CTE homogeneity of ZERODUR® in the ELT century

The excellent homogeneity of the coefficient of thermal expansion (CTE) is the most advantageous property of ZERODUR^®. Many publications show that the typical CTE homogeneity is in the one digit ppb/K range for parts up to 4.2 m in diameter. Among these evaluations, the CTE homogeneity of two 1.5 m diameter blanks has been characterized with 90 CTE samples evenly spread on 10 cm spatial scale in 2008. Both measurements showed a CTE homogeneity of better 10 ppb/K. In 2016 SCHOTT introduced an advanced dilatometer setup with an improved repeatability. Therefore the extensive CTE evaluation from 2008 was repeated in 2019. The larger statistical reproducibility variations of the earlier dilatometers led to an obscuration of the real homogeneity values, which had been much better. With Monte Carlo simulations this can be demonstrated to be strongest for the most homogeneous items. The results are provided in this paper. SCHOTT started to produce 798 ZERODUR^® mirror blanks for the primary mirror of the ELT M1, 39 m in diameter, in 2019. In January 2019 SCHOTT has already successfully finished and delivered the ZERODUR^® mirror blank for the secondary mirror (M2) 4.25 m in diameter of the extremely large telescope (ELT) of ESO, followed by a ZERODUR^® mirror blank for the tertiary mirror (M3) 4 m in diameter in January 2020. All ELT mirror blanks have stringent CTE homogeneity requirements. Therefore, it is a good time to give an update on the CTE homogeneity of ZERODUR^® in the ELT century.