Stress mirror polishing for large lightweight mirrors for space application : Space and Earth observations

Les grands miroirs primaires des futurs grands observatoires spatiaux sont développés de manière à augmenter le rapport rigidité / masse primordiale pour réduire les coûts et maintenir une bonne tenue mécanique tout en gardant une très grande qualité de surface optique. La mise en place des structures d’allègement, le choix du matériau et la méthode de polissage sont des paramètres essentiels pour atteindre ces exigences. Le polissage sous contraintes (SMP) est une technique qui présente de nombreux avantages dont la capacité d’asphériser une optique de grand diamètre sans générer de défauts de hautes fréquences spatiales et en un minimum de temps grâce à l’utilisation d’un outil de pleine taille. Dans ce manuscrit, le polissage sous contraintes d’un grand miroir allégé spatial est présenté en deux parties principales. Dans la première partie, je fais une étude sur le polissage sous contraintes d’un miroir allégé monolithique traditionnel en corrigeant le design pas à pas. Dans un premier temps, j’agis sur le miroir de façon externe avec une contreforme et, dans un second temps, je redéfinis le modèle du miroir pour le rendre compatible à la production d'une asphère de type axisymétrique par SMP. La deuxième grande partie sépare les fonctions de polissage et de rigidification. En premier lieu, l’optimisation de forme assistera la démarche théorique dans la recherche de la forme optimale du miroir pour atteindre l'objectif d’asphérisation. En second lieu, l’optimisation topologique me donnera les outils afin de trouver la structure allégée idéale pour répondre aux spécifications en termes de première fréquence de vibration mais aussi de réduction de la masse totaleLarge primary mirrors for future space obstervatories are developed by increasing the ratio stiffness/weigth which is critical to reduce the cost and to maintain a very light structure with a high mechanical rigidity. The implementation of ligthweight structure, the choice of the material and the polishing technique are essential to reach the specifications required. Stress mirror polishing (SMP) technique provides many advantages such as an effective way to aspherize a large optical substrate without generating high spatial frequencies errors in a minimum time period thanks to a full-size polishing tool. In this manuscript, the stress mirror polishing of a large lightweight space mirror is presented in two main parts.In the first part, I study the SMP method on a classic lightweight monolithic mirror by rectifying the model step by step. In a first instance, I operate on the mirror in an external way with an oppposite shape structure and then, subsequently, I redefine the mirror design to produce an axisymetric aspheric. The second part handles the antagonistic aspect between the SMP technique and the high rigidity of lightweight mirror by introducing a functional decoupling. I separate, in this section, polishing and rigidification functions. Concerning the polishing function, I use a shape optimisation algorithm to assist the theoretical approach to find the optimal mirror shape in order to reach the aspherisation goal. For the rigidication function, the tools given by a topology optimisation algorithm to converge toward the ideal lightweight structure is very valuable to meet the space requirements in terms of first vibration frequency and lightened structur

Stress mirror polishing for future large lightweight mirrors: design using shape optimization

International audienceThis study proposes a new way to manufacture large lightweight aspherics for space telescopes using Stress Mirror Polishing (SMP). This technique is well known to allow reaching high quality optical surfaces in a minimum time period, thanks to a spherical full-size polishing tool. To obtain the correct surface' aspheric shape, it is necessary to define precisely the thickness distribution of the mirror to be deformed, according to the manufacturing parameters. We first introduce active optics and Stress Mirror Polishing techniques, and then, we describe the process to obtain the appropriate thickness mirror distribution allowing to generate the required aspheric shape during polishing phase. Shape optimization procedure using PYTHON programing and NASTRAN optimization solver using Finite Element Model (FEM) is developed and discussed in order to assist this process. The main result of this paper is the ability of the shape optimization process to support SMP technique to generate a peculiar aspherical shape from a spherical optical surface thanks to a thickness distribution reshaping. This paper is primarily focused on a theoretical framework with numerical simulations as the first step before the manufacturing of a demonstrator. This "two-steps" approach was successfully used for previous projects

Elastic bending of Variable Curvature Mirrors: validation of a simplified analytical method

International audienceThis work explores the variable curvature mirror's (VCM) elastic bending rules through modeling it as a thin elastic plate with an exponential thickness distribution actuated with a uniform pressure under simply supported boundary conditions. By using the small-parameter method, the general analytical expression of a plate's deflection is worked out. The results calculated by the analytical solution are compared to the finite element analysis of a VCM model with the same specific parameters. We demonstrate that the two have a good correlation with the one the other. This analytical solution is an effective way to predict a VCM's deflection

A methodology to design optical systems with curved sensors

International audienceCurved sensors are a suitable technological solution to enhance the vast majority of optical systems. In this work, we show the entire process to create curved sensor-based optical systems and the possibilities they offer. This paper defines the boundaries of the reachable curvatures for a full range of monolithic sensors. We discuss how the curved focal plane shape is related to the imaged scenes and optical parameters. Two camera prototypes are designed, realized and tested, demonstrating a new compact optical architecture for a 40 degree compact objective, as well as a wide field fisheye zoom objective using a convex sensor to image a 180 degree field of view

Methodology to design optical systems with curved sensors

Curved sensors are a suitable technological solution to enhance the vast majority of optical systems. In this work, we show the entire process to create curved sensor-based optical systems and the possibilities they offer. This paper defines the boundaries of the reachable curvatures for a full range of monolithic sensors. We discuss how the curved focal plane shape is related to the imaged scenes and optical parameters. Two camera prototypes are designed, realized, and tested, demonstrating a new compact optical architecture for a 40 deg compact objective as well as a wide-field fisheye zoom objective using a convex sensor to image a 180 deg field of view