By Paul Ogilvie | Updated Aug 30, 2022
Infrared telescopes operate on the same optical principles as their visible‑light counterparts: a system of lenses or mirrors concentrates incoming radiation onto a detector array. These arrays are typically built from mercury‑cadmium telluride (HgCdTe), a superconductor alloy that offers high sensitivity across the near‑ and mid‑infrared bands. Because ambient heat can overwhelm the faint cosmic signal, the detectors must be cooled to cryogenic temperatures—often with liquid nitrogen or helium—bringing them close to absolute zero. For example, the Spitzer Space Telescope, launched in 2003 as the largest infrared observatory at the time, maintained its optics at –273 °C and operates in an Earth‑trailing heliocentric orbit to avoid terrestrial thermal interference.
Water vapor in the Earth’s atmosphere absorbs most extraterrestrial infrared photons, so effective ground‑based telescopes are positioned at high, dry sites. The Mauna Kea Observatory in Hawaii sits at 4 205 m and offers a clear, arid sky for infrared work. Atmospheric turbulence is further mitigated by airborne platforms: the Kuiper Airborne Observatory (KAO) flew from 1974 to 1995, providing a brief, above‑atmosphere window for infrared studies. Eliminating atmospheric effects entirely, space‑based missions are the gold standard. The Infrared Astronomical Satellite (IRAS), launched in 1983, expanded the known catalog by about 70 percent and laid the groundwork for subsequent infrared space telescopes.
Infrared detectors can reveal celestial bodies that are too cool—or thus too dim—to register in visible light, such as exoplanets, brown dwarfs, and certain nebulae. Moreover, because infrared wavelengths are longer than visible photons, they can penetrate interstellar gas and dust that scatter or absorb shorter wavelengths. This capability allows astronomers to peer into heavily obscured regions, including the central bulge of the Milky Way, and to map star‑forming complexes that are invisible to optical telescopes.
The universe’s ongoing expansion stretches light from distant objects toward longer wavelengths—a process known as redshift. As a result, photons that were emitted in the visible range billions of years ago arrive at Earth shifted into the infrared. Infrared observatories thus act as time machines, capturing radiation that originated during the universe’s infancy and providing a direct window into its earliest epochs.
