Search

International audience; In this paper, we present the design and prototyping of the HARMONI Adaptive Optics Calibration Unit (AOCU). The AOCU consists of a set of on-axis sources (covering 0.5-2.4 µm) with a controllable wavefront shape. It will deploy into the instrument focal plane to inject calibration light into the rest of the system. The AOCU supports all-natural guide-star wavefront sensors for SCAO, HCAO, and LTAO. The AOCU will be used to calibrate the WFSs, the internal interaction matrices of HARMONI, measure and compensate NCPAs between AO dichroics and the science detectors, and calibrate the pointing model zero position. The illumination assembly of the AOCU will consist of six diffraction-limited sources and a resolved source coupled into fibres. Because of the wide range of wavelengths and the spatial separations requirements, we use two endlessly single-mode fibres and a multimode fibre. In addition, several LED sources need to be coupled efficiently into the single-mode fibres. In this paper, we present the general AOCU design using off-the-shelf with a focus on the illumination and source module.

International audience; HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics (AO) modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for Final Design Reviews.The SCAO system for HARMONI is based on a pyramid wavefront sensor (PWFS) operating in the visible (700 – 1000 nm). Previous implementations on very large telescopes have demonstrated the challenges associated with optimising PWFS performance on-sky, particularly when operated at visible wavelengths. ELT operation will pose further challenges for AO systems, particularly related to the segmentation of the telescope and the control of badly seen ‘petal modes’. In this paper we investigate these challenges in the context of the HARMONI SCAO system. We present the results of end-to-end simulations of our baseline approach, using a coupled control basis to avoid the runaway development of petal modes in the control loop. The impact of key parameters are investigated and methods for optical gain compensation and optimisation of the control basis are presented. We discuss recent updates to the control algorithms and demonstrate the possibility of improving performance using a form of super resolution. Finally, we report on the expected performance across a range of conditions.

International audience; This paper describes the outcomes of a study funded by the European Space Agency aimed at identifying the technical challenges and trade-offs at the system level, performing preliminary designs of an active correction loop for large deployable telescopes, and defining technological roadmaps for the development of the active correction loop for the selected designs. This study has targeted two very different application cases, one for High Resolution Earth Observation from Geostationary orbit (called GeoHR, with a 4m diameter entrance pupil) and one for Science missions requiring very large telescopes (with a up to 18 m diameter entrance pupil) with high-contrast imaging capabilities for exo-Earth observations and characterization. For both application cases, this paper first summarizes the mission objectives and constraints that have influence on the telescope designs. It then presents the high-level trade-offs that have been led and the optical and mechanical design that have been developed, including the deployable aspects. Finally, the performance assessment is presented, and is the basis for the justification of an active optics correction chain, with a preliminary set of requirements for typical components of the system. The presentation is concluded with proposed technological roadmaps that aim to allow the development of the building blocks on which the next generation instruments will be able to rely on.

HARMONI is the adaptive optics assisted, near-infrared and visible light integral field spectrograph for the Extremely Large Telescope (ELT). A first light instrument, it provides the work-horse spectroscopic capability for the ELT. As the project approaches its Final Design Review milestone, the design of the instrument is being finalized, and the plans for assembly, integration and testing are being detailed. We present an overview of the instrument's capabilities from a user perspective, provide a summary of the instrument's design, including plans for operations and calibrations, and provide a brief glimpse of the predicted performance for a specific observing scenario. The paper also provides some details of the consortium composition and its evolution since the project commenced in 2015.

International audience; The volume available on-board small satellites limit the optical aperture to a few centimetres, which limits the Ground-Sampling Distance (GSD) in the visible to approximately 3 m at 500 km. We present the latest development of a laboratory demonstrator for a deployable telescope that will triple the achievable ground resolution and quadruple the photometric capability from a CubeSat imager. In this paper, we present the overall opto-mechanical design of a Cassegrain telescope with a segmented primary mirror with a 30 cm baseline. The segments are folded for launch and unfold in space. To enable diffraction-limited imaging, piston, tip, and tilt (PTT) on each of the mirror segments should be below 12 nm RMS. The key challenge is to ensure phasing, and this precision level will require an active phasing stage. We present laboratory results of deployment and active phasing of the primary mirror segments. The initial deployment is performed using shaped memory alloy that deploy mirror segments. We demonstrate a repeatability below ±4.5 µm, enabling the four PSFs (one for each mirror segment) to be imaged on the detector simultaneously. An alignment step using compact and calibrated capacitive sensors allows for a control of the mirror positions in PTT below the wavelength. Finally, we investigate the sensitivity of misalignments of a deployable secondary mirror and show that it is well within reach of the technology developed in this study.

International audience; Context. Extremely Large Telescopes have chosen the Pyramid wavefront sensor (PyWFS) over the more widely used Shack-Hartmann WaveFront Sensor (SHWFS) to perform their Single Conjugate Adaptive Optics (SCAO) mode. The PyWFS is a Fourier-filtering based sensor, which has proven to be strongly efficient for astronomical purposes. However, it shows non-linearity behaviors that lead to a reduction of its sensitivity when working around a non-null phase. This effect, called Optical Gains (OG), degrades the performance of the closed loop and prevents accurate correction of Non-Common-Path Aberrations (NCPA). Aims. We aim at computing these so-called OG with a fast and agile technique in order to control the PyWFS measurements for adaptive optics closed-loop systems. Methods. Thanks to a new theoretical description of the PyFWS, which uses a convolutional model to describe the sensor, we analytically predict the behavior of the PyWFS in closed-loop operation. This model allows us to explore the impact of residual phases on the properties of the PyFWS measurements in terms of sensitivity, and associated OG. The proposed method relies on the knowledge of the residual phase statistics and allows to automatically estimate the current OG. End-to-End numerical simulations are used to validate our predictions and test the relevance of our approach. Results. We show that an accurate estimation of the OGs is possible by only using the AO telemetry data to derive statistical information of the turbulence. The method is then fully non-invasive. We further show that by only having an estimation of the current Fried parameter r 0 and the basic system characteristics, OGs can be estimated within 10% accuracy. The proposed method applies to Pyramid WFS, but also to any Fourier-Filtering WFS suffering from OG variations.; Le contexte. Les télescopes extrêmement grands ont choisi le capteur de front d'onde Pyramid (PyWFS) plutôt que le capteur Shack-HartmannWaveFront (SHWFS) plus largement utilisé pour effectuer leur mode SCAO (Single Conjugate Adaptive Optics). Le PyWFS est un capteur basé sur un filtrage de Fourier, qui s'est avéré très efficace à des fins astronomiques. Cependant, il montre des comportements de non-linéarité qui conduisent à une réduction de sa sensibilité lors du travail autour d'une phase non nulle. Cet effet, appelé Gains optiques (OG), dégrade les performances de la boucle fermée et empêche la correction précise des aberrations non communes (NCPA). Objectifs. Nous visons à calculer ces soi-disant OG avec une technique rapide et agile afin de contrôler les mesures PyWFS pour les systèmes en boucle fermée d'optique adaptative. Méthodes. Grâce à une nouvelle description théorique du PyFWS, qui utilise un modèle convolutionnel pour décrire le capteur, nous prédisons analytiquement le comportement du PyWFS en fonctionnement en boucle fermée. Ce modèle nous permet d'explorer l'impact des phases résiduelles sur les propriétés des mesures PyFWS en termes de sensibilité et OG associées. La méthode proposée s'appuie sur la connaissance des statistiques de phase résiduelle et permet d'estimer automatiquement l'OG actuel. Des simulations numériques de bout en bout sont utilisées pour valider nos prédictions et tester la pertinence de nos approches. Résultats. Nous montrons qu'une estimation précise des OG est possible en utilisant uniquement les données de télémétrie AO pour dériver des informations statistiques de la turbulence. La méthode est alors totalement non invasive. Nous montrons en outre qu'en n'ayant qu'une estimation du paramètre de Fried r0 actuel et des caractéristiques de base du système, les OG peuvent être estimés avec une précision de 10%. La méthode proposée s'applique à Pyramid WFS, mais également à tout WFS à filtrage de Fourier subissant des variations OG.

International audience; As already noticed in other telescopes, the presence of large telescope spiders and of a segmented deformable mirror in an Adaptive Optics system leads to pupil fragmentation and may create phase discontinuities. On the ELT telescope, a typical effect is the differential piston, where all disconnected areas of the pupil create their own piston, unseen locally but drastically degrading the final image quality. The poor sensitivity of the Pyramid WFS to differential piston will lead to these modes been badly seen and therefore badly controlled by the adaptive optics (AO) loop. In close loop operation, differential pistons between segments will start to appear and settle around integer values of the average sensing wavelength. These additional differential pistons are artificially injected by the adaptive optics control loop but do not have any real physical origin, contrary to the Low Wind Effect. In an attempt to reduce the impact of unwanted differential pistons that are injected by the AO loop, we propose a novel approach based on the pair-wise coupling of the actuators sitting on the edges of the deformable mirror segments. In this paper, we present the correction principle, its performance in nominal seeing condition, and its robustness relative to changing seeing conditions, wind speed and natural guide star magnitude. We show that the edge actuator coupling is a simple and robust solution and that the additional quadratic error relative to the reference case (i.e. no spiders) is of only 40 nm RMS, well within the requirements for HARMONI.