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In 2016, Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO. The secondary mirror is a 4-meter convex mirror, the world's largest convex precision mirror ever made. This mirror is currently in its final phase of polishing. It is regularly measured on a dedicated interferometric test bench specifically designed to achieve the best accuracy on such a large mirror. The mirror is quite thin compared to its diameter and one challenge was to design a supporting tool able to hold the mirror during the interferometric measurements without stress and deformation. The thickness is about 100 mm for a 4m diameter, leading to a ratio of 40 between thickness and diameter. Such a thin mirror requires a very good control on the force field applied during the measurement, which shall be exactly the same as the one seen in the telescope. An accuracy on each force applied in the range of 0.1 to 0.01% is requested. In order to guarantee this performance, the full system had to be thoroughly designed and tested, to avoid any bias, which could degrade the optical performances at the customer's facility. In this paper, we present the specification and the design of the supporting tool, along with the validation results obtained on sub-systems and finally obtained with the full mirror. The ELT M2 is now close to its final shape and any defects coming from the supporting tool can be easily detected. Thanks to that, it was possible to demonstrate the performance achieved in use.
Monin et al. (Mon,) studied this question.