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Characterization and mitigation of Laser—Guide—Star aberrations

Abstract 155

Submitted by Olivier LARDIÈRE


O. Lardière, R. Conan, C. Bradley, K. Jackson, P. Hampton


Adaptive Optics laboratory, Mechanical Engineering Department, University of Victoria


Sodium Laser Guide Stars (LGS) induce optical aberrations in adaptive optics (AO) systems.The artificial star is elongated due to the sodium layer thickness, and the variations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots.

In closed-loop AO systems, these errors generate spurious optical aberrations, termed LGS aberrations, especially with ELTs for which the spot elongation is greater. According to analytical models and experimental results obtained with the University of Victoria LGS bench demonstrator, we characterized the main LGS aberrations and present here two options to mitigate them.

For Centre-of-Gravity-based (CoG) Shack-Hartmann (SH) wavefront sensors (WFS), the main source of LGS aberrations is the threshold applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed "radial thresholding", is presented, canceling out most of the LGS aberrations without altering the centroid measurement accuracy.

The second option, applicable to any kind of LGS WFSs, is a temporal filtering of the LGS aberrations. This filtering requires a low bandwidth natural guide star (NGS) WFS in addition to the LGS WFS. A simple formalism and experimental results are presented for both CoG and Matched-Filter-based SH-WFSs.