1. In an optical correlation system wherein the position of an object within a search area x-y is represented by the position of a correlation image in correlation space, the image being formed by a focused beam of light, means for detecting the position of the correlation image comprising; a beam splitter in the beam of light to form two light beams, multiple imaging diffraction grating means in each of the two light beams arranged to display two image patterns in respective correlation planes, each image pattern comprising a series of closely spaced spots arranged in a straight line, and a photoelectric detector in each correlation plane, each detector comprising a linear photodiode array positioned orthogonally to and intersecting the image pattern in its respective correlation plane so that a photodiode in each array is illuminated according to the position of the correlation image thereby providing a pair of electrical signals representing the coordinate position of the correlation image.