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The success of a space-borne direct-imaging mission pursuing earth-sized exoplanets in the habitable zone hinges on the ability to achieve high contrast over a maximum field of view. Coronagraphic instruments designed to address this challenge suffer from optical aberrations and rely on focal-plane wavefront control to suppress the resulting speckles and widen the search area. Even small-featured quasi-static speckles--which may obscure or be confused with a planet--must be suppressed to the order of 10-10 over the search region, placing extreme demands on the deformable mirrors (DMs) used to implement the closed-loop control, both in wavefront requirements and actuation resolution. The ideal DM for focal-plane wavefront control has high surface quality and is capable of high-precision, low-stroke actuation. Conventional mirror technologies such as MEMS DMs, with heritage in ground-based adaptive optics instruments that correct for dynamic atmosphere-induced aberrations, are nominally at and provide high-stroke, high-resolution control but at a cost of precision and surface quality. We present a new technology currently under development at Princeton, which features a ferrofluid-supported optical surface with local magnetic actuation. The actuation is transferred to the optical surface through a liquid medium which continuously supports it, decoupling the nominal surface profile from the actuator configuration and eliminating quilting. Additionally, the device carries tunable nominal optical power via regulation of the ferrofluid pressure, permitting a degree of high-fidelity low-order wavefront control impossible with current instrumentation. We report on the continuing technological growth of the prototype device, including progress with actuation, metrology, and modeling of the DM response.
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