Study of a Large Deformable Mirror Concept

University dissertation from Department of Astronomy and Theoretical Physics, Lund University

Abstract: It is attractive to integrate a large deformable mirror for adaptive optics into an astronomical telescope rather than using relay optics within an auxiliary instrument. However, the resulting large deformable mirror can be expensive, particularly for extremely large telescopes. We have pursued a low-cost approach using force actuators connected to the back of the deformable mirror through suction cups. This innovative concept for attachment of force actuators does not require high mechanical tolerances. Use of inexpensive voice-coil actuators and a thin mirror leads to a poorly damped system with many structural eigenfrequencies within the desired bandwidth. A feedback signal (in addition to the one from the wavefront sensor) is introduced by electro-mechanical sensors placed at the back of the deformable mirror. Using these sensors, stiffness and damping are added to the mirror through feedback loops. We introduce a local control concept with actuator families that have predetermined force patterns. Use of actuator families reduces crosstalk between adjacent actuators and prevents excitation of a number of low-order eigenmodes. This strategy can be seen as extending Saint-Venant`s principle beyond the static case. Thus, low-order eigenmodes are only weakly excited by actuation, leading to significant cost reduction for the sensors. The suggested sensors are of the electret microphone type. We present an integrated model of our suggested deformable mirror concept, which we use to demonstrate the controllability of the proposed first experimental laboratory setup. The experimental setup encompasses a partially illuminated large deformable mirror, where some force actuators are replaced by dummy actuators. From the experiment, key features, such as local control performance, dynamic range, controllability and robustness of the deformable mirror can be evaluated.

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