Symposium on Synthetic Aperture Optics

Abstract

The papers presented in this document were given at a technical session in conjunc- tion with the dedication of the Optical Sciences Center on January 22, 1970. Session chairman was the Honorable Harry Davis, Deputy Under Secretary of the Air Force. Mr. Davis was an appropriate choice for the chairman, as he has been active in stimulating much of the current thought and effort in this area. The concept of synthesizing a large optical aperture from a collection of smaller, related apertures has appealed to optical physicists for a long time. The difficulties of manufacturing a large telescope or other optical system using a single massive blank for the primary objective are in themselves extreme. In the case of a large astronomical telescope, even though the primary image- forming element may be monolithic, it cannot be rigid to the tolerances required. As a result, most telescopes are active in the sense that the primary mirror is supported on a mount that adjusts to differing mechanical or thermal loads. The development of large space telescopes will lead ultimately to actively controlled mirrors in the adverse environment of any useful orbit. Under such conditions, it is possible to conceive of a synthetic aperture telescope consisting of a set of reflectors, mounted on an actively controlled mechanical system, that can respond to environmental changes. In this way, proper phasing of the optical elements can be achieved for significant exposure times. In fact, some of the phasing can be done optically in the region of the image plane, which is typically much smaller than the apertures themselves. In addition to the above passive systems, active systems using laser illumination with either spatial or temporal integration can be devised and are close to realization. In effect, these would be optical radar systems, with all of the advantages of coherent detection, along with the fineness of resolution available at optical wavelengths. The state of the art in the field has not reached the engineering stage. Indeed, at present we cannot speak even of a developmental stage. However, the time is fast approaching when synthetic aperture optical systems are going to be realized in practice. Atmospheric limitations imply that the first applications will occur in space, but certainly not before the techniques are well explored on the ground, perhaps in the far infrared region where the resolution limitation of the atmosphere is not so restrictive as in the visible range. Before this happens, technological developments in several areas are required. Needed are active control and phasing of the optics, post-processing of the sampled pictures, and an over-all concept for system design. Any interested person can likely add more areas of technology that would be stretched to attain the desired goal. This meeting was organized to permit discussion of the state of the art in this area and to encourage further work in the field. Appended to this set of papers is a bibliography on the subject of optical synthetic apertures, which was prepared by Mr. R. Brian Hooker, currently a graduate student at the Optical Sciences Center. The list should be useful as a starting point for anyone who wishes to delve deeper into the subject. -R. R. Shannon