The Infrared Array Camera (IRAC)

The Infrared Array Camera (IRAC) is one of Spitzer's three science instruments. It is an imaging camera, designed to detect light at near- and mid-infrared wavelengths - in other words, light with wavelengths between 3.6 and 8.0 microns (1 micron is one-millionth of a meter). It is a general-purpose camera that is used by observers for a wide range of astronomical research programs.

Unlike a normal camera, which has a single detector array, and is sensitive to a wide range of different wavelengths of light, IRAC is a four-channel camera, meaning that it has four different detectors, each measuring light at one particular wavelength. It takes simultaneous images at wavelengths of 3.6, 4.5, 5.8 and 8.0 microns, and each of the four detector arrays in the camera is 256 x 256 pixels in size. IRAC uses two different types of material in the detector arrays: the two shorter-wavelength channels (3.6 and 4.5 microns) have detectors made of indium and antimony, and the two longer-wavelength channels (5.8 and 8.0 microns) have detectors that have been specially treated with arsenic. The only moving part on the whole of the IRAC instrument is the camera shutter, which actually has not been used while Spitzer is in-flight.

IRAC is the only one of Spitzer's instruments that will still function once the liquid helium cryogen runs out. The 5.8 and 8.0 micron detectors will be too warm to be useful for scientific observations, but the 3.6 and 4.5 micron detectors will still work at peak performance for several more years.

Caption: Black Widow nebula captured by Spitzer's IRAC.
Credit: NASA/JPL-Caltech/E. Churchwell (University of Wisconsin-Madison) and the GLIMPSE Team