The present generation of 8-10m diameter is fully operational and has allowed to access to several new discoveries (extra solar systems…). Projects of a new generation of extremely large telescopes are currently studied. One of them is the ESO’s European Extremely Large Telescope (E-ELT) project. This telescope will get a 42 m diameter primary mirror and its first light is planned for 2017. The optical scheme of this telescope includes an adaptive relay unit (ARU) for the adaptive and active optics. This ARU will include a deformable mirror (DM) in order to correct high orders of atmospheric turbulence. However, new dimensions of this mirror (diameter : 2.5 m with about 8000 actuators getting actuator strokes as high as 60 µm PV) may lead to some design issues. So, in the case of E-ELT’s DM, we will have to face to a scale problem to predict its behaviour when working at atmospheric turbulence frequencies (corresponding to a sampling frequency in the 100 and 1000 Hz range).
The last 4 years, the lab has contributed to CILAS’s proposals for studies in phase A (concept study of ARU) and B (preliminary design study of M4 mirror). In 2006, we had proposed mechanical and optical concepts of ARU for the concept study in phase A of this unit. We are now involved in studies of the DM in phase B at different levels:Assistance to critical reviews of definition and conception of M4’s subsystems :
- Expertise in mechanics and metrology
- Expertise in the characterisation of dynamic behaviour of M4 and its prototype
- Expertise in the possible use of active or passive control techniques
These last 2 points are studied under the workpackage “Analysis of dynamic behaviour of M4AM and of its prototype” and under PhD study of R Gasmi, led in collaboration with CILAS and ONERA. During the first step of these studies, we focused on FE modelling of M4 and its prototype in order to identify eigenmodes, to check the influence of design parameters on these previous modes and to help design, by optimization tools, M4’s prototype. Frequency responses are being calculated.
In a second step, we plan to check the dynamic behaviour of the prototype by experimental techniques and to study, on collaboration with ONERA, the relevance of using some vibration control techniques in this structure.