Design, Fabrication, and Validation of an Ultra-Lightweight Membrane Mirror

摘要

Large aperture optical quality primary mirrors have been developed which are extremely lightweight (areal densities less than 1kg/m~2) made from stretched reflective polymer membranes. However, aberrations induced by boundary support errors and pressurization of a flat membrane do not produce a perfect parabolic shape. Modeling studies have shown that active boundary control can be very effective in correcting certain types of figure errors typically seen in membrane mirrors. This paper validates these design studies by applying boundary control on a 0.25-meter pressure augmented membrane mirror (PAMM). The 0.25 meter PAMM was fabricated as a pathfinder for a larger prototype. A combination of displacement actuators and electrostatic force actuators were used to control the shape of the mirror. A varied thickness stress coating prescription was developed by a SRS/AFRL team using nonlinear membrane theory. Based on modeled data, the stress coating should force the membrane into a parabolic shape when pressurized, as opposed to a spherically aberrated shape characteristic of a pressurized flat membrane. Test data from the 0.25-meter PAMM proved that the varied thickness stress coating allows for a better shape than the uniform coating.