Space telescopes require large primary mirrors within a demanding thermal environment: observatories at L2 orbit provide a stable environment with a drawback of very low temperature. Besides, it is necessary to limit as far as possible the mirrors mass while withstanding launch loads and keeping image quality within a cryogenic environment.
ZERODUR is a well-known material extensively used for large telescope. Alcatel Alenia Space and Sagem/REOSC have combined their respective skills to go further in the lightweighting ratio of large mirror (36 kg/m2 on 1.5 m2) through a detailed design, performance assessment and technology demonstration with breadboards.
Beyond on a large mirror detailed design supported by analysis, a ZERODUR mock-up has been manufacturing by Sagem/REOSC to demonstrate the achievability of the demanding parameters offering this high lightweighting ratio.
Through the ISO experience on mirror attachments, a detailed design of the mirror fixation has been done as well. A full size mock-up has been manufactured and successfully tested under thermal cycling and static loading.
Eventually, the ZERODUR stability behavior within this large temperature range has been verified through thermal cycling and image quality cryotest on a flat mirror breadboard.
These developments demonstrate that ZERODUR is a good candidate for large space cryogenic mirrors offering outstanding optical performances associated to matured and proven technology and manufacturing process.
Due to more and more stringent requirements for observation missions, diameter of primary mirrors for space telescopes is increasing. Difficulty is then to have a design stiff enough to be able to withstand launch loads and keep a reasonable mass while providing high opto-mechanical performance. Among the possible solutions, Thales Alenia Space France has investigated optimization of ZERODUR mirrors. Indeed this material, although fragile, is very well mastered and its characteristics well known. Moreover, its thermo-elastic properties (almost null CTE) is unequalled yet, in particular at ambient temperature. Finally, this material can be polished down to very low roughness without any coating. Light-weighting can be achieved by two different means : either optimizing manufacturing parameters or optimizing design (or both). Manufacturing parameters such as walls and optical face thickness have been improved and tested on representative breadboards defined on the basis of SAGEM-REOSC and Thales Alenia Space France expertise and realized by SAGEM-REOSC. In the frame of CNES Research and Technology activities, specific mass has been decreased down to 36 kg/m2. Moreover SNAP study dealt with a 2 m diameter primary mirror. Design has been optimized by Thales Alenia Space France while using classical manufacturing parameters – thus ensuring feasibility and costs. Mass was decreased down to 60 kg/m2 for a gravity effect of 52 nm. It is thus demonstrated that high opto-mechanical performance can be guaranteed with large highly lightweighted ZERODUR mirrors.
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.