The Berkeley infrared spatial interferometer: A heterodyne stellar interferometer for the mid-infrared

A detailed description is given of the Infrared Spatial Interferometer (ISI ), developed at the Space Sciences Laboratory of the University of Californ ia at Berkeley, which is a high spatial resolution interferometer for mid-i nfrared wavelengths. The instrumentation, its capabilities and performance, data analysis, science program, and future plans are all discussed. The sy stem's use of heterodyne detection, analogous to that of a modem radio inte rferometer, is also compared with the homodyne or direct methods more commo nly encountered in the visible and infrared. The ISI has been operating pro ductively on Mount Wilson for the past 10 years measuring materials immedia tely surrounding stars and their changes as well as some stellar diameters. The new spectral capabilities described here, a recent increase in baselin e length, and the upcoming expansion to a closure-phase imaging array provi de important additional types of measurements.