Performance of the multiple mirror telescope (MMT) VIII. MMT as an optical-infrared interferometer and phased array

Large gains in spatial resolution have been achieved at 0.5 and 5.0 µm by operating the MMT as a two-element interferometer with a maximum baseline of 6.9 m. Measurements of the resulting Michelson fringes at 5.0 µm have determined pathlength errors within the MMT and characterized pathlength stability versus elevation angle, temperature, and perturbations of the optical elements. At 0.5 μm, a “coherent beam-combiner” has successfully reconstructed a phased entrance pupil achieving a field >5 arcsec in diameter with 15 milli-arcsec resolution. Scientific measurements have fully resolved the 0.024 arcsec diameter disk α Tau (Aldebaran); resolved the 0.1 arcsec binary, β Tau; and provided new measurements of the spectroscopic binary, α Aur (Capella). Instrument designs employing only two additional reflections in each beam have been developed to phase the entire MMT for simultaneous applications at optical and infrared wavelengths. When configured as a phased array of four or six elements, the MMT responds to the full range of spatial frequencies present in a filled 6.9 m aperture with only a small degree of redundancy. A fully phased MMT possesses significant advantages for low background infrared photometry.