PROCEEDINGS ARTICLE | July 20, 2010
Proc. SPIE. 7739, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation
KEYWORDS: Telescopes, Mirrors, Diamond, Polishing, Nickel, Silicon, Interferometry, Aluminum, Cryogenics, Temperature metrology
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<sup>-6</sup> K<sup>-1</sup>), 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.
