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infrared telescope(flux collector) A reflecting telescope used to detect and study infrared radiation from space; refractors cannot be employed because glass is opaque at these wavelengths. The mirror surface does not, however, need to be made to the same accuracy as one used for optical work because of the longer wavelengths of infrared radiation. Since neither the eye nor the photographic plate is sensitive to long-wavelength infrared radiation, special infrared detectors are required, such as photovoltaic and photoconductive detectors and bolometers. The detectors, and in some cases the telescope optics, must be cooled to liquid-helium or liquid-nitrogen temperatures to reduce thermal emission from the instruments themselves and hence noise. As an alternative to active cooling by cryogenic liquids, space telescopes can be allowed to cool radiatively, i.e. passively, by being shielded from direct sunlight.
Infrared telescopes can detect sources partly or totally obscured at optical wavelengths by interstellar dust, and can identify those that are true infrared sources rather than highly reddened ordinary stars. Because water vapor in the Earth's atmosphere absorbs infrared radiation (see atmospheric windows), the most advanced work is carried out with specially designed telescopes, such as the UK Infrared Telescope, at high-altitude observatories or with airborne or space observatories, such as KAO, IRAS, and ISO. Infrared telescopes can operate day and night.