Multiple mirror telescope as a phased array telescope

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Abstract
By adjusting the optical path lengths of its individual beams, it is possible to make the multiple mirror telescope (MMT) into a phased array with a 6.86-m base line. A coherent phased focus can be achieved with tilted focal planes if the tilt angle is chosen so that the internal phase differences exactly compensate the external phase differences. This amounts to a slight change in configuration so that the beams are brought together at f/8.39 rather than the originally designed f/9. We summarize experiments which have used the MMT subapertures as a phased array and as a coherent phased telescope and present a simple analysis of the titled focal plane geometry for coherent observation. The phased operation of the MMT is important not only for obtaining high angular resolution but also for obtaining the higher detection sensitivity which results from the better discrimination against the sky emission background for IR diffraction-limited images. Full-aperture (six-beam) diffraction-limited results for the unresolved source Gama Orionis, the well-known close binary Capella, and the resolved red supergiant Betelgeuse (including a diffraction-limited differential speckle image of the latter) are presented as preliminary demonstration of the potential capabilities of this configuration.

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