Proc. of SPIE Vol. 7013 70130D-1

The ISI (Infrared Spatial Interferometer) involves three movable telescopes of Pfund design, each in trailers, each with a radiation collecting mirror 65 inches in diameter. The telescopes can be moved, and then set down on concrete pads for stability. They are located on Mt. Wilson, where pads have been arranged to give baselines varying from 4 to approximately 75 meters and oriented in various directions. Initial work, between 1988 and 2002, was done with two telescopes; the third telescope was added in 2003. Description and Functioning of the Interferometer System. The telescopes involve heterodyne detection of stellar signals at wavelengths near 11 μm, with laser local oscillators which in the detectors beat with stellar signals. Beat frequencies are generated and amplified over a frequency range of approximately 3 x 10 Hz giving a detected bandwidth of twice that because of the superposition of upper and lower sidebands. This bandwidth is limited by the response time of the solid state detectors. Each telescope has its own CO2 laser local oscillator, but laser signals are sent between the three telescopes, allowing the three lasers to be locked in phase with small frequency differences chosen to be in the range 50 – 200 Hz. Stellar signals from each pair of telescopes are combined, producing interference signals centered on the frequency differences of the two lasers in the two telescopes. A picture of two telescopes separated by 4 meters, the minimum baseline, is shown in Figure 1. Figure 2 gives a schematic of a single telescope. Figures 3 and 4 show schematics of the signal arrangement for producing interference fringes. There are advantages and disadvantages to heterodyne detection at infrared wavelengths. The primary disadvantage is lower sensitivity than most direct detections. Heterodyne detection has a fundamental minimum noise corresponding to a noise