Radial thresholding to mitigate Laser-Guide-Star aberrations on Centre-of-Gravity-based Shack-Hartmann wavefront sensors

TL;DR

This paper introduces radial thresholding, a novel method to reduce laser guide star aberrations caused by sodium layer fluctuations, improving the accuracy of Shack-Hartmann wavefront sensors in adaptive optics.

Contribution

The paper proposes a new radial thresholding technique that mitigates LGS aberrations without compromising centroid measurement accuracy.

Findings

Radial thresholding significantly reduces LGS aberrations.

Experimental results confirm improved wavefront sensing accuracy.

The method is effective across various sodium layer conditions.

Abstract

Sodium Laser Guide Stars (LGSs) are elongated sources due to the thickness and the finite distance of the sodium layer. The fluctuations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots, and generate spurious optical aberrations in closed--loop adaptive optics (AO) systems. According to an analytical model and experimental results obtained with the University of Victoria LGS bench demonstrator, one of the main origins of these aberrations, referred to as LGS aberrations, is not the Centre-of-Gravity (CoG) algorithm itself, but the thresholding applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed ``radial thresholding'', is presented here, cancelling out most of the LGS aberrations without altering the centroid measurement accuracy.