Gerard R. Lemaitre

Recording method for obtaining high-resolution holographic gratings through use of multimode deformable plane mirrors

We propose a new way of recording in which a spherical blank and a deformable mirror are used to obtain high-resolution holographic gratings. The reflection of one of the two laser recording waves upon this mirror provides the deformations necessary to image correction to as much as seventh-order aberrations inclusively.

Illustration of the use of multimode deformable plane mirrors to record high-resolution concave gratings: results for the Cosmic Origins Spectrograph gratings of the Hubble Space Telescope

To illustrate the efficiency of using a deformable plane mirror to record holographic gratings, we have computed the three gratings for the Cosmic Origins Spectrograph. Their working conditions are severe, since they have to correct the residual spherical aberration of the Hubble Space Telescope. Nevertheless, all images obtained are largely diffraction limited with regard to the resolution.

Stress polishing of E-ELT segment at LAM: full-scale demonstrator status

The Laboratoire dAstrophysique de Marseille (LAM) is involved in the prototyping of a full scale demonstrator for stress polishing of segments for the European Extremely Large Telescope (E-ELT). Stress polishing method is developed at LAM since more than 40 years, and this mature technology has recently been used with success for VLT instruments. Stress polishing is now considered as a promising manufacturing method for mass production of large off axis mirrors, specifically for ELT segments. This powerful method, based on elasticity theory, allows the generation of super-smooth off-axis aspherics with a minimal amount of high spatial frequency ripples by spherically polishing a warped blank with a full-sized tool. Thanks to the simple spherical polishing, the operation time can be strongly reduced compared to the time-consuming sub-aperture tool methods of grinding and polishing. The goal is to rapidly converge to less than 1 micron RMS of optical quality on a circular blank which will be finally cut hexagonally and finished using Ion Beam Finishing. In this paper we will present the status of the demonstrator and the design of the warping harness prototype that must be able to precisely warp the circular blank.

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.

Highly variable curvature mirrors for the Very Large Telescope Interferometer

The design of two holosteric configurations have been optimized for a maximum center/edge deflexure of 400 micrometers . Their thickness distribution is given for active zones of 16 mm in diameter. The curvature action is obtained from an air-pressure chamber that generates onto the rear side of the mirrors (1) a uniform pressure up to 9 Atm or (2) a central force up to 11 daN. The control of the curvature is made by an accurate pressure gauge. Some preliminary results are shown as obtained on metal prototype VCMs from a first fabrication cycle as well as an X-ray device for testing the machining validity of the boundaries at the edge of mirrors.

Active optics and corrective holographic gratings: a general recording method applied to COS/HST 2003

Aberration corrected gratings are useful to design spectrographs having the minimum of optical surfaces, thus minimizing light loss such as needed for astronomical instruments.

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.

Stress Polishing of Aspherical Surfaces for VLT AO Instrumentation

Stress polishing methods are developed for the manufacturing of a large aspherical thin shell for the VLT Deformable Secondary Mirror and for the manufacturing of toric mirrors for the VLT SPHERE instrument.

Fabrication of a highly variable curvature mirror: testing and results

A variable curvature mirror is a powerful device that can increase the field of view of optical interferometers. Such a mirror is being developed for the coherent combined focus of the European Southern Observatory Very Large Telescope Interferometer. The variable focal length permits positioning of the pupil image of an individual telescope at a precise location after the delay-line. This property is necessary to exactly remap homothetically the output pupil configuration at the image beamcombiner. Given the large zoom range that is needed in the delay line, when the mirror is not stressed the optical surface is a plane while it is convex with f/2.5 at maximum stress. The mirror itself is a very small stainless steel meniscus, with a 300 micrometers thickness, because only the high elasticity of such material allows to achieve the full domain of curvature. The thickness distribution of the meniscus is calculated using elasticity theory and a set of non-linear equations valid for the case of a large deformation. The realization of this micro-optic active device requires advanced techniques in optical fabrication and in particular high precision manufacturing with numerical command lathe. This article also presents the testing of this highly Variable Curvature Mirror and the surface quality obtained within the full curvature range.