Minimizing the bimetallic bending for cryogenic metal optics based on electroless nickel

Abstract

Ultra-precise metal optics are key components of sophisticated scientific instruments in astronomy and space applications. Especially for cryogenic applications, a detailed knowledge and the control of the coefficient of thermal expansion (CTE) of the used materials are essential. Reflective optical components in IR- and NIR-instruments primarily consist of the aluminum alloy Al6061. The achievable micro-roughness of diamond machined and directly polished Al6061 does not fulfill the requirements for applications in the visible spectral range. Electroless nickel enables the reduction of the mirror surface roughness to the sub-nm range by polishing. To minimize the associated disadvantageous bimetallic effect, a novel material combination for cryogenic mirrors based on electroless nickel and hypereutectic aluminum-silicon is investigated. An increasing silicon content of the aluminum material decreases the CTE in the temperature range to be considered. This paper shows the CTE for aluminum materials containing about 42 wt% silicon (AlSi42) and for electroless nickel with a phosphorous content ranging from 10.5 to 13 %. The CTE differ to about 0.5 × 10-6 K-1 in a temperature range from -185 °C (LN2) to 100 °C. Besides, the correlations between the chemical compositions of aluminum-silicon materials and electroless nickel are shown. A metrology setup for cryo-interferometry was developed to analyze the remaining and reversible shape deviation at cryogenic temperatures. Changes could be caused by different CTE, mounting forces and residual stress conditions. In the electroless nickel layer, the resulting shape deviation can be preshaped by deterministic correction processes such as magnetorheological finishing (MRF) at room temperature

Athermal metal optics made of nickel plated AlSi40

“…NiP layer that are suitable for diamond turning and post polishing vary between 11 wt% and 13 wt% phosphorous content. Within these limits, the CTE are in a tolerance of < 0.5 × 10 -6 K -1 [9], which is verified experimentally. In principle, the CTE of electroless NiP can be manipulated and tailored to a target value during the plating process by controlling the deposition parameters.…”

“…The material matches the CTE of the electroless nickel polishing layer with 1 × 10 -6 K -1 in between the manufacturing tolerance of the materials. It can be tailored by adjusting the silicon content of the bulk material as well as by modifying the phosphorous content of the NiP layer [9]. …”

Precision manufacturing of a lightweight mirror body made by selective laser melting

“…Due to this mismatch shape changes of the part occur when thermal loads are applied (bimetallic bending). By using aluminum with 40 wt% silicon (AlSi40), this thermal mismatch is reduced to a minimum [6]. Beside the optical and mechanical performance of metal mirrors, their weight is an important factor when highly dynamic scanning applications are desired or when mirrors are used in optical systems in space.…”

Design study for an active metal mirror: sub-system of a correction chain for large UVOIR space telescopes

“…Ultra-precision single point diamond turning is used to improve the figure of the mirror surface just before the electroless nickel plating. Nickel-Phosphorous is perfectly thermally adapted to the AlSi-composite [8] and provides sufficient hardness to achieve the extraordinary results in figure and roughness that are indispensable for the UVOIR. The transmission elements should be designed monolithically as well could be manufactured with common technologies as milling and electron discharge machining or with additive manufacturing technologies.…”

“…Otherwise, there is no reason to start such cost-intensive projects. The most effective way to identify more Earth-like planets is to enlarge the primary mirror to 4,8,12 or 16 meters. However, in doing so, the aspect ratio of the primary mirror will increase and hence suffer drastically from gravity release after reaching space surface distortions due to thermal changes and vibrations.…”