Design of Composite Surrogate Mirror Support Structure

Three multimaterial support structures were designed and built to support 200 mm (8 in.) diameter "surrogate" mirrors for the deployable optical telescope (DOT) ground experiment at the Air Force Research Laboratory (AFRL/VS). A "spider-truss" structure was selected for its stiffness. Materials used included "Invar" alloy and AS4-epoxy composite rods manufactured by the "pultrusion" process, for their low thermal expansion coefficients, and aluminum for its machinability and low density. The composite rods were pultruded at AFRL/VS, and their coefficients of thermal expansion (CTE) were determined by a laser interferometry method. Finite-element method (FEM) analysis of the vibration modes of the structure was conducted to determine the optimum selection of materials. FEM analyses were also used to determine the mirror surface figure errors caused by ±5°C temperature fluctuation, as a function of the materials chosen. The paper discusses the rationale for the materials selection and design issues.