Development of an interferometric test bench for the manufacturing of the ELT secondary mirror

R. Mercier-Ythier; H. Monin; G. Foucaud; C. Beudy; N. Lefeuvre; P. Wuillaume; J. L. Carel; M. C. Le-Clech; C. Baptista; C. Frapolli; J. Hatzigeorgopoulos

doi:10.1117/12.3022722

17 June 2024 Development of an interferometric test bench for the manufacturing of the ELT secondary mirror

R. Mercier-Ythier, H. Monin, G. Foucaud, C. Beudy, N. Lefeuvre, P. Wuillaume, J. L. Carel, M. C. Le-Clech, C. Baptista, C. Frapolli, J. Hatzigeorgopoulos

Author Affiliations +

Proceedings Volume 13021, Optical Fabrication and Testing VIII; 130210E (2024) https://doi.org/10.1117/12.3022722
Event: SPIE Optical Systems Design, 2024, Strasbourg, France

Abstract

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.

Conference Presentation

(2024) Published by SPIE. Downloading of the abstract is permitted for personal use only.

Citation Download Citation

R. Mercier-Ythier, H. Monin, G. Foucaud, C. Beudy, N. Lefeuvre, P. Wuillaume, J. L. Carel, M. C. Le-Clech, C. Baptista, C. Frapolli, and J. Hatzigeorgopoulos "Development of an interferometric test bench for the manufacturing of the ELT secondary mirror", Proc. SPIE 13021, Optical Fabrication and Testing VIII, 130210E (17 June 2024); https://doi.org/10.1117/12.3022722

ACCESS THE FULL ARTICLE

INSTITUTIONAL
Select your institution to access the SPIE Digital Library.

SELECT YOUR INSTITUTION

PERSONAL
Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.

PERSONAL SIGN IN

No SPIE Account? Create one

PURCHASE THIS CONTENT

SUBSCRIBE TO DIGITAL LIBRARY

50 downloads per 1-year subscription

Members: $195

Non-members: $335 ADD TO CART

25 downloads per 1 - year subscription

Members: $145

Non-members: $250 ADD TO CART

PURCHASE SINGLE ARTICLE

Includes PDF, HTML & Video, when available