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More telescopes than photons: beam multiplexing in imaging interferometers

Imaging with interferometers is dramatically improved with increasing numbers of telescopes, because the number of baselines sampled increases quadratically with the number of telescopes. The Magdalena Ridge Observatory Interferometer (MROI) is designed so that it is possible in theory to combine the beams from all 10 telescopes simultaneously with one another to form fringes on 45 different baselines. All-in-one combination with this large number of beams is problematic for both technical and fundamental physical reasons. We propose instead to use a number of smaller beam combiners, each of which combines a subset of telescopes together at any given time. This requires an "optical switchyard" which is able to switch permutations of beams between different combiners while maintaining alignment in the tilts and pistons of the beams. We describe here the design for the switchyard for MROI. The design has a low-angle-of-incidence geometry which allows high optical throughput and polarisation fidelity. We describe the criteria driving the design at both the geometric and mechanical level and explain how these are met using our proposed implementation.