PISTON ERROR ESTIMATION METHOD FOR SEGMENTED APERTURE OPTICAL SYSTEMS WHILE OBSERVING ARBITRARY UNKNOWN EXTENDED SCENES

Relative piston misalignments between segments of a multisegment optical system, such as a segmented primary mirror, are reduced by a method in which elements of an arbitrary unknown extended scene image carried by an input beam are eliminated from the beam's error information in performing an error calculation. After appropriately processing the beam with the specialized sensor elements, spatial frequency domain representations of a variety of optical images are obtained which correspond to both individual segments, and to multiple combinations of segments of the optical system; each combination is preferably composed of two individual segments. The spatial frequency domain representations of the individual segments are then subtracted from the representations of the segment combinations to obtain spatial frequency domain functions for the combinations. These in turn are compared by a cross-coherence technique to derive spatial domain differences of piston differences misalignments between the combinations, from which the piston errors between individual segments can be calculated via a reconstruction matrix. The calculated piston errors are then used to make a compensating adjustment to the piston positions of the individual segments in the optical processing element.