13 September 2012 Mitigation of transient meteor events in sodium layer by TMT NFIRAOS
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NFIRAOS Small meteors usually bum up near the bottom of the sodium layer. Meteor trails can lead to temporary dra­ matic changes in the altitude of the sodium layer. This altitude change is very rapid, typically over 1 second, and after some unpredictable period of 10-20 seconds, can transition back to the nominal mean altitude also in about 1 second. The altitude change is very drastic and can jump by up to 1 km which, on the face of it, would cause 4 micrometers defocus errors on LGS WFS measurements for a 30-m telescope, unless properly tracked. Measurements by the UBC Lidar detected 20 meteor trails I hour, and of these, 1-2 are significant events. We report on a full end-to-end Simulink simulation for TMT NFIRAOS including: meteor events measured by the UBC Lidar; on-instrument NGS focus sensor running at 90 Hz (median sky coverage frame rate); optimal temporal blending with LGS WFS focus measurements; LGS WFS centroiding matched filter update and Truth WFS update very 3s; full trombone servo model including non-linear focus range vs stage position. We optimized our control architecture and traded off motor power dissipation versus residual wavefront error and Shack-Hartmann spot displacement and found range tracking errors induce 12 nm WFE in normal conditions and brief (Is) jumps of 30-80 nm WFE at the beginning and ending of meteor transients.
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Glen Herriot, Craig Irvin, "Mitigation of transient meteor events in sodium layer by TMT NFIRAOS", Proc. SPIE 8447, Adaptive Optics Systems III, 844734 (13 September 2012); doi: 10.1117/12.924926; https://doi.org/10.1117/12.924926

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