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Owen Brown

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Proceedings Article | 13 December 2020 Poster + Paper

Fast modulation and dithering for the NFIRAOS pyramid wavefront sensor

Edward Chapin, David Andersen, Owen Brown, Jeffrey Crane, Adam Densmore, Jennifer Dunn, Tim Hardy, Glen Herriot, Dan Kerley, Olivier Lardiere, Jean-Pierre Véran

Proceedings Volume 11451, 114514V (2020) https://doi.org/10.1117/12.2560418

KEYWORDS: Wavefront sensors, Modulation, Thirty Meter Telescope, Mirrors, Sensors, Adaptive optics, Stars, Optical testing, Phase shift keying, Laser guide stars

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The Narrow Field InfraRed Adaptive Optics System (NFIRAOS) for the Thirty Meter Telescope (TMT) will use a natural guide star (NGS) Pyramid Wavefront Sensor (PWFS). A 32-mm diameter Fast Steering Mirror (FSM) is used to modulate the position of the NGS image around the tip of the pyramid. The mirror traces out a circular tip/tilt pattern at up to 800 Hz (the maximum operating frequency of NFIRAOS), with a diameter chosen to balance sensitivity and dynamic range. A circular dither pattern at 1/4 the modulation frequency is superimposed to facilitate optical gain measurements. The timing of this motion is synchronized precisely with individual exposures on the PWFS detector, and must also be phased with other wavefront sensors, such as Laser Guide Star Wavefront Sensors (LGSWFS) and the On-Instrument Wavefront Sensors (OIWFS) of NFIRAOS client instruments (depending on the observing mode), to minimize latency. During trade studies it was decided to pursue a piezo actuator from Physik Instrumente (PI) using a monocrystalline piezo material, as more conventional polycrystalline devices would not meet the lifetime, stroke, and frequency requirements. Furthermore, PI claims excellent stability and hysteresis with similar piezo stages, rendering sensor feedback unnecessary. To characterize the performance of this mechanism, and to verify that it can function acceptably in open-loop, we have operated the stage on a test bench using a laser and high-speed position sensing devices (PSDs) both at room temperature and at the cold -30C operating temperature of NFIRAOS. We have also prototyped the software and hardware triggering strategy that will be used to synchronize the FSM with the rest of NFIRAOS.

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