Glassy thin shells are key components for the development of adaptive optics and are part of future and innovative projects such as ELT. However, manufacturing thin shells is a real challenge. Even though optical requirements for the front face - or optical face - are relaxed compared to conventional passive mirrors, requirements concerning thickness uniformity are difficult to achieve. In addition, process has to be completely re-defined as thin mirror generates new manufacturing issues. In particular, scratches and digs requirement is more difficult as this could weaken the shell, handling is also an important issue due to the fragility of the mirror. Sagem, through REOSC program, has recently manufactured different types of thin shells in the frame of European projects - E-ELT M4 prototypes and VLT Deformable Secondary Mirror (VLT DSM).
The Megajoule laser (LMJ) project was launched in 1995 by the French Atomic Energy Commission and is aimed at developing a facility to achieve inertial confinement fusion. The LMJ architecture is based on 176 laser beamlines. To provide these 176 high-powered beams when required for subsequent operations, one of the main issues consists of aligning reliably the laser Transportation Section (TS) not only during the normal operation of the installation but also during the power rising of each laser chain, its initial alignment and after major maintenance. They also must be compatible with the fact that the entire installation should be maintained by a limited staff. Consequently, the goals of the techniques involved in this processing design are essentially robust detection and identification of the relevant items of information present in images, but also the reduction of the number of parameters accessible to the operators.
This paper provides a general overview of how the TS is aligned before focusing on the image processing techniques developed to identify and measure the beam centering, since the major difference between the LIL and LMJ TS is the type of centering detector.
These techniques have been developed and tested thoroughly against sets of up to 57 images representing both nominal and extreme conditions acquired during recent experiments on the Laser Integration Line (LIL). This facility is fully consistent with the LMJ requirements, a complete laser chain with 4 beamlines. After presenting the basic design principles, we focus on the performances as demonstrated and measured.