Focal-plane Cn2(h) profiling based on single-conjugate adaptive optics compensated images.

Knowledge of the atmospheric turbulence in the telescope line-of-sight is crucial for wide-field observations assisted by adaptive optics (AO), particularly tomodel how the point spread function (PSF) elongates across the field of view (FOV) owing to the anisoplanatism effect. The extraction of key astronomical parameters accounts on an accurate representation of the PSF, which call for an accurate anisoplanatism characterisation. This one is, however, a function of the |$C_n^2(h)$| profile, which is not directly accessible from single-conjugate AO telemetry. It is possible to rely on external profilers, but recent studies have highlighted discrepancies of more than 10 per cent with AO internal measurements, while we aim at better than 1 per cent accuracy for PSF modelling. In order to tackle this limitation, we present focal-plane profiling (FPP) as a |$C_n^2(h)$| profiling method that relies on post-AO focal-plane images. We demonstrate that such an approach complies with a 1 per cent level of accuracy on the |$C_n^2(h)$| estimation and establish how this accuracy varies regarding the calibration star magnitudes and their positions in the field. We highlight the fact that photometry and astrometry errors caused by PSF mis-modelling reach respectively 1 per cent and 50 μas using FPP on a Keck baseline, with a preliminary calibration using a star of magnitude H = 14 at 20 arcsec. We validate this concept using Canada's NRC–Herzberg HeNOS testbed images by comparing FPP retrieval with alternative |$C_n^2(h)$| measurements on HeNOS. The FPP approach allows the |$C_n^2(h)$| to be profiled using the SCAO systems and significantly improves the PSF characterization. Such a methodology is also ELT-size-compliant and will be extrapolated to tomographic systems in the near future. [ABSTRACT FROM AUTHOR]