BACKGROUND LIMITED INFRARED AND SUBMILLIMETER INSTRUMENTS

The temperature and emissivity of infrared and submillimeter telescope s are basic parameters that drive the optical and thermal design of as tronomical space projects. They determine also, among other parameters , the self-emission of the instrument and the photon noise produced by this radiation on the detectors. By comparing the telescope brightnes s with that of the sky in the 1 mu m-1cm cm wavelength range, general conditions for background limited photometry are derived. For lambda < 0.4 mm, temperature is the driving parameter, and for lambda > 0.4 mm , temperature and emissivity have equivalent importances. It can be sh own on actual projects that these two regimes determine different opti cal and thermal concepts. Although based on a simplistic approach, thi s work intends to help designers to handle some basic system parameter s of infrared and submillimeter instruments.