ELT-scale elongated LGS wavefront sensing: on-sky results

Context. Laser guide stars (LGS) allow adaptive optics (AO) systems to reach greater sky coverage, especially for AO correcting the atmospheric turbulence on large fields of view. However LGS suffer from limitations, among which is their apparent elongation can 20 arcsec when observed with aperture telescopes such as European Southern Observatory 39 m telescope. The consequences this extreme have been studied in simulations and laboratory experiments, although never on-sky, yet understanding mitigating those effects key taking full advantage Extremely Large Telescope (ELT) six LGS. Aims. In paper we study impact wavefront sensing an ELT-scale elongated using on-sky data obtained demonstrator CANARY William Herschel telescope (WHT) ESO Wendelstein unit. simultaneously a natural star superimposed launched placed 40 away WHT pupil. Methods. Comparison wavefronts measured each allows determination error breakdown sensing. With breakdown, isolate contribution its impact. We also investigate truncation or undersampling spots. Results. successfully used sensor (WFS) drive loop during operations, but it necessitated regular calibrations non-common path aberrations WFS arm. off-line processing collected separate term encapsulating dynamic quasi-static component. measure errors varying 0 nm 160 rms are able link strength spot elongation. significant vary between 200 depending conditions. They increase by much 70 over course 10 m. do not observe any spots pixel scales 1.95″, while width half maximum varies 1.7″ 2.2″; however, appear truncating These view smaller than 10.4″ 15.6″, spots’ elongations. Translated ELT observing at zenith, elongations long 23.5″ must be accommodated, corresponding field 16.3″ if most put across diagonal subaperture.