Large amounts of low thermal expansion material are required for the upcoming ELT projects. The main mirror is designed using several hundreds of hexagonal 1.4 m sized mirror blanks. The M2 and M3 are monolithic 4 m class mirror blanks. The mirror blank material needs to fulfill tight requirements regarding CTE specification and homogeneity.

Additionally the mirror blanks need to be dimensionally stable for more than 30 years. In particular, stress effects due to the changes in the environment shall not entail shape variation of more than 0.5 μm PV within 30 years.

In 2010 SCHOTT developed a physically based model to describe the thermal and mechanical long time behavior of ZERODUR. The model enables simulation of the long time behavior of ZERODUR mirror blanks under realistic mechanical and thermal constraints. This presentation shows FEM simulation results on the long time behavior of the ELT M1, M2 and M3 mirror blanks under different loading conditions. Additionally the model results will be compared to an already 15 years lasting long time measurement of a ZERODUR sample at the German federal physical standardization institute (PTB).

In recent years SCHOTT pushed the push rod dilatometer measurement technology to its limit. With the new Advanced Dilatometer CTE measurement accuracies of +- 3 ppb/K and reproducibilities of better 1 ppb/K have been achieved. The new Advanced Dilatometer exhibits excellent long time stability.

Date Published: 22 July 2016
PDF: 16 pages
Proc. SPIE 9912, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II, 99120J (22 July 2016); doi: 10.1117/12.2231086

Published in SPIE Proceedings Vol. 9912:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation II
Ramón Navarro; James H. Burge, Editor(s)