Abstract
Observational astronomy has striven for better telescopes with higher resolutions from its start. This needs ever-larger mirrors with stable high precision surfaces. The extremely low expansion glass ceramic ZERODUR® enables such mirrors with more than 50 years of significant improvements in size and quality since its development. We provide a survey of the progress achieved in the last 15 years. The narrowest coefficient of thermal expansion (CTE) tolerance is now ±7 ppb / K. It is possible to adapt the material for lowest expansion to temperature-time courses given for special environments. At cryo temperatures, expansion is low and adaptable. Improved measurement capabilities allow for absolute CTE uncertainty of 3 ppb / K and reproducibility of 1 ppb / K (2σ). The influence of ionizing radiation on the surface figure integrity is subject to new investigations. Improved measurement capabilities increase the reliability of structure designs. Some outstanding examples are given for applications of ZERODUR in astronomy and in the very important high technology industry. The progress of thermal expansion homogeneity, the mechanical strength of ZERODUR, production capabilities in melting, precision machining, light-weighting, and dimensional metrology is presented in the second part of the paper.
Highlights
The lithium-aluminum-silicate (LAS) glass ceramic ZERODUR® (Trademark of SCHOTT AG, Germany) is a material full of innovations even more than 50 years after its introduction
A glass ceramic is a material between two extremes: glass and crystal, which are free from each opposite configuration
Glass ceramics consist of a glassy phase that is interspersed with tiny crystals
Summary
The lithium-aluminum-silicate (LAS) glass ceramic ZERODUR® (Trademark of SCHOTT AG, Germany) is a material full of innovations even more than 50 years after its introduction. This is, for example, the very high controllability of the absolute value and the outstanding homogeneity of its key property For optical glass, this is the index of refraction, and for ZERODUR, it is its low thermal expansion. The first part concentrates on the low thermal expansion under ambient conditions, within the cryo temperature range, and on the expansion measurement methods used It treats the resistance of ZERODUR against ionizing radiation present in space applications. The second part highlights the homogeneity of the thermal expansion, the material homogeneity, and the mechanical strength It presents the progress in manufacturing technology that enables extremely light-weighted mirror blanks and large 4-m mirror blanks to generate surface geometry and texture ready for polishing
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