Mutual coherence function in radio astronomy

The van Cittert-Zernike theorem of classical, optical coherence theory, which does not describe fully the earth-rotation, aperture-synthesis imaging technique used in radio astronomy, is broadened to include finite-frequency bandwidths, extended radiation source distributions, and interferometer baselines which do not always lie in a plane. The extended formation explicitly describes some of the observed imaging aberrations so that compensation is made possible. In the light of the results it can be shown how measurements at different frequencies can reduce the number of separate baselines needed for unambiguous imaging.