Flat mirror optics to study extra-solar terrestrial planets from space

NASA is planning the Terrestrial Planet Finder (TPF) mission to nd and study Earth-like planets of other stars. Its goal is to detect directly their thermal radiation, and it will have the spectroscopic sensitivity to nd strong atmospheric features, such as atmospheric water vapor (indicating oceans) and chemical signs of life, like Earth's biogenically-generated ozone. The TPF con guration currently under study is similar to ground based interferometers, with an array of afocal telescopes relaying beams to a central interferometric station where destructive interference of the starlight allows detection of the much fainter planetary signal. In this paper we consider a di erent implementation strategy which fully exploits the space environment. The primary optical elements are simply free ying at mirrors made from thin stretched membrane of gossamer weight. The constellation of ats would be viewed by a single wideeld telescope with aperture no bigger than one at, separately orbiting some distance away. Starlight directed from all the at elements into the viewing telescope would be combined at the focal plane by small-scale interferometer optics. At the size envisaged for TPF, four 4-m ats in a 100-m constellation would be viewed by a 4-m telescope several km distant. A more powerful interferometric successor, well suited for detailed atmospheric studies of the most promising planets found by TPF, would have a dozens of 8 m ats re ecting into a single, NGST-sized combining telescope. The use of at primary elements to study extra-solar planets is not limited to interferometry. True imaging telescopes with very large, nearlylled aperture can be made with a primary collector synthesized from many at segments, and the wavefront corrected at a scalloped tertiary mirror.