Showing papers by "Katie M. Morzinski published in 2008"

Characterizing MEMS deformable mirrors for open-loop operation: High-resolution measurements of thin-plate behavior

Abstract: New concepts for astronomical adaptive optics are enabled by use of micro-electrical mechanical systems (MEMS) deformable mirrors (DMs). Unlike traditional DMs now used in astronomical AO systems, MEMS devices are smaller, less expensive, and exhibit extraordinarily repeatable actuation. Consequently, MEMS technology allows for novel configurations, such as multi-object AO, that require open-loop control of multiple DMs. At the UCO/Lick Observatory Laboratory for Adaptive Optics we are pursuing this concept in part by creating a phaseto- voltage model for the MEMS DM. We model the surface deflection approximately by the thin-plate equation. Using this modeling technique, we have achieved open-loop control accuracy in the laboratory to ~13-30 nm surface rms in response to ~1-3 μm peak-to-valley commands, respectively. Next, high-resolution measurements of the displacement between actuator posts are compared to the homogeneous solution of the thin-plate equation, to verify the model's validity. These measurements show that the thin-plate equation seems a plausible approach to modeling deformations of the top surface down to lateral scales of a tenth actuator spacing. Finally, in order to determine the physical lower limit to which our model can be expected to be accurate, we conducted a set of hysteresis experiments with a MEMS. We detect only a sub-nanometer amount of hysteresis of 0.6±0.3 nm surface over a 160-volt loop. This complements our previous stability and position repeatability measurements, showing that MEMS DMs actuate to sub-nanometer precision and are hence controllable in open-loop.

ViLLaGEs: Opto-Mechanical Design of an on-sky visible-light MEMS-based AO system

Abstract: Visible Light Laser Guidestar Experiments (ViLLaGEs) is a new Micro-Electro Mechanical Systems (MEMS) based visible-wavelength adaptive optics (AO) testbed on the Nickel 1-meter telescope at Lick Observatory. Closed loop Natural Guide Star (NGS) experiments were successfully carried out during engineering during the fall of 2007. This is a major evolutionary step, signaling the movement of AO technologies into visible light with a MEMS mirror. With on-sky Strehls in I-band of greater than 20% during second light tests, the science possibilities have become evident. Described here is the advanced engineering used in the design and construction of the ViLLaGEs system, comparing it to the LickAO infrared system, and a discussion of Nickel dome infrastructural improvements necessary for this system. A significant portion of the engineering discussion revolves around the sizable effort that went towards eliminating flexure. Then, we detail upgrades to ViLLaGEs to make it a facility class instrument. These upgrades will focus on Nyquist sampling the diffraction limited point spread function during open loop operations, motorization and automation for technician level alignments, adding dithering capabilities and changes for near infrared science.

Empirical measurement of MEMS stroke saturation, with implications for woofer-tweeter architectures

Abstract: The Gemini Planet Imager (GPI) is currently in production for the Gemini Telescope in Chile. This instrument will directly image young jovian exoplanets, aided by a micro-electrical mechanical systems (MEMS) deformable mirror (DM). Boston Micromachines MEMS mirrors operate thousands of actuators to provide a well-sampled correction at high spatial frequencies. However, because MEMS stroke alone is insufficient to fully correct the atmosphere in the near-IR on an 8-meter telescope, a dual-mirror system is planned for GPI: The MEMS is used as a 'tweeter' to correct the higher spatial frequencies while a separate 'woofer' DM will be used to correct the lower frequencies. During operation at GPI, any saturated actuators would scatter starlight into the dark hole instead of allowing it to be removed coronagraphically; thus, stroke saturation on the MEMS is tolerated only at the 5-sigma level. In the Laboratory for Adaptive Optics, we test the ability of the MEMS to counter atmospheric turbulence. The MEMS shape is set to random iterations of woofer-corrected Kolmogorov phase screens with varying woofer sizes. We find that, for r0 = 10 cm, saturation decreases from several percent to a few tenths of a percent (∼3-sigma) when using a 100cm-pitch woofer. The MEMS we tested has 0.2 mm inter-actuator stroke for a 200V-range. Nonetheless, saturation (when it occurs) appears to be due to low-order peak-to-valley stroke even in the woofer-corrected case. Gemini characteristically has r0 = 15 cm, so future work includes extrapolating to find where the 5-sigma saturation level occurs.

Application of Hartmann linear calibrations to ViLLaGEs

Abstract: We attempt to linearize the output of the Shack-Hartmann wavefront sensor in the ViLLaGEs instrument ViLLaGEs (Visible Light Laser Guidestar Experiments) is a MEMS-based Adaptive Optics system on the 1 - meter Nickel telescope at Lick Observatory meant to provide correction at visible wavelengths with a 9x9 subaperture Hartmann sensor We estimate that the open-loop accuracy of ViLLaGEs is ~40 nm We "calibrate" the Hartmann linearity by raster scanning a tip/tilt mirror downstream of an internal fiber and inverting the resulting signal, forming a lookup table of unbiased tilts From this calibration, we conclude that nonlinearity is a minor effect in the open-loop operation of ViLLaGEs, on the order of ~15 nm We show through simulations of Shack-Hartmann sensors that this error is likely due to an internal pupil mask not physically conjugate to the telescope pupil We test the resulting lookup table on an internal "turbulator" in ViLLaGEs, or a rotating plate meant to simulate the wind-driven atmosphere, and find that the Strehls with and without the lookup table are indistinguishable