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Increasing the phased field-of-view of large distributed aperture telescope arrays

There are some unique optical design considerations in the design of a distributed aperture telescope array consisting of a number of individual afocal collector telescopes and a single combining telescope. As might be expected, diffraction limited imagery over the field of view requires the correction of spherical aberration, coma, astigmatism and field curvature. However, there is an additional requirement on the design of the collectors. Coherent addition of the light from the collectors requires that the ratio of the sine of the input and output chief ray angles be essentially constant over the field of view. This output angle restriction is equivalent to requiring the collector telescopes to have a specific variation in angular magnification with field. Variation of magnification with field is, by definition, image distortion. With simple two and three mirror afocal collector telescope designs, there are insufficient design degrees of freedom to control distortion in addition to the other aberrations. In this paper, we present a modification to a three-mirror collector design that permits the image distortion to be adjusted in a controlled manner without influencing the other third order, Seidel aberrations. This modification permits a large distributed aperture telescope array to be designed that has both a simple optical form and a much larger phased (i.e. corrected) field of view. As a design example, we investigate the effect of the distortion correcting element on the size of the achievable field of view for a 45 meter diameter distributed aperture telescope array populated with 8 meter diameter collectors.