Abstract

The infrared spatial interferometer (ISI) is a high-resolution aperture synthesis imaging system for the 10-μm region located at Mt. Wilson Observatory since Jan. 1988. The compactness of the novel-design Pfund telescopes allows all optics and electronics to be mounted in two semitrailers.^1,2 Each of the two telescopes consists of a fixed f/3.14 parabolic mirror with 1.65-m aperture and a 2-m diam flat mirror in an altaz fork mount, which tracks the source. Hererodyne detection is achieved using LN₂ cooled HgCdTe photodiodes and phase-locked CO₂ laser local oscillators. The quantum efficiency including telescope transmission losses is ~25%, and the IF passband is 200–2000 MHz. Phase fluctuations on the LO signal caused by the atmosphere between the telescopes are compensated with a path length control system. As in many radio interferometers the fringes are stopped with a lobe rotator acting on the LO system. Correlation of the received signals over the IF passband is maintained with an IF delay line. A newly costructed He–Ne laser metrology system ensures that the telescopes can be pointed to a precision of ~0.01 sec of arc. Critical internal distances can also be monitored with this system. In addition the telescopes are equipped with conventional optical encoders. Although designed for baselines up to 1000 m, the initial range is 4–35 m yielding angular resolutions of 0.25–0.03 sec of arc.

© 1988 Optical Society of America