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Description
Laser Guide Stars (LGS) allow Adaptive Optics (AO) systems to reach higher sky coverage and correct the atmospheric turbulence on wider field of views. However LGS suffer from limitations, among which is their apparent elongation which can reach 20 arcseconds when observed with large aperture telescopes such as the European Southern Observatory's 39m telescope. The consequences of these extreme elongations have been studied in simulations and lab experiments, but never on-sky. Yet understanding and mitigating those effects is key to taking full advantage of the Extremely Large Telescope (ELT) six LGS. In this paper, we study the impact of wavefront sensing with an ELT-scale elongated LGS using data obtained on-sky with the AO demonstrator CANARY on the William Herschel telescope (WHT) and ESO's Wendelstein LGS unit. CANARY observed simultaneously a natural guide star and a superimposed LGS launched from a telescope placed 40 m away from the WHT pupil. Comparison of the wavefronts measured with each guide star allows to build an error breakdown of the elongated LGS wavefront sensing. With this error breakdown, we isolate the contribution of the LGS elongation and study its impact. We also investigate the effects of truncating or undersampling the LGS spots. We successfully used the elongated LGS wavefront sensor (WFS) to drive the AO loop during on-sky operations, but it necessitated regular calibrations of the non-common path aberrations on the LGS WFS arm. In the off-line processing of the data collected on-sky, we separate the error term encapsulating the impact of LGS elongation in a dynamic and quasi-static component. We measure errors varying from 0 to 160nm rms for the dynamic error and are able to link it with turbulence strength and spot elongation. The quasi-static errors are significant and vary between 20 to 200nm rms depending on the conditions. They also increase by as much as 70nm in the course of 10 min. We do not observe any impact when undersampling the spots with pixel scales as large as 1.95 arcseconds but significant errors appear when truncating the spots. These errors appear for field of views smaller than 10.4 to 15.6 arcseconds, depending on the spots elongations. Translated to the ELT observing at zenith, elongations as long as 23.5 arcseconds must be accommodated, corresponding to a field of view of 16.3 arcseconds if the most elongated spots are put in the diagonal of the subaperture.
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