Christophe Devilliers

Mirror actively deformed and regulated for applications in space: design and performance

Abstract. The need for both high quality images and lightweight structures is one of the main drivers in space telescope design. An efficient wavefront control system will become mandatory in future large observatories, retaining performance while relaxing specifications in the global system’s stability. We present the mirror actively deformed and regulated for applications in space project, which aims to demonstrate the applicability of active optics for future space instrumentation. It has led to the development of a 24-actuator, 90-mm-diameter active mirror, able to compensate for large lightweight primary mirror deformations in the telescope’s exit pupil. The correcting system has been designed for expected wavefront errors from 3-m-class lightweight primary mirrors, while also taking into account constraints for space use. Finite element analysis allowed an optimization of the system in order to achieve a precision of correction better than 10 nm rms. A dedicated testbed has been designed to fully characterize the integrated system performance in representative operating conditions. It is composed of: a telescope simulator, an active correction loop, a point spread function imager, and a Fizeau interferometer. All conducted tests demonstrated the correcting mirror performance and has improved this technology maturity to a TRL4.

Space active optics: performance of a deformable mirror for in-situ wave-front correction in space telescopes

MADRAS (Mirror Active, Deformable and Regulated for Applications in Space) project aims at demonstrating the interest of Active Optics for space applications. We present the prototype of a 24 actuators, 100 mm diameter deformable mirror to be included in a space telescopes pupil relay to compensate for large lightweight primary mirror deformation. The mirror design has been optimized with Finite Element Analysis and its experimental performance characterized in representative conditions. The developed deformable mirror provides an efficient wave-front correction with a limited number of actuators and a design fitting space requirements.

Si3N4 ceramic application for large telescope development results

Thales-Alenia-Space has identified the ceramic Si3N4 as an interesting material for the manufacturing of stiff , stable and lightweight truss structure for future large telescopes. Si3N4 ceramic made by FCT has been selected for its own intrinsic properties (high specific Young modulus, low CTE, very high intrinsic strength for a ceramics) and its cost effective beams manufacturing capabilities. In order to qualify beam and beams end fittings for future large and thermo-elastical stable truss structure for space telescope, full development and tests activities have been performed. Manufacturing process has been optimized in order to obtain a very high reliable strength. Full scale beams with thin wall have been manufactured and tested in bending and in tension. Full scale beam assembly with integrated junctions have been manufactured and tested up to ultimate loads and have been space qualified. Beams end fittings made also in Si3N4 and its direct bolting capabilities have been also space qualified by tests. Beside this qualification for current space telescope, developments are continuing thank to CNES R&T to develop high loaded brazed junction between Si3N4 parts, enhanced thermal conductivity and mechanical strength through Si3N4 formulation and manufacturing process tuning.

SPIRALE: the first all-Cesic® telescopes orbiting earth

SPIRALE is a French Earth observation demonstration project consisting of two satellites. In support of the project and under contract with Thales Alenia Space, ECM manufactured two fully integrated all-Cesic® telescopes, composed of super-light-weighted complex monolithic structures, including two off-axis aspheric mirrors per telescope with integrated interfaces for mounting. The all-Cesic telescope assembly was tested under shock and vibration loads, and by exposure to realistic in-flight thermal environments. In this paper we describe the space-qualified process of manufacturing such high-precision space telescopes based on our Cesic® technology; the advantages of our Cesic® technology compared to traditional materials, such as metals or glass ceramics; and some of the test results. This project demonstrates that all-Cesic® telescopes have great potential for future space applications, especially under cryogenic conditions, due to their athermal characteristics and the great versatility of the Cesic® manufacturing process.

Silicon nitride ceramic development in Thales Alenia Space : qualification achievement and further developments for future applications

Dealing with ceramic materials for more than two decades, Thales Alenia Space – France has identified Silicon Nitride Si3N4 as a high potential material for the manufacturing of stiff, stable and lightweight truss structure for future large telescopes. Indeed, for earth observation or astronomic observation, space mission requires more and more telescopes with high spatial resolution, which leads to the use of large primary mirrors, and a long distance between primary and secondary mirrors. Therefore current and future large space telescopes require a huge truss structure to hold and locate precisely the mirrors. Such large structure requires very strong materials with high specific stiffness and a low coefficient of thermal expansion (CTE). Based on the silicon nitride performances and on the know how of FCT Ingenieurkeramik to manufacture complex parts, Thales Alenia Space (TAS) has engaged, in cooperation with FCT, activities to develop and qualify silicon nitride parts for other applications for space projects.

New design and new challenge for space large ultralightweight and stable Zerodur mirror for future high resolution observation instruments

Space telescopes pupil diameter increases continuously to reach higher resolutions and associated optical scheme become more sensitive. As a consequence the size of these telescopes but also their stability requirements increase. Therefore, mass of space telescopes becomes a strong design driver to be still compatible with price competitive launcher capabilities. Moreover satellite agility requirements are more and more severe and instruments shall be compatible with quick evolution of thermal environment.

Silicon nitride for lightweight stiff structures for optical instruments

Due to their very specific set of material properties, silicon nitride and silicon carbide have gained a lot of interest in the last 20 years. Moreover, many new approaches in technical equipment and processes were enabled with corresponding research and production activities. Also large efforts were made at FCT during the last years, to get able to supply even very large and complex shaped components made of sintered silicon carbide (SSiC) and of gas pressure sintered silicon nitride (GPSN) ceramics. This approach has opened new applications and markets for such ceramic materials. On the other side, designers and engineers are now allowed to think much more complex in designing of ceramic components. In this paper, a new rapid prototyping routine for very complex components as well as the corresponding materials will be presented. Components for optical equipment in innovative avionic and space applications, and more conventional technologies are described. Not only their unique key intrinsic properties, like high Youngs Modulus, very low CTE, very high strength and fracture toughness for a ceramic but also newly developed and adopted shaping, sintering and machining technologies in both green and sintered state have let to highly valued products. This enabled FCT to offer Carl Zeiss Optronics using silicon nitride for a newly designed, very complex housing structure of an avionic pod camera. Due to a very low CTE, high stiffness and less weight, an improved performance was reached. Also Thales Alenia Space is engaged since some years in activities to develop and qualify Silicon nitride ceramics for space projects. Extremely stiff, very lightweight and large truss space structures with a very low CTE, high rigidity and no outgasing for satellites can now be realized. Deep tests sequence has been performed to qualify truss beams and end fittings made in the same material. Also advanced dynamic testing equipment for avionic turbine blades requires new approaches. In cooperation with TIRA a series of shaker heads were developed which can operate at much higher frequencies and so reduce fatigue testing time and costs. Last but not least, highly precise and thin walled disc structures with diameters up to 380 mm are produced for wafer handling and testing equipment.