Design of composite surrogate mirror support structure

Three multimaterial support structures were designed and built to support 2 00 mm (8 in.) diameter "surrogate" mirrors for the deployable optical teles cope (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 "p ultrusion" process, for their low thermal expansion coefficients, and alumi num for its machinability and low density. The composite rods were pultrude d at AFRL/VS, and their coefficients of thermal expansion (CTE) were determ ined by a laser interferometry method. Finite-element method (FEM) analysis of the vibration modes of the structure was conducted to determine the opt imum selection of materials. FEM analyses were also used to determine the m irror surface figure errors caused by +/-5 degreesC temperature fluctuation , as a function of the materials chosen. The paper discusses the rationale for the materials selection and design issues.