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Commande haute performance des systèmes d’optique adaptative classique - des grands aux extrêmement grands télescopes (ELT)

Abstract : Astronomical Adaptive optics (AO) systems enable to compensate for the degradation induced by atmospheric turbulence on images recorder by ground-based telescopes. In classical AO, a Deformable Mirror (DM) compensates for the disturbance in real time, using measurements of wavefront deformation provided by a Wavefront Sensor (WFS). The most commonly used AO controller is a pure integral action regulator (integrator). AO systems feedback loops exhibit delays. In order to improve performance, model-based controllers have been proposed, enabling to predict the disturbance and compensate for the global delay. The models proposed in this PhD dissertation rely on the frozen flow assumption (Taylor hypothesis), where every atmospheric layer is viewed as the realization of a random field shifted by the wind. To develop high performance controllers, we seek to account for this hypothesis, which has the serious drawback of naturally leading to non spatially localized models.We focus on Linear Quadradic Gaussian (LQG) predictive controllers based on a state-space representation of the adaptive optics loop and relying on spatial and temporal priors on the turbulence (in particular the Cn2 and wind profiles). We propose new models which take into account the frozen flow hypothesis and are localized in the telescope pupil, where the turbulent phase is represented on a zonal basis (sampled in the pupil). These localized models include a maximum a posteriori extrapolation of phase points outside the telescope pupil. We also propose a so-called « LQG tiède » control, where the phase is estimated in a modal (Zernike) basis and the prediction is performed in a zonal basis. In all these developments, the dynamical disturbance models are autoregressive processes of order 1 (AR1) or of order 2 (AR2).A performance comparison is presented between the various LQG controllers proposed here (with multi-layer reconstruction or a reconstruction of the resulting phase, in a zonal or Zernike basis), the integrator and other model-based controllers (Distributed Kalman Filter, LQG based on an AR2 model in Zernike basis, zonal SA-LQG). Two application cases are considered: a classical AO astronomy case, with a VLT- NAOS type AO system, and a case of low-orbit satellite observation, where the turbulance dynamics are much stronger, due to the satellite motion. Accounting for frozen flow enables to increase performance by a few Strehl points in the VLT-NAOS case. For the satellite case, preliminary results show a dramatic improvement brought by the new zonal controllers over the integrator, with more than 40 Strehl points in some configurations.The AO systems of the next generation telescopes will feature dimensions far greater than for previously mentioned systems, with around 5000 actuators and 10000 WFS measurements for the Extremely Large Telescope (ELT) in classical AO mode. Looking for solutions suited for ELT dimensions allowed us to develop new sparse models in Karhunen-Loève basis, the identification of which is very fast. Performance comparison between those controllers, our new zonal controllers and an integral action regulator are conducted in a classical AO HARMONI-like case. Our so called « Lazy SA-LQG » zonal controller with oversampling of four points per subaperture and the LQG controller in Karhunen-Loève basis turn out to provide a very good balance between performance and complexity. New possibilities to design sparse basis adapted to state-space representations in ELT dimensions are discussed as perspectives for model-buiding in Karhunen-Loève basis.
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Submitted on : Tuesday, July 13, 2021 - 2:36:11 PM
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  • HAL Id : tel-03285570, version 1

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Léonard Prengere. Commande haute performance des systèmes d’optique adaptative classique - des grands aux extrêmement grands télescopes (ELT). Optique [physics.optics]. Université Paris-Saclay, 2021. Français. ⟨NNT : 2021UPAST028⟩. ⟨tel-03285570⟩

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