Fresnel imager

rdf:langString Fresnel imager

rdf:langString A Fresnel imager is a proposed ultra-lightweight design for a space telescope that uses a Fresnel array as primary optics instead of a typical lens. It focuses light with a thin opaque foil sheet punched with specially shaped holes, thus focusing light on a certain point by using the phenomenon of diffraction. Such patterned sheets, called Fresnel zone plates, have long been used for focusing laser beams, but have so far not been used for astronomy. No optical material is involved in the focusing process as in traditional telescopes. Rather, the light collected by the Fresnel array is concentrated on smaller classical optics (e.g. 1/20 of the array size), to form a final image. The long focal lengths of the Fresnel imager (a few kilometers) require operation by two-vessel formation flying in space at the L2 Sun-Earth Lagrangian point. In this two-spacecraft formation-flying instrument, one spacecraft holds the focussing element: the Fresnel interferometric array; the other spacecraft holds the field optics, focal instrumentation, and detectors.