Origins of sparse aperture imaging

Sparse aperture imaging has its roots in the work on optical interferometry by Fizeau and Michelson over one-hundred years ago, and the development of radio astronomy nearly fifty years later. In their quest for higher angular resolution at meter wavelengths radio astronomers were forced to seek alternatives to filled aperture telescopes. Radio interferometers, as these instruments are called, measure the complex visibility, which is the Fourier transform of the source brightness distribution. The earliest instruments measured only the amplitude of the visibility, and it was several years before phase measurements became routine. Both the amplitude and phase of the visibility are needed to produce images of a complex source by Fourier inversion. It wasn't until the 1970s and 1980s that the technology was available to allow modern optical stellar interferometers to be built. This paper traces the history of sparse aperture imaging from early radio measurements to the current generation of ground-based optical interferometers, and discusses their general principles of operation.