Donald T. Gavel

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

High-contrast imaging of extrasolar planet candidates around a main-sequence star has recently been realized from the ground using current adaptive optics (AO) systems. Advancing such observations will be a task for the Gemini Planet Imager, an upcoming extreme AO instrument. High-order tweeter and low-order woofer deformable mirrors (DMs) will supply a >90%-Strehl correction, a specialized coronagraph will suppress the stellar flux, and any planets can then be imaged in the dark hole region. Residual wavefront error scatters light into the DM-controlled dark hole, making planets difficult to image above the noise. It is crucial in this regard that the high-density tweeter, a micro-electrical mechanical systems (MEMS) DM, have sufficient stroke to deform to the shapes required by atmospheric turbulence. Laboratory experiments were conducted to determine the rate and circumstance of saturation, i.e. stroke insufficiency. A 1024-actuator 1.5-microm-stroke MEMS device was empirically tested with software Kolmogorov-turbulence screens of r(0) =10-15 cm. The MEMS when solitary suffered saturation approximately 4% of the time. Simulating a woofer DM with approximately 5-10 actuators across a 5-m primary mitigated MEMS saturation occurrence to a fraction of a percent. While no adjacent actuators were saturated at opposing positions, mid-to-high-spatial-frequency stroke did saturate more frequently than expected, implying that correlations through the influence functions are important. Analytical models underpredict the stroke requirements, so empirical studies are important.

Laser guide star adaptive optics at Lick Observatory

We present an overview of the adaptive optics system at the Shane telescope (ShaneAO) along with research and development efforts on the technology and algorithms for that will advance AO into wider application for astronomy. Diffraction-limited imaging and spectroscopy from ground based large aperture telescopes will open up the opportunity for unprecedented science advancement. The AO challenges we are targeting are correction down to visible science wavelengths, which demands high-order wavefront correction, and dim object viewing over the whole sky, which demands bright artificial laser beacons. We discuss our ongoing development of MEMS based AO correction, woofer-tweeter architecture, wind-predictive wavefront control algorithms and a guide star laser tuned for optical pumping of the sodium layer. We present the latest on-sky results from the new AO system and present status and experimental plans for the optical pumping guide star laser.

Bulk Wind Estimator Performance for AO Systems

We use the Cramer-Rao lower bound to find that the error in a bulk wind estimator is dependent on both the signal-to-noise ratio at the wavefront sensor and the spatial frequency content of the wavefront.