Coronagraphs are a promising solution for the next generation of exoplanet imaging instrumentation. While a coronagraph can have very good contrast and inner working angle performance, it is highly sensitive to optical aberrations. This necessitates a wavefront control system to correct aberrations within the telescope. The wavefront requirements and desired search area in a deformable mirror (DM) demand control of the electric field out to relatively high spatial frequencies. Conventional wisdom leads us to high stroke, high actuator density DMs that are capable of reaching these spatial frequencies on a single surface. Here we model a different architecture, where nearly every optical surface, powered or unpowered, is a controllable element. Rather than relying on one or two controllable surfaces for the success of the entire instrument the modeled instrument consists of a series of lower actuator count deformable mirrors to achieve the same result by leveraging the conjugate planes that exist in a coronagraphic instrument. To make such an instrument concept effective the imaging optics themselves must become precision deformable elements, akin to the deformable secondary mirrors at major telescope facilities. Such a DM does not exist commercially; all current DMs, while not necessarily incapable of carrying optical power, are manufactured with flat nominal surfaces. This simplifies control and manufacturing, but complicates their integration into an optical system because there is oftentimes a need to pack several into collimated space. Furthermore, high actuator count DMs cannot approximate low order shapes such as focus or tip-tilt without significant mid-spatial frequency residuals, which is not acceptable for a coronagraphic high-contrast imager. The ability to integrate the wavefront control system into the nominal coronagraphic optical train simplifies packaging, reduces cost and complexity, and increases optical throughput of any coronagraphic instrument. This adds redundancy, increases controllability of the complex aberrations, and mitigates both cost and risk associated with a single high-actuator count device that the entire instrument performance relies on. Here we simulate an optical system with a combination of controllable imaging optics both with and without a high order DM at the pupil. This example instrument is based loosely on the current DM technology being considered for the WFIRST CGI, and is merely an example of a larger trade study to be done to optimally balance actuator requirements, controllability, and wavefront quality. The relative performance of each configuration with regard to contrast, achievable bandwidth, and redundancy is discussed. The overall performance enhancements and risk associated with actuator failures on the assumed DM technology is also evaluated.
Deformable mirrors (DMs) are an enabling and mission-critical technology in any coronagraphic instrument designed to directly image exoplanets. A new ferro fluid deformable mirror technology for high-contrast imaging is currently under development at Princeton, featuring a flexible optical surface manipulated by the local electromagnetic and global hydraulic actuation of a reservoir of ferro fluid. The ferro fluid DM is designed to prioritize high optical surface quality, high-precision/low-stroke actuation, and excellent low-spatial-frequency performance - capabilities that meet the unique demands of high-contrast coronagraphy in a space-based platform. To this end, the ferro-fluid medium continuously supports the DM face sheet, a configuration that eliminates actuator print-through (or, quilting) by decoupling the nominal surface figure from the geometry of the actuator array. The global pressure control allows independent focus actuation. In this paper we describe an analytical model for the quasi-static deformation response of the DM face sheet to both magnetic and pressure actuation. These modeling efforts serve to identify the key design parameters and quantify their contributions to the DM response, model the relationship between actuation commands and DM surface-profile response, and predict performance metrics such as achievable spatial resolution and stroke precision for specific actuator configurations. Our theoretical approach addresses the complexity of the boundary conditions associated with mechanical mounting of the face sheet, and makes use of asymptotic approximations by leveraging the three distinct length scales in the problem - namely, the low-stroke (~nm) actuation, face sheet thickness (~mm), and mirror diameter (cm). In addition to describing the theoretical treatment, we report the progress of computational multi physics simulations which will be useful in improving the model fidelity and in drawing conclusions to improve the design.
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.