Background suppression in double-beam interferometry

An optical background suppression technique is described in which the lower spatial frequencies of a background field are effectively suppressed, thereby enhancing the detectability of localized sources. The technique is particularly applicable for use in image-forming double-beam interferometers in which the individual detector elements are matched to the diffraction-limited resolution of the instrument. The technique uses pairs of optical systems whose modulation transfer functions are suitably matched, and a number of these pairs are described.