The Infrared Spatial Interferometer (ISI) is an astronomical interferometer

Astronomical interferometer

An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution by means of interferometry....

 array of three 65 inch (1.65 m) telescopes operating in the mid-infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

. The telescopes are fully mobile and their current site on Mount Wilson

Mount Wilson (California)

Mount Wilson is one of the better known peaks in the San Gabriel Mountains, part of the Angeles National Forest in Los Angeles County, California. It is the location of the Mount Wilson Observatory and has become the astronomical center of Southern California with and telescopes, and and tall...

 allows for placements as far as 70 m apart, giving the resolution of a telescope of that diameter. The signals are converted to radio frequencies through heterodyne

Heterodyne

Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...

 circuits and then combined electronically using techniques copied from radio astronomy

Radio astronomy

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

. ISI is run by the UC Berkeley Space Sciences Laboratory

Space Sciences Laboratory

The Space Sciences Laboratory is an Organized Research Unit of the University of California, Berkeley. It is located in the Berkeley Hills above the university campus...

. The longest (70m) baseline provides a resolution of 0.003 arcsec

Arcsec

Arcsec may refer to:* arcsecond, a unit of angular measurement* arcsecant, an inverse trigonometric function...

 at a wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of 11 micrometers. On July 9, 2003, ISI recorded the first closure phase

Closure phase

The closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...

 aperture synthesis

Aperture synthesis

Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...

measurements in the mid infrared.