In many optical imaging situations (e.g., space optics or infrared imaging systems), the lens is nearly diffraction limited so that object resolution is limited by the diameter of the primary lens or mirror. Thus, to increase object resolution, either the lens diameter must be increased or the wavelength decreased. Since the wavelength reduction is often impractical, systems designers must increase the lens diameter to see the smaller objects. The weight, size, and cost of an optical system are probably the limiting factors in most applications. Thus, some sort of synthetic aperture optical system becomes a candidate for solving the problem of achieving higher resolution. Various kinds of optical synthetic apertures have been discussed in the literature, including interferometry, feedback-controlled optics, imaging with partially filled apertures, and coherent optical aperture synthesis.
Historically, one of the earliest demonstrations of optical aperture synthesis was the Michelson stellar interferometer. This instrument was used by Michelson and Pease in the 1920s to measure the angular diameter of the star Betelgeuse. The interferometer consisted of outrigger mirrors in front of a telescope, which were moved apart until the fringe contrast became zero in the focal plane. By utilizing the inverse Van Cittert-Zernike theorem of coherence theory, this mirror separation may be used to determine the angular diameter of the star. (In fact, Michelson actually used a direct convolution in the image space, so that the fringe contrast is zero when the spacing of the fringes is the same as the size of the image of the star.) This concept was utilized by radio astronomers in the 1950s to measure the angular diameter of stars in the radio universe. Since these radio measurements require aperture diameters of miles or more to resolve the biggest radio stars, aperture synthesis with these radio interferometers made such measurements feasible. In the late 1950s and early 1960s, the side-looking radar system was developed at the University of Michigan.
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