Development of a Sparse Aperture Test-Bed Utilizing Pupil-Plane Imaging
AIR FORCE RESEARCH LAB WRIGHT-PATTERSON AFB OH SENSORS DIRECTORATE
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Sparse aperture imaging provides the opportunity to achieve high resolutions image with groups of smaller apertures rather than a large single aperture. Eliminating large monolithic apertures provides both a cost and weight reduction among other possible benefits. Imaging with sparse apertures requires that both amplitude and phase of the incident field be captured so that an effectively larger aperture can be synthesized. Amplitude is easily captured using traditional imaging systems however capturing phase requires more advanced methods such as interferometry or phase retrieval algorithms. The design of sparse aperture imaging arrays has previously been discussed theoretically and through computer simulations however experimental verification is still needed. A sparse test bed is proposed and initial designs are discussed. The test bed provides the ability to experiment with various sparse aperture array patterns as well as sub-aperture spacing, allowing comparison of results with theory. The first step in construction of the test beds is the design of individual telescopes which will allow the field to be recorded at each of the sub-apertures by utilizing digital holography, a method referred to as pupil plane imaging. Pupil plane imaging using spatial heterodyning is described and the theory is laid out, which will allow for the recovery of the target field from a recorded interference pattern. It is demonstrated how a pupil plane imaging telescope can be used to perform lens-less imaging, effectively forming the image digitally. Expressions, which simplify focusing the resulting image on the target, are derived and experimental results are provided. Compact telescopes are developed which will provide the foundation of a sparse aperture test bed and image results are given for a prototype telescope.
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