What is it about?
In order to enable very sharp high-resolution images a telescope mirror must have a very precise and stable reflecting surface. In order to avoid mechanical warping a mirror needs a minimum thickness. This makes it more sensitive to changing ambient temperature. Even if temperature would be constant throughout the volume different parts with different expansion coefficients CTE will expand non-uniformly thus warping the mirror. This means that not only a material with extremely low thermal expansion is required. It must be also extremely homogenous. Today the CTE variation of ZERODUR® lies below 5 ppb/K. This was checked for long and short distances in radial and axial directions. The improved measurement capabilities allow reducing a systematic error, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR® is outstanding. A new method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and measurement capability and capacity is strongly extended. Improved grinding methods provide surface figure and texture of large blanks in a quality that allows starting directly with polishing saving time and costs. Filigree structures of ZERODUR® with 90% weight reduction are well-suited for space mirrors
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Why is it important?
The article gives an overview over the achievements with the ultra low thermal expansion material ZERODUR from SCHOTT AG Germany. The second part shows the extremely high homogeneity of the low expansion on small scales of 1 cm and across 4 m. New expansion measurement equipment allows proving the homogeneity, which was partly covered by measurement uncertainty in the past. High homogeneity and reproducibility holds also for other properties such as material continuity, refractive index, density and the elasticity properties Young’s modulus and Poisson’s ratio. New data and a new prediction calculation method allows applying ZERODUR at much higher mechanical loads. Progress in mechanical shaping enables highly light-weighted mirror structures and large items finished close to dimensions precision needed for polishing verifiable across 4 m diameter. The overview is meant to encourage users with extreme requirements to consider applying this outstanding material.
Perspectives
For me this material ZERODUR is a bundle of wonders. The first wonder is its mere existence. Found by a serendipity event it combines glass and crystal in way thought to be impossible before. The second one is the low expansion based on the negative expansion of the nano-crystals. The third one is the possibility to produce items of more than 8 m diameter from this material. The fourth wonder is that it can be controlled so precisely in its properties, that the extremely low expansion can be reproduced to a very high degree. The fifth is the extreme homogeneity with respect to its expansion even across 8 m diameter. This holds also for other bulk material properties, one can read a newspaper through 1 m of Zerodur (ok in the near IR). The sixth is that it allows minimum breakage strength for long time mechanically loaded applications. The seventh wonder is that a brittle material such as Zerodur can be ground to achieve extremely light-weighted structures withstanding high accelerations. The eighth wonder is that is can be polished to (sub?)nanometer figures and roughness. The ninth wonder is that it can be recoated on and on without degradation of the surface. The tenth wonder is that there is still no obstacle in sight, which could prevent further developments.
Dr Peter Hartmann
SCHOTT AG retired
Read the Original
This page is a summary of: Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 2, Journal of Astronomical Telescopes Instruments and Systems, June 2021, SPIE,
DOI: 10.1117/1.jatis.7.2.020902.
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