BRDF performance of highly polished Al6061T6 and impact on TMA performance

Aluminum telescopes are usually considered only for IR wavelengths, due to the inability of aluminum to be surfaced to the surface quality and roughness needed to minimize scatter at visible wavelengths. Yet Al6061T6, a material routinely specified into IR optical sensors due to its heritage, high yield strength, and fracture toughness, has been economically surfaced to equivalent mid and high spatial frequency error as glass surfaces typically specified into visible wavelength sensors. However, microroughness data can correlate poorly with scatter performance where microdefects dominate scatter. To show the performance of aluminum surfaces for visible wavelength applications, BRDF data is measured for low roughness Al6061T6 surfaces. The impact of the BRDF derived scatter for Al6061T6 is evaluated for a nominal TMA telescope.