Development and verification of the largest cryogenic mid IR aluminium concave mirror for METIS showing excellent wavefront performance

Mirka Lewińska-Maresca; Daan Zaalberg; Erik Tabak; Niels Tromp; Dennis Dolkens; Tibor Agocs; Murat Durkut; Martin Lemmen; Jeff Lynn; Ivan Lloro; Menno Schuil; Menno de Haan; Rik ter Horst; Eddy Elswijk

doi:10.1117/12.2629984

29 August 2022 Development and verification of the largest cryogenic mid IR aluminium concave mirror for METIS showing excellent wavefront performance

Mirka Lewińska-Maresca, Daan Zaalberg, Erik Tabak, Niels Tromp, Dennis Dolkens, Tibor Agocs, Murat Durkut, Martin Lemmen, Jeff Lynn, Ivan Lloro, Menno Schuil, Menno de Haan, Rik ter Horst, Eddy Elswijk

Author Affiliations +

Proceedings Volume 12188, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V; 121881P (2022) https://doi.org/10.1117/12.2629984
Event: SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal, Québec, Canada

Abstract

METIS, the cryogenic mid-infrared imager and spectrograph designed for the ESO’s Extremely Large Telescope (ELT) in Chile, is designed to optimally utilize the high resolution provided by the large aperture of the ELT. To enable high contrast imaging, the wavefront errors in the Common Fore Optics (CFO) of METIS must be minimized. In this paper, we focus on the design, the development and verification of the largest, 260 mm diameter curved mirror (CM1) of the CFO. The mirror is made of Rapidly Solidified Aluminium (RSA) Al6061 T6 alloy to achieve a homogenous shrinkage of the instrument and as such facilitate room-temperature instrument alignment and verification. The strict requirement on the maximum surface form error of 30 nm RMS is challenging to meet with metallic mirrors. Thanks to the careful integral design of the mirror, its mount and its manufacturing, the sensitive degrees of freedom of the mirror surface are precisely tuned considering the various contributors. Using numerical optimization, the aberrations related to gravity, assembly loads and minute shrinkage differences are greatly reduced. Finally, by developing a manufacturing process based on inhouse milling, thermal treatments, diamond turning, in-house polishing and gold coating, the final mirror is manufactured. Interferometric tests show good agreement with the numerical predictions of the surface form error and confirm the excellent performance of the CM1 mirror. This is one of the state-of-art examples of metal mirrors designed, manufactured and assembled to such an accuracy.

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Mirka Lewińska-Maresca, Daan Zaalberg, Erik Tabak, Niels Tromp, Dennis Dolkens, Tibor Agocs, Murat Durkut, Martin Lemmen, Jeff Lynn, Ivan Lloro, Menno Schuil, Menno de Haan, Rik ter Horst, and Eddy Elswijk "Development and verification of the largest cryogenic mid IR aluminium concave mirror for METIS showing excellent wavefront performance", Proc. SPIE 12188, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation V, 121881P (29 August 2022); https://doi.org/10.1117/12.2629984

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