The Compound Parabolic/elliptic Lightshades: Near-Optimal Shading for Cold Radiators 2000-01-2278
This paper presents the Compound Parabolic Lightshade geometry as a candidate for shading of cold passive radiators in space. Use of solid-state detectors is increasing for space-based scientific observation of the earth and in astronomy. Many detectors require cooling to reduce background noise and damage from the radiation environment in space, with target temperatures anywhere from 270°K down to 100°K or lower. A passive radiator system is a desirable choice for detector cooling, but parasitic thermal loads from the earth, sun and nearby surfaces are a design challenge, especially at lower detector system temperatures. Conical or trapezoidal geometries are often used for shades against direct solar and earth thermal radiation.
The Compound Parabolic (CP) geometry, originally developed for non-imaging concentrators of low-level electromagnetic radiation, may also be used as a near-optimal shade for a cold radiator. Its advantages include complete rejection and sharp cutoff of all radiation entering the shade beyond a specific acceptance angle from the axis, and rejection of almost all parasitic radiation originating from the shade interior surface except by a direct view. The paper presents design principles for two- and three-dimensional CP geometry, and compares its shading performance both qualitatively and quantitatively with that of other shade geometries, including the closely related Compound Elliptical (CE) shape. Effects of fabrication errors in the CP geometry and in modeling approximations are quantified by ray-trace results. Finally, successful flight experience from CP shades on the Submillimeter-Wave Astronomy Satellite and analysis results from the CE shade on the Meteosat Second Generation Satellite are discussed.