12.2.1. 3C and 3CR Surveys
The 3CR catalog (Bennett, 1962) is based on the original 3C survey (Edge et al., 1960) made at Cambridge at 159 MHz using a complex interferometer system. This survey was preceded by the 2C survey made with the same instrument at 81 MHz with a resolution two times poorer in each coordinate. The 2C catalog contained 1936 sources, but owing to the poor resolving power, it became clear at an early stage that many of these sources were not real, and were due to blends of two or more sources in the primary antenna beam. Moreover, except for the strongest sources the determination of the flux density and angular coordinates was poor. The 3C survey with twice the resolution contained only 471 sources and was considerably more reliable. Nevertheless, because of the relatively poor primary resolving power, there were still large errors in the positions and flux densities. In particular, it was frequently uncertain in which lobe of the interference pattern a particular source was located, and this introduced large positional uncertainties.
In order to reduce these uncertainties an additional survey was made at 178 MHz using a large parabolic cylinder antenna. The narrow E-W beam of this antenna eliminated nearly all of the lobe ambiguities of the original 3C catalog. The data from the two surveys were combined to form the then most reliable radio source catalog - the Revised 3C or 3CR Catalogue.
The same parabolic cylinder antenna was later used together with a smaller moveable antenna as an aperture synthesis instrument (see Chapter 10) to make a complete high-resolution survey of the northern sky (the 4C survey), which contains about 2000 sources.