The ZERODUR production, consisting of established processes used in the manufacturing of high homogeneous optical glasses, results in excellent blanks with low stress birefringence, striae content and outstanding homogeneity of the coefficient of thermal expansion. For future extremely large telescope projects like OWL (OverWhelmingly Large Telescope) or TMT (Thirty Meter Telescope), with at least several hundreds of mirror blanks, the material homogeneity within a single blank is extremely important. Previously, the stress birefringence of 2m class mirror blanks could be reduced to amounts far below our catalog values. Striae in ZERODUR, if present, are weak band-like density fluctuations within the material with only minimum influence on the properties of the material. This paper presents the results of dilatometric measurements on the influence of standard grade striae within ZERODUR on the homogeneity of the CTE. All CTE measurements have been carried out using our new dilatometer setup with improved reproducibility.
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