Stretched membrane with electrostatic curvature (SMEC): A new technology for ultra-lightweight space telescopes

Very large space telescopes with primary mirrors made of flat segments have been recently proposed. The segments would be extremely lightweight, made like pellicles from stretched, reflective membranes. Here we consider the use of such membrane primary mirrors in which slight concave curvature is induced by electrostatic force, by application of a potential difference between the membrane and a control electrode behind. In this way segmented spherical or paraboloidal primaries of long focal length can be made directly, eliminating the correction optics needed when flat segments are used. The electric potential would be spatially and temporally controlled to obtain uniform curvature despite non-uniformity in membrane tension, to create slight asphericity if needed and to provide active damping of vibrations. We report the operation of a small prototype telescope with a SMEC primary. A design for a 3.2-m space prototype is described, based on the two-mirror anastigmat of Schwartzschild and Couder. A 400 m radius primary is combined with a single concave secondary flying in formation at a distance of 190 m. Together they form a wide-field, well-corrected focus at f/30. A larger telescope with much higher diffraction limited resolution could be made simply by adding more segments of the 27 m diameter parent primary to the formation. The TPF nulling interferomer could be made with similarly sized elements in a 100 m formation, configured as segments of a kilometer focal length parent. A very large SMEC telescope might be made from a continuous 10×1000 m membrane, rolled up for launch. Cryogenic cooling for all these configurations would be accomplished by additional spacecraft in the formation to block direct solar illumination of the telescope elements.