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High-contrast imaging of extrasolar planet candidates around a main-sequencestar has recently been realized from the ground using current adaptive optics(AO) systems. Advancing such observations will be a task for the Gemini PlanetImager, an upcoming "extreme" AO instrument. High-order "tweeter" and low-order"woofer" deformable mirrors (DMs) will supply a >90%-Strehl correction, aspecialized coronagraph will suppress the stellar flux, and any planets canthen be imaged in the "dark hole" region. Residual wavefront error scatterslight into the DM-controlled dark hole, making planets difficult to image abovethe noise. It is crucial in this regard that the high-density tweeter, amicro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deformto the shapes required by atmospheric turbulence. Laboratory experiments wereconducted to determine the rate and circumstance of saturation, i.e. strokeinsufficiency. A 1024-actuator 1.5-um-stroke MEMS device was empirically testedwith software Kolmogorov-turbulence screens of r_0=10-15cm. The MEMS whensolitary suffered saturation ~4% of the time. Simulating a woofer DM with ~5-10actuators across a 5-m primary mitigated MEMS saturation occurrence to afraction of a percent. While no adjacent actuators were saturated at opposingpositions, mid-to-high-spatial-frequency stroke did saturate more frequentlythan expected, implying that correlations through the influence functions areimportant. Analytical models underpredict the stroke requirements, so empiricalstudies are important.

Stroke saturation on a MEMS deformable mirror for woofer-tweeter adaptive optics