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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. 1/20th 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.〔(【引用サイトリンク】publisher=harvard.edu )〕 ==Advantages== * A Fresnel imager with a sheet of a given size has vision just as sharp as a traditional telescope with a mirror of the same size, though it collects about 10% of the light.〔(【引用サイトリンク】publisher=science.slashdot.org )〕 * The use of vacuum for the individual subapertures eliminates phase defects and spectral limitations, which would result from the use of a transparent or reflective material.〔 * It can observe in the ultraviolet and infrared, in addition to visible light.〔 * It achieves images of high contrast, enabling observation of a very faint object in the close vicinity of a bright one.〔 * Since it is constructed using foil instead of mirrors, it is expected to be more lightweight, and therefore less expensive to launch, than a traditional telescope.〔(【引用サイトリンク】author=David Shiga )〕 * A 30-metre Fresnel imager would be powerful enough to see Earth-sized planets within 30 light years of Earth, and measure the planets' light spectrum to look for signs of life, such as atmospheric oxygen. The Fresnel imager could also measure the properties of very young galaxies in the distant universe and take detailed images of objects in our own solar system.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fresnel Imager」の詳細全文を読む スポンサード リンク
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