4 - Aperture synthesis

Summary

Aperture synthesis Aperture synthesis is the way that the Van Cittert–Zernike theorem (section 3.3) is used in practice to get higher resolution images than a single large aperture will allow. Although Michelson's stellar interferometer was the first implementation of the concept, and occurred before the formalization of coherence theory, its real application began in the late 1940s in radio astronomy, where today it is responsible for almost all high-resolution images. At radio frequencies the problem of getting high angular resolution is very acute because of the long wavelengths involved, and Michelson's stellar interferometer inspired Martin Ryle (1952) to use the same idea in radio astronomy. And so, while optical aperture synthesis languished for 60 years for lack of suitable electronics, the radio astronomical applications of the technique blossomed. There are several excellent texts on the theory and practice of aperture synthesis, mainly directed to the radio regime, such as Thompson (2001), Perley and Schwab (1989) and Rohlfs (1996), which will give the reader insight not only into the principles but also the techniques involved.

The optics of aperture synthesis

Suppose we measure the complex spatial coherence function due to a distant source using two receivers separated by a vector r lying in a plane normal to its direction in inertial space. Following this observation, we can change the vector and make another measurement, and so on, until a sufficiently large bank of data for γ (r) is accumulated.