A Review Of Partially-Filled, Synthetic-Aperture, Imaging Systems

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Abstract
Large optical apertures can be synthesized by utilizing a phased array of smaller elements. The phasing is realized by ensuring that the individual elements are made from the same lens or mirror and properly positioned. The synthesis of MTF for various geometrical arrays (Cross, Covington-Drane, Golay Six, Thin Annulus, etc.) of such partially filled apertures has been demonstrated experimentally with incoherent illumination. Post processing techniques were used to remove the deleterious effect in the image due to auto-correlation of the individual elements. The results demonstrated that the resolution of the equivalent full aperture is achievable by the MTF synthesis which arises from cross correlation of the various elements. The limitations due to misalignment of the various elements in six degrees of freedom were investigated theoretically, and experimentally verified for both refractive and reflective elements. The experimental verification of these tolerances required interferometric measurements in the focal plane. Systems Analysis showed that the aperture must be designed so that the synthesized MTF has a modulation higher than the detector noise threshold at all spatial frequencies (AIM curve). In general, the technique does not work unless the radiation is spatially incoherent. To illustrate this, experiments were performed with laser illumination. The aperture did not synthesize unless the laser was rendered incoherent by utilizing moving diffusers or scanning techniques. Synthesis with laser illumination was achieved in a non-partially filled mode by phasing with holographic arrays. Finally, some experimental results on super-resolving pupils are shown and discussed.

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