The function of the cryogenic limb-scanning interferometer/radiometer (CLIR) is to observe infrared emissions of the earth's upper atmosphere from space. The earth's surface is an extended source of intense background radiation with a small angular separation from the desired scene. The CLIR employs an off-axis Gregorian telescope whose primary mirror and baffles are cooled by an open-cycle cryogenic system. A system of specular annular baffles has been designed to minimize both stray light problems and cryogen consumption by retromapping the aperture into itself. Off-axis rays which enter the aperture and strike the baffles are reflected so that they pass out of the aperture again and are not absorbed on cryogenic surfaces. The edge ray principle is used to configure the specular baffle surfaces. The baffle which lies closest to the aperture is an ellipsoid whose foci trace out the circular aperture on revolution about the axis. Its theoretical "ray trace" efficiency is 100 percent. A subsequent baffle has an elliptical cross section whose near focus traces out the central hole in the ellipsoidal baffle and whose far focus traces out the aperture. Its theoretical efficiency is about 90 percent. These baffles reduce the earth radiation heat load on the cryogenic cooler by an order of magnitude, changing it from the dominant cause of cryogen consumption to a relatively small effect. An aperture shield is also desirable to reduce cryogen consumption, stray light, and contamination.
James C. Bremer,
"Baffle Design For Earth Radiation Rejection In The Cryogenic Limb-Scanning Interferometer/Radiometer," Optical Engineering 22(1), 221166 (1 February 1983). https://doi.org/10.1117/12.7973068