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Infrared Astronomy is the last part of the electromagnetic spectrum to be developed. Over much of this spectral range, stretching from the visible to ~ 300 microns, absorption by water vapour and other gases in the atmosphere precludes observations from the ground except in those spectral regions where the absorption is low and so called 'windows' exist, such as at 10 and 20 microns. No good windows exist at longer wave-lengths where astronomical bodies at temperatures of a few tens of degrees radiate most of their energy. So, for much of the infrared region it is important to be able to get above the atmosphere. This can be done in a number of ways. To date observations have been made from aeroplanes, ballons and rockets and plans are well advanced for an infrared survey satellite and the use of the space shuttle. Each type of vehicle has its own particular advantages and disadvantages. Thus the use of an aeroplane, for example NASA's C141 (the Kuiper Airborne Observatory) in which a 1 metre telescope has been mounted, is very convenient and is ideally suited to making broad band observations. However, it is at a disadvantage for high resolution observations due to the lines from the remnant water vapour in the atmosphere and here observations from a ballon borne telescope are preferable but somewhat more difficult to make. Rockets are good in as much as they get above essentially all the atmosphere but the observation time is short. The main driver for an infrared satellite, which is a joint Dutch/US/UK enterprise, is to make an all sky IR survey at wavelengths between 8 and 120 microns. There is no doubt that a comprehensive survey out to such long wavelengths is urgently needed. Another urgent requirement is to achieve high angular resolu-tion at the longer wavelengths and this is the purpose of mounting a 3m. dish onto the shuttle. The equivalent NASA proposal is for a somewhat smaller telescope cooled to cryogenic temperatures to reduce the photon noise and so achieve a higher detector sensitivity.
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