Emmanuel Hugot

In-flight aberrations corrections for large space telescopes using active optics

The need for both high quality images and light structures is a constant concern in the conception of space telescopes. The goal here is to determine how an active optics system could be embarked on a satellite in order to correct the wave front deformations of the optical train. The optical aberrations appearing in a space environment are due to mirrors deformations, with three main origins: the thermal variations, the weightlessness in space with respect to the Assemblage, Integration and Testing (AIT) conditions on ground and the use of large weightlighted primary mirrors. We are developing a model of deformable mirror as minimalist as possible, especially in term of number of actuators, which is able to correct the first Zernike polynomials in the specified range of amplitude and precision. Flight constraints as weight, volume and power consumption have to be considered. Firstly, such a system is designed according to the equations from the elasticity theory: we determine the geometrical and mechanical characteristics of the mirror, the location of the forces to be applied and the way to apply them. The concept is validated with a Finite Element Analysis (FEA), allowing optimizing the system by taking into account parameters absent from the theory. At the end of the program the mirror will be realized and characterized in a representative optical configuration.

Toric mirrors and active optics: degenerated configuration for spherical monomode deformable mirrors

Providing toric mirrors with Active Optics techniques will allow generating aspheric surfaces which optical quality avoid high spatial frequencies errors. In order to demonstrate the feasibility of this technique, a link has been established between analytical calculations, finite element modelling and experimental validation. A particular configuration of a flat mono-mode deformable mirror (MDM), called degenerated configuration, has been analytically calculated, showing how to generate a third order astigmatism aberration (Astm 3) by active deformation. This mirror has been manufactured and tested. A finite element model has been produced in order to correlate simulations with experiments. The deformed optical surface is projected on a Zernike polynomial base, indicating that Astm 3 mode is, within a very high precision, the only aberration generated on the optical surface. Another spherical concave MDM has been modelled as a VLT-SPHERE toric mirror of diameter 133mm, to demonstrate the feasibility of toric surfaces from active optics deformation of a spherical shell. Projection on Zernike base shows that the simulated deformed surface is a combination of a sphere and a quasi pure Astm 3 mode, corresponding to a toroidal surface. Other terms generated, like Astm 5, could benefit of a fine adjustment from the geometry of the substrate.

Stress polishing of toric mirrors for the VLT-SPHERE common path

The very challenging goal of the Vlt-Sphere instrument, Exoplanet direct detection and characterisation, requires high contrast imaging and extreme adaptive optics.1 In order not to limit the overall imaging performances of the instrument, all the optics in the common path optical train need to be of the better quality over each range of spatial frequencies. Three Toric mirrors are placed in the common path to relay the beam to the deformable mirror and to the instruments. This paper details the Stress polishing principle developed at Laboratoire dAstrophysique de Marseille (Lam) to get the better optical quality on the toric surfaces, using a spherical polishing with full size tools. The elasticity theory giving the optimisation of the blank geometry to be warped during the stress polishing process is detailed from analytical calculation to finite element analysis. The use of an angular thickness distribution allows us to reach the better optical quality of the deformation by canceling higher order terms. We also present the polishing results for the 366mm diameter Toric Mirror manufacturing.

Active Optics Theory : Compensation of aberration using the single actuator principle

Active Optics allows the possibility of using the generation of complex variable optical surfaces to keep the optical layout of future instruments relatively simple, something which could be of great interest to future telescopes such as E-Elt, Tmt. The aim of this article is to describe the development of the single actuator - single mode principle that makes it possible to generate single optical modes on a circular mirror using a single actuator at a specific location. We show the progress from design analysis (elasticity theory, finite element analysis etc) through to experimental validation for Variable Curvature Mirrors and Variable Astigmatism Mirrors. Current and future applications of these active mirrors are discussed in the framework of the EAGLE instrument for E-Elt and we present plans for further development of the technique.

Active Optics techniques and complex instrumentation for future ELTs

In the frame of the future European Extremely Large Telescope, the Laboratoire d’Astrophysique de Marseille is developing manufacturing methods and complex instrumentation for astronomy, based on the active bending of mirrors.

Space Active Optics: toward optimized correcting mirrors for future large spaceborne observatories

Wave-front correction in optical instruments is often needed, either to compensate Optical Path Differences, off-axis aberrations or mirrors deformations. Active optics techniques are developed to allow efficient corrections with deformable mirrors. In this paper, we will present the conception of particular deformation systems which could be used in space telescopes and instruments in order to improve their performances while allowing relaxing specifications on the global system stability. A first section will be dedicated to the design and performance analysis of an active mirror specifically designed to compensate for aberrations that might appear in future 3m-class space telescopes, due to lightweight primary mirrors, thermal variations or weightless conditions. A second section will be dedicated to a brand new design of active mirror, able to compensate for given combinations of aberrations with a single actuator. If the aberrations to be corrected in an instrument and their evolutions are known in advance, an optimal system geometry can be determined thanks to the elasticity theory and Finite Element Analysis.

Development and performance of the EAGLE active optics LGS WFS refocusing system

We designed, developed, and tested a Variable Curvature Mirror (VCM) as an active refocusing system for the Laser Guide Star (LGS) Wave Front Sensor (WFS) of the E-ELT EAGLE instrument [1]. This paper is the second of two from our team on this R&D activity: Hugot et al. this conf. [2] presented the mirror design and performance simulations. Here, we report on the fabrication integration, testing and performance of the VCM system. During this activity, we developed all necessary parts for the VCM system: a metallic mirror, its housing and mounts, a computer-controlled pressure system, an internal metrology, a testbench etc. The functional testing of the VCM system is successful: we can control the internal pressure to less than 1 mBar, and measure the mirror displacement with a 100 nm accuracy. The mirror displacement is a near-linear and well-simulated function of internal pressure for the desired range of focus. The intrinsic optical quality of the mirror meniscus is well within the specifications. Once mounted in its housing, we observe additional mechanical constraints for the current design that generate optical aberrations. We measured the amplitude of the Zernike modes, and we showed that the axisymetric terms display a variation trend very similar to simulations, with amplitude close to simulations. All these results are very promising for a design of focus compensation without any moving part.

Toward high-dynamic active mirrors for LGS refocusing systems

In the frame of the E-ELT-EAGLE instrument phase A studies, we designed a convex VCM able to compensate for the focus variation on the Laser Guide Star (LGS) wavefront sensor, due to the elevation of the telescope and the fixed sodium layer altitude. We present an original optical design including this active convex mirror, providing a large sag variation on a spherical surface with a 120mm clear aperture, with an optical quality better than lambda/5 RMS up to 820μm of sag and better than lambda/4 RMS up to 1000μm of sag. Finite element analysis (FEA) allowed an optimisation of the mirrors variable thickness distribution to compensate for geometrical and material non linearity. Preliminary study of the pre-stressing has also been performed by FEA, showing that a permanent deformation remains after removal of the loads. Results and comparison with the FEA are presented in the article of F.Madec et al (AS10-7736-119, this conference), with an emphasis on the system approach.