Design of an active metal mirror for large space telescopes

Manufacturing telescopes with 4, 8 or 16 meter apertures is the most effective way to gather the light of faint exoplanets or look back towards the Big Bang. However, ultra-high optical quality large mirrors drastically increase the mass of such instruments - if made conventionally. Thinner, and hence lighter, primary mirrors suffer from gravity release, temperature changes and misalignment during launch. The resulting surface distortions as well as inherent surface errors which arise during manufacturing can be reduced by the implementation of active optics. We designed an active metal mirror as a key element for active optics in space. Our goal was to develop an ultra-stable, set-and-forget, lightweight active mirror with good wavefront correction performance. A simulation routine was developed to investigate the dependency between geometric parameters of the Deformable Mirror (DM) and the residual surface error after correction of Zernike modes. With the final 25 actuator mirror design we can achieve residual errors of less than 10 nm RMS for individual Zernike modes for an optical pupil of 103 mm diameter. It is able to withstand quasi-static launch loads, is insensitive to temperature changes and we can limit the overall weight to 2500 g including actuators and mirror mount.