Scalable, cost-effective, high-actuator-count deformable mirrors for astronomical adaptive optics

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Over the past decade, adaptive optics (AO) has become indispensible as a means to compensate for aberrations introduced by atmospheric turbulence in large ground-based telescopes. The resulting gains in resolution are especially important for large telescopes. Indeed, because of their greater aperture, large ground-based telescopes have exceeded in narrow fields the fidelity possible with orbiting observatories and have led to exciting recent advances in the observation of exoplanets, characterization of planetary rings and atmospheres, and studies of galactic structure. The universal optical component that allows for this wavefront compensation is a deformable mirror (DM), which must operate at frequencies above 1 kHz and whose actuator count scales with the area of the telescope primary mirror. Indeed, DMs with tens of thousands of actuators are required for planned extremely large telescopes (ELTs) - those with apertures greater than ~20-m. The technology to make high-actuator density DMs with high production yield and therefore low cost does not exist today. A main objective of work planned by Dr. T. Bifano of Boston University is to develop such manufacturing technologies using a microelectromechanical systems (MEMS) approach. A critical failure mode of current large-format DMs has been the thousands of fragile electrical traces that route signals along the front surface from bonding pads located near the periphery of the module. This problem worsens rapidly as the actuator count increases, culminating in a yield of less than 1% for the latest attempts at constructing a high-count DM for the Gemini Planet Imager (GPI). Dr. Bifano's plans to circumvent this problem involve replacing the dense network of surface traces with through-wafer interconnects and bonding to a backside package. While this basic technique is not unprecedented in modern electronics manufacturing, technical challenges include the very large voltages (~250V) that must be endured by the silicon substrate to achieve the required DM stroke of ~3.5 microns. However, the payoff is large, since the availability of reliable high-actuator-count DMs would catalyze advances in promising imaging techniques such as Multi-Object AO and Extreme AO. An outcome of the proposed project will be fully functional MEMS DM prototypes with 2048 actuators, evaluated at a leading astronomical AO test bed. Funding for development high actuator count DMs for next-generation AO is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.