Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO in 2016. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. As of spring 2024, this mirror is in its final phase of polishing. It is regularly controlled on a dedicated interferometric test bench (ITB) specifically designed to achieve the best accuracy on such a large mirror. The test bench was thoroughly validated in September 2023 and all its requirements were demonstrated. This bench includes a convex-concave test plate together with a unique custom interferometer system, both specifically designed and manufactured for this test bench. In fact, the specific requirements of this very large mirror forbid the use of off-the-shelf component or interferometer and it was necessary to design a specific interferometer to cope with all the requirements. Furthermore, it was necessary to manufacture an almost perfect 2-m reference plate to control the mirror. It took about 3 years to manufacture this reference plate with the requested accuracy.
The test bench makes a measurement on 24 sub-pupils which are stitched together to reconstruct the final surface error. The full aperture measurement and the map stitching lasts around 2 hours. Specific sensors are used to monitor the stitching process and the stitching algorithm had to be adapted in order to fulfil the stringent performance requirements.
In this paper, we present the main features of the bench, the alignment strategy, the test results obtained on the bench and the stitching algorithm developed to reconstruct the surface error.
Safran Reosc was awarded the manufacturing of the Secondary Mirror of the Extremely Large Telescope by ESO in 2016. The secondary mirror is a 4-meter convex mirror, the world’s largest convex precision mirror ever made. As of spring 2024, this mirror is in its final phase of polishing. It is regularly controlled on a dedicated interferometric test bench (ITB) specifically designed to achieve the best accuracy on such a large mirror. The test bench was thoroughly validated in September 2023 and all its requirements were demonstrated. This bench includes a convex-concave test plate together with a unique custom interferometer system, both specifically designed and manufactured for this test bench. In fact, the specific requirements of this very large mirror forbid the use of off-the-shelf component or interferometer and it was necessary to design a specific interferometer to cope with all the requirements. Furthermore, it was necessary to manufacture an almost perfect 2-m reference plate to control the mirror. It took about 3 years to manufacture this reference plate with the requested accuracy. The test bench makes a measurement on 24 sub-pupils which are stitched together to reconstruct the final surface error. The full aperture measurement and the map stitching lasts around 2 hours. Specific sensors are used to monitor the stitching process and the stitching algorithm had to be adapted in order to fulfil the stringent performance requirements. In this paper, we present the main features of the bench, the alignment strategy, the test results obtained on the bench and the stitching algorithm developed to reconstruct the surface error.
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.
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