using the designator 4CT.