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Proceedings Paper

A novel athermal approach for high-performance cryogenic metal optics
Author(s): Ralf-Rainer Rohloff; Andreas Gebhardt; Veit Schönherr; Stefan Risse; Jan Kinast; Sebastian Scheiding; Thomas Peschel
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Paper Abstract

This paper describes a new athermal approach for high performance metal optics, particularly with regard to extreme environmental conditions as they usually may occur in terrestrial as well as in space applications. Whereas for mid infrared applications diamond turned aluminium is the preferred mirror substrate, it is insufficient for the visual range. For applications at near infrared wavelengths (0.8 μm - 2.4 μm) as well as at on cryogenic temperatures (-200°C) requirements exist, which are only partially met for diamond turned substrates. In this context athermal concepts such as optical surfaces with high shape accuracy and small surface micro-roughness without diffraction effect and marginal loss of stray light, are of enormous interest. The novel, patented material combination matches the Coefficient of Thermal Expansion (CTE) of an aluminium alloy with high silicon content (AlSi, Si ≥ 40 %) as mirror substrate with the CTE of the electroless nickel plating (NiP). Besides the harmonization of the CTE (~ 13 * 10-6 K-1), considerable advantages are achieved due to the high specific stiffness of these materials. Hence, this alloy also fulfils an additional requirement: it is ideal for the manufacturing of very stable light weight metal mirrors. To achieve minimal form deviations occurring due to the bimetallic effect, a detailed knowledge of the thermal expansion behavior of both, the substrate and the NiP layer is essential. The paper describes the reduction of the bimetallic bending by the use of expansion controlled aluminium-silicon alloys and NiP as a polishing layer. The acquisition of CTE-measurement data, the finite elements simulations of light weight mirrors as well as planned interferometrical experiments under cryogenic conditions are pointed out. The use of the new athermal approach is described exemplary.

Paper Details

Date Published: 20 July 2010
PDF: 14 pages
Proc. SPIE 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation, 77394E (20 July 2010); doi: 10.1117/12.856983
Show Author Affiliations
Ralf-Rainer Rohloff, Max-Planck-Institut für Astronomie (Germany)
Andreas Gebhardt, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)
Veit Schönherr, Max-Planck-Institut für Astronomie (Germany)
Stefan Risse, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)
Jan Kinast, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)
Sebastian Scheiding, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)
Thomas Peschel, Fraunhofer-Institut für Angewandte Optik und Feinmechanik (Germany)

Published in SPIE Proceedings Vol. 7739:
Modern Technologies in Space- and Ground-based Telescopes and Instrumentation
Eli Atad-Ettedgui; Dietrich Lemke, Editor(s)

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