An infrared telescope is one that examines the infrared (IR) radiation emitted by astronomical bodies. Infrared radiation is a part of the electromagnetic spectrum between visible light and microwaves. This part of the spectrum can roughly be divided into the shorter near-IR and longer far-IR wavelengths. Traditional optical telescopes are capable of making observations in the near-IR range. Specially designed infrared telescopes are usually used to observe in the far-IR wavelengths.
There are three general types of infrared telescopes, differentiated by where they are deployed. Earth-based infrared telescopes are located at high altitude in arid climates. Infrared radiation is less energetic than visible light, and is particularly subject to being absorbed by atmospheric water vapor. Little far-IR radiation makes it through the atmosphere. The James Clerk Maxwell Telescope(JCMT)is an infrared telescope built at Mauna Kea Observatory in Hawaii at an altitude of 13,425ft (4092m).
Air-borne infrared telescopes rise above more of the earth's atmosphere. Originally, these were carried aloft by balloon. Current versions are equipped within the bodies of jet aircraft. The Stratospheric Observatory for Infrared Astronomy (SOFIA) is built into the body of a Boeing 747. This device features a 9ft (2.7 m) primary mirror.
Deployment of infrared telescopes in space eliminates the problem of atmospheric absorption. The Infrared Astronomical Satellite (IRAS) was the first space telescope to survey the sky at infrared wavelengths. A more recent space-based IR telescope was the Wide-field Infrared Survey Explorer (WISE). It was launched in 2009 and remained in service until 2011. Space-based infrared telescopes typically have a short service life because the required coolants are quickly exhausted.
At the heart of each of these systems is an astronomical mirror used to gather the radiation and reflect it to an observation point. Various instruments can be used in observation, the most common being an infrared camera and solid-state infrared detectors. One of the greatest challenges in designing and implementing an infrared telescope is the cooling problem. The instrumentation must be cooled to within a few degrees of absolute zero. At warmer temperatures, radiation from the telescope itself interferes with observation.
The purpose of far-IR observation is to reveal that which can not be seen in visible light. Clouds of dust and gas in our own and neighboring galaxies emit far-IR radiation. Stellar nurseries can be detected by the heat of contracting protostars before they begin to emit visible light. Infrared telescopes extend our sight beyond visible wavelengths, just as radio telescopes extend it beyond the infrared.