Abstract
Observational astronomy has sought better telescopes with higher resolution from its beginning. This needs ever-larger mirrors with stable, high-precision surfaces. The extremely low-expansion glass ceramic ZERODUR® has enabled such mirrors for more than 50 years with significant improvements in size and quality since then. We provide a survey of the progress achieved in the last 15 years. Equally important as the thermal expansion coefficient CTE is its homogeneity. The CTE variation in 4-m mirror blanks lies below 5 ppb / K in radial and axial directions on large and short scales. Improved measurement capabilities allow reduced bias, which in the past made variations look greater than they were. Isotropy and uniformity of ZERODUR are outstanding. A method for lifetime calculation increases reliability considerably with respect to mechanical loads. The production and metrology capability and capacity are greatly extended. Surface figure and texture of large blanks allow starting directly with polishing. Filigree structures with up to 90% weight reduction are well-suited for space mirrors. The progress with low thermal expansion and its measurement, the insensitivity of ZERODUR against ionizing radiation in space, and outstanding application examples are presented in the first part of our review.
Highlights
Fifty years after its introduction, one could expect that all properties of a material are explored and well known
The first one shows it in lateral illumination free from inclusions, the second one is the image on the shadow-graph screen free from striae, and the third one is the refractive index homogeneity map based on the wave front image of a laser interferometer revealing optical homogeneity as good as that of the best optical glass
Even though ZERODUR is a young material with 50+ years of existence, it is sometimes regarded as a classical material with all properties well known, settled, and fixed
Summary
Fifty years after its introduction, one could expect that all properties of a material are explored and well known. The progress is not restricted to only one property It covers a wide range including the key property of low thermal expansion as well as homogeneity, radiation resistance, mechanical strength, and workability to large mirrors and other large, lightweight structures. Many outstanding properties of optical glass can be found in ZERODUR: 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 low thermal expansion. This is the index of refraction, and for ZERODUR it is low thermal expansion Further parallels are their very high internal material quality (low bubble and inclusion, striae, and stress content) and the capability to produce large (meter-sized) items. The second part highlights the homogeneity of the thermal expansion and the material homogeneity, the mechanical strength and progress in manufacturing technology of extremely lightweighted mirror blanks, or large 4-m mirror blanks ready for polishing
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