Abstract
In the framework of an assessment of optical polishing techniques, ESA has signed a contract with Media Lario to deliver two 250 mm mirrors with a common optical design to be polished down to very tight surface requirements. NOPT mirrors are respectively made of Zerodur and AlSi alloy with electroless nickel and will they be polished by means of bonnet polishing. Mirrors are light-weighted up to 20kg/m2; the mechanical and optical design is proven to be scalable up to 1m CA surfaces. This paper reviews the mirrors opto-mechanical design, introduce the polishing and metrology strategy while highlighting the differences and the common point in fabricating such mirror in Zerodur and metal.
Highlights
Modern polishing techniques for high performance optics often use of small area tools, in contrast to classical large area polishing techniques, where the complete optical surface to be polished or large part of is covered by the polishing tool, limiting the available surface shapes to plane or spherical ones
Using local area polishing techniques enables the polishing of aspherical and freeform surface shapes. In this framework ESA has signed a contract with Media Lario to deliver two 230 mm clear aperture (CA) mirrors polished down to very tight specifications by means of Zeeko bonnet polishing process [1]
Media Lario has developed through the years different polishing processes specific both for Zerodur and AlSi electroless nickel plated mirrors able to bring down the surface errors to the requirements
Summary
Modern polishing techniques for high performance optics often use of small area tools, in contrast to classical large area polishing techniques, where the complete optical surface to be polished or large part of is covered by the polishing tool, limiting the available surface shapes to plane or spherical ones. Using local area polishing techniques enables the polishing of aspherical and freeform surface shapes. In this framework ESA has signed a contract with Media Lario to deliver two 230 mm clear aperture (CA) mirrors polished down to very tight specifications (shown in Table 1) by means of Zeeko bonnet polishing process [1]. Mirrors will be light-weighted up to an areal density of 20 kg/m2 and will be fabricated one in Zerodur and one in AlSi aluminium alloy coated with a polishing layer of electroless Nickel. The use of the AlSi alloy, that is CTE matched with the Nickel coating, allows the fabrication of an almost athermal design of the mirror as for the case of Zerodur. The polishing process, the light weighting and mechanical design of the mirror are proven to be scalable up 1 m CA [2]
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