Atmospheric turbulence profiling with multi-aperture scintillation of a Shack–Hartmann sensor
ABSTRACT
Adaptive optics (AO) systems that use tomographic estimation of the three-dimensional structure of atmospheric turbulence require the vertical atmospheric turbulence profile, which describes turbulence strength as a function of altitude as prior information. We propose a novel method to reconstruct the profile by applying a multi-aperture scintillation sensor (MASS) method to scintillation data obtained by a Shack–Hartmann wavefront sensor (SH-WFS). Compared with a traditional MASS, which uses atmospheric scintillation within four concentric annular apertures, the new method utilizes scintillation in several hundreds of spatial patterns, which are created by combinations of SH-WFS subapertures. Accuracy of the turbulence profile reconstruction is evaluated with Bayesian inference, and it is confirmed that the turbulence profile with more than 10 layers can be reconstructed because of the large number of constraints. We demonstrate the new method with a SH-WFS attached to the 51-cm telescope at Tohoku University and we confirm that the general characteristics of the atmospheric turbulence profile are reproduced.
1 INTRODUCTION
The fluctuation of the refractive index in the Earth’s atmosphere distorts the wavefront or equiphase surface of starlight and causes blurring of the stellar image. Adaptive optics (AO) systems realize diffraction-limited spatial resolution images with ground-based large aperture telescopes. In AO systems, by using a natural guide star (NGS) or an artificial laser guide star (LGS) as a reference source, the distortion of a wavefront is measured by a wavefront sensor (WFS) and corrected by a deformable mirror (DM) in a time-scale of ∼1 ms.
In the last decade, in order to improve the performance of AO systems that use a single LGS, which is affected by the cone effect (Tallon & Foy 1990) and angular anisoplanatism (Stone et al. 1994), AO systems using multiple LGSs and WFSs have been demonstrated or developed for the 8-m class of telescopes (e.g. Marchetti et al. 2007; Arsenault et al. 2012; Lardière et al. 2014; Rigaut et al. 2014; Minowa et al. 2017). These systems measure the wavefront distortion in several lines of sight and reconstruct the distortions optimized in the direction of science objects using tomographic estimation, aiming for AO correction of the lower wavefront error, with laser tomography adaptive optics (LTAO), or a wider field of view with multiconjugate adaptive optics (MCAO; Beckers 1988; Rigaut & Neichel 2018), ground-layer adaptive optics (GLAO; Rigaut 2002; Tokovinin 2004), multi-object adaptive optics (MOAO; Hammer et al. 2004; Vidal, Gendron & Rousset 2010). The tomographic turbulence estimation is essential technique for next-generation giant segmented mirror telescopes (GSMTs), which have 30-m class primary mirrors.
The tomographic estimation of the three-dimensional turbulence structure requires prior information of the strength of atmospheric turbulence as a function of altitude, which is called the atmospheric turbulence profile. A tomographic reconstruction matrix is computed from the positions of guide stars and the vertical atmospheric turbulence profile. Imperfect prior information of the turbulence profile causes tomographic error, which accounts for a large fraction of the total AO error budget (Gilles, Wang & Ellerbroek 2008). Because the atmospheric turbulence profile varies with time, the tomographic reconstruction matrix should be updated in a time-scale of tens of minutes, which corresponds to the typical time-scale of the profile time evolution (Gendron et al. 2014; Farley et al. 2020).
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