A support structure for a compliant deformable mirror

The Navy's ground-based optical interferometer requires 10 discrete reflections for each of its six stations that transport stellar radiation into a six-way beam combiner where the modulated beams are overlapped in a collinear fashion and fringes obtained for analysis. Wavefront aberrations, introduced at each reflection from non-perfect mirrors, reduce the quality of fringe contrast and adversely affect the final science results. In practice, mirror fabrication and mounting methods generate small surface irregularities that produce aberrations in the reflected wavefront beam. Under multiple reflection scenarios, these errors do not necessarily cancel one another, and can increase the resultant wavefront distortion. In a previous paper, we showed a single-force actuator acting on the back surface of an 8-inch diameter Zerodur® mirror will achieve a canceling deformation in the reflective surface that substantially reduces the combined wavefront aberrations resulting from a 7-reflection beam. Our finite element model demonstrated that the peak-to-valley difference can be reduced from 210 nm to 55 nm. In this paper, we extend our previous work to include a support structure to contain the deforming mirror and analyze its interaction and effect on the corrected wavefront. Our design used the mechanical advantage gained from a tuned flexure plate with a simple motorized screw actuator applied to the back mirror surface to achieve an 87:1 deflection ratio on the front mirror surface. A practical design is proposed, the support structure and mirror analyzed using the finite element method, and the results presented and discussed.