Wavefront Control Strategies for Large Active Thin Shell Primaries with Unimorph Actuators

Abstract

This paper presents various aspects of the wavefront control strategies for an ultra-lightweight composite reflector made of polymers for the large primary of a space telescope, and the shape control is made by a set of patterned unimorph strain actuators attached to the reflector. It starts with an analytical investigation of the mechanical behaviors of a strain-actuated curved shell, resulting in the accurate prediction of typical features, such as the damped wave deformation at the transition between electrodes and the limited morphing amplitude of a “print-through” actuation, which indicates that the curvature-induced rigidity deteriorates the performances of the forming accuracy of the active reflector and the morphing stroke of the actuators. The morphing capabilities are evaluated with both petal-like segmented and monolithic configured reflectors by numerical tests on forming target shapes of Zernike modes with various patternings of electrodes, and the structural dynamics are examined. Finally, a compound control strategy is proposed, which uses a deformable relay mirror to compensate for the residual surface error corrected partially by the active unimorph primary mirror, showing a great relaxation of the shape error budget of the thin-shell primary, especially for on-axis observation.