Abstract
The U.C. Berkeley Infrared Spatial Interferometer (ISI), previously described by Hale et al. as a two-telescope stellar interferometer operating in the mid-infrared regime of 9-12 μm, is now testing a recently constructed third telescope and centralized laser local oscillator and beam combining facilities for operation of a three-telescope system with phase closure. This new system will allow the ISI Array to make measurements of stellar photospheric and dust asymmetries in the mid-IR, measurements which have previously been lacking. The new facilities and instrumentation required for three-way heterodyne beam combination and fringe detection, together with techniques for measuring the closure phase, are described. Both hardware and software methods of beam combination are used for producing three simultaneous fringe visibilities and a closure phase measurement. Additionally, the phase closure signal-to-noise of pairwise and of all-in-one beam combination are contrasted.
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