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Innovative Calibration Strategies for Large Adaptive Telescopes with Pyramid Wave-Front Sensors

Abstract : The new generation of Extremely Large Telescopes (ELT) will provide an optical resolution never achieved before for ground-based observation. However, in order to fully benefit from the potential of these telescopes, the scientific instruments will rely on complex Adaptive Optics Systems (AO) to correct for the optical aberrations due to the atmospheric turbulence. These AO instruments will all include Pyramid Wave-Front Sensor (PWFS) in their design as these WFS provide a gain in sensitivity with respect to the historical Shack-Hartmann WFS (SH). The cost of this gain in sensitivity comes with a higher operational complexity as the sensor exhibits a modal linearity and sensitivity that depends on both the seeing conditions and level of AO correction itself, the so-called optical gains of the PWFS. Coupled to this very first technical challenge, the future ELT will provide a constrained environment for the AO calibration with a large number of degrees of freedom, that will have to be calibrated often without any external calibration source and unprecedented distances between Deformable Mirror (DM) and AO instruments. This will induce differential motions and thus opto-mechanical conjugation errors between WFS and DM. Regular evolution of these so-called mis-registrations are then to be expected during the observations. They have to be monitored and compensated as they will highly affect the AO performance or lead to loop instability that will jeopardize the scientific observations. To address these operational constraints, we propose to consider a pseudo synthetic approach where calibration data are generated from a synthetic model, identifying the model parameters from experimental inputs. Such strategy is already used at the Adaptive Optics Facility working with SH-WFS. For PWFS, synthetic-based calibration have already been performed on several existing systems but a tracking of the mis-registration parameters during the operation is still to be investigated. As part of my PhD studies, I first developed a pseudo-synthetic model of the AO system of the Large Binocular Telescope that included the modelling of a PWFS and Adaptive Secondary Mirror. The purpose of this model was to generate a pseudo synthetic Interaction Matrix that could be used on the real system and identify the key-ingredients to efficiently model the PWFS. The model has been experimentally validated at the telescope and provided the same level of AO performances as a measured interaction matrix, demonstrating the high accuracy of the model. For this experiment, to tune the parameters of the model, we had access to a full interaction matrix measured at the telescope which will not be the case of the future ELT. The second part of my PhD was focused on optimizing the identification of the mis-registration parameters to allow a regular tracking of the parameters during the operation. We identified two strategies to provide an online tracking of the parameter. The first one is invasive and consists in dithering well selected signals with a low amplitude on the DM during the operations. This method appears to be robust to the different observing conditions and we demonstrated that the perturbation can be reduced to a few signals only, selected to maximize the sensitivity to the mis-registrations. This method has to be applied with the constraints of minimizing the impact on the scientific path that has to be carefully evaluated. Another strategy consists in accumulating enough telemetry data to retrieve a noisy interaction matrix that is used to give an estimation of the mis-registration parameters. This non invasive method appears to be attractive as no perturbation on the scientific path is required. The purpose of our research was to understand the physics that underpin the estimation of this noisy interaction matrix to identify the domain of validity of the method depending on the observing conditions, especially when considering its application with a PWFS.
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Submitted on : Monday, January 13, 2020 - 2:32:00 PM
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  • HAL Id : tel-02390861, version 3

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Cedric Taïssir Heritier. Innovative Calibration Strategies for Large Adaptive Telescopes with Pyramid Wave-Front Sensors. Instrumentation and Methods for Astrophysic [astro-ph.IM]. Aix Marseille Université, 2019. English. ⟨tel-02390861v3⟩

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