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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The room‐temperature refractive index is measured for three different prior cooling rates (approximately 10, 50, and 250 K/h) for two glasses especially developed for precision molding. The empirical logarithmic relationship between the cooling rate and the refractive index is also reproduced for the comparatively high cooling rate of ca. 250 K/h. The same relationship is found in a simulation of these cooling rates by the semiempirical Tool–Narayanaswamy–Moynihan model for structural relaxation, with the necessary parameters obtained from differential scanning calorimetry and temperature jump experiments. The measured and the simulated refractive indices coincide within the limits of experimental error. The results demonstrate that the index drop, which is observed when these glasses are first cooled at a regular optical cooling rate (e.g., 2 K/h), and then precision molded (typical cooling rate 1000 K/h), can be understood considering the concepts of structural relaxation.

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