Abstract: Calibration is provided for a diffuser panel (30) on board a spacecraft (10) employed in measuring the intensity of sunlight reflected from subjects on the earth. The calibration is accomplished by comparing the intensity of solar radiation reflected from the panel with the intensity of the solar radiation incident directly from the sun. The comparison is obtained by directing both radiation into an integrating sphere (60) through separate ports (62, 64) which are sized to admit substantially equal amounts of power of the reflected and the direct radiations. A detector (76) detects an average value of intensity of the reflected radiation while the direct rays are excluded by a shutter (94). Upon an opening of the shutter and a deflecting of the diffuser panel from the calibrating position, the detector detects the average value of radiation intensity from the direct rays of the sun.

Abstract: An optical spectrometer system has a central axis, a detector having a detector surface disposed at one end of the system and located at a focal plane of the optical spectrometer system. A cold stop is associated with the detector for permitting entrance of target source radiation onto the detector surface while blocking background radiation from surroundings. A cooling device is coupled to the detector for cooling it and the cold stop to a predetermined low temperature. A flat plate is provided and has an spectrometer slit formed in it, the plate having a flat surface facing the detector, the flat surface being coated with a highly reflective, low emissivity material. Fore-optic focusing lenses or mirrors are located in front of the flat plate for focusing radiation onto the slit on the flat plate between the fore-optic focusing and the collimating lenses. The collimating lenses between the flat plate and the cold stop collimate the light from the slit and form a pupil at the cold stop.