Minimizing high spatial frequency residual in active space telescope mirrors

Abstract

The trend in future space telescopes is towards large apertures and lightweight, rib-stiffened, and actively controlled deformable mirrors. These mirror architectures permit the development of segmented and deployed primary mirrors that lead to tremendous advancement in space telescope performance. Rib-stiffened and discretely actuated deformable mirrors have been shown to effectively mitigate common low order disturbances, but they are inevitably plagued by the "correction limit," or the extent to which the actuators can correct for a given shape disturbance. Improving the correctability of deformable mirrors requires understanding the origins of the correction limit, and optimizing the mirror design accordingly. This thesis details efforts to evaluate the mirror correction limit and the three predominant high spatial frequency mirror surface residual components: actuation-induced dimpling, manufacturing-induced print-through, and disturbance-induced uncorrectable error. The methods for simulating each effect are discussed, and an objective function is developed to quantify the effects of these residual components to gage the performance of each mirror design. A gradient descent algorithm is combined with the parametric capability of the Modular Optical Space Telescope (MOST) modeling tool to allow rapid trade space navigation and optimization of the mirror design across variations in mirror areal density, f-number, structural mass fractions, and rib aspect ratio. These optimization routines yield more advanced design heuristics that improve upon the simplified design techniques that are typical in industry. By forming the heuristics in terms of minimum machinable rib thickness, these new design relationships produce mirrors that satisfy manufacturing constraints and minimize uncorrectable high spatial frequency error.