Checking the base of the flux density calibration scale

The sensitivity of large radio telescopes cannot easily be calculated theoretically. Therefore, the flux density calibration of radio astronomical observations at centimeter wavelengths is usually done by comparing the observed antenna temperatures of selected sources (“calibrators”) with their assumed flux density. The probably most influential flux density scale in use is the one of Baars et al. (Astronomy & Astrophysics 61, p. 99, 1977). It is primarily based on the measurements of four strong radio sources, namely Cas A, Cyg A, Tau A, and Vir A, which have been observed using antennas with well known characteristics (e.g. horn antennas, dipoles). In a second step, the flux densities of these sources have been connected to observations of a set of weaker and more compact secondary calibrators that are better suited for large and sensitive radio telescopes. This set is now commonly used for the flux density calibration of large radio astronomical antennas.