The need and desire for large-scale reflectors is immediate and long lasting. Therefore engineers and designers are turning toward processes that produce reflectors much different than the conventional ground glass mirror. This paradigm shift encompasses many new and emerging technologies, including, but not limited to, pressure-augmented stress-coated membrane mirrors.
Recent research has centered on determining the proper amount of stress (from the coating) to apply to a membrane substrate to produce a near-net shape that can be augmented with positive pressure to conclude in the smallest figure error. The bulge test has been applied to membrane samples of seven inch diameter, both uncoated and after coating, and central displacements used as data points when coupled with the finite element code ABAQUS to determine strain and stress values. These values are then correlated to the coating process to determine a 'coating prescription' by which that state of minimal figure error can be attained.
Vibration testing in vacuum also shows promise as an effective method to determine the amount of stress present in the coated membrane. The shifts in natural frequencies of a coated membrane versus its uncoated self are unique and indicative of the stress increase by the addition of the coating. These natural frequencies are input into theoretical and ABAQUS models to determine strain and stress. This method is used to provide confidence with the bulge test results.
Triton Systems Inc. has teamed with Virginia Tech to develop smart active materials for mirror shape control and stabilization systems utilized in thin film, space-based, polymeric mirrors. The development of novel lightweight space-qualified optics and support structures is of vital importance to science, industry, and national defense. Primary mirrors are one of the main drivers of the mass of space based optical systems. Therefore, lightweight optics is an essential component to reducing launch costs while increasing payload utility. Electroactive polymers represent a special class of “smart materials” whose electronic and physical properties such as conductivity, charge distribution, and shape can be changed in response to the environment (voltage and stress). The ability of electroactive polymers to change structure within a matrix in response to electrical stimulation has several applications for large ultra-lightweight optics. The Triton/VT team has begun the development of castable electroactive materials that do not depend on aqueous systems. These novel non-aqueous materials allow both an increase in voltage limits, and the ability to be used in environments where water would rapidly evaporate. These materials will allow the adjustment of the shape, to remove aberrations, as well as solve issues such as damping vibrations after re-pointing of large space telescopes.