Long-term reliability of large ULE mirror blanks

Suresh T. Gulati; Mary J. Edwards

doi:10.1117/12.319294

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25 August 1998 Long-term reliability of large ULE mirror blanks

Suresh T. Gulati, Mary J. Edwards

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Proceedings Volume 3352, Advanced Technology Optical/IR Telescopes VI; (1998) https://doi.org/10.1117/12.319294
Event: Astronomical Telescopes and Instrumentation, 1998, Kona, HI, United States

Abstract

ULE^TM, the titania-silica binary glass with zero expansion coefficient, is an ideal material for large telescope mirror blanks due to the unique combination of its optical, thermal and mechanical properties--together with the ease of fabrication--which help meet performance and durability requirements in a cost-effective manner. Indeed, the 8m class Subaru and Gemini telescope mirror blanks have been fabricated successfully from ULE glass and will be in full operation in the not too distant future. This paper will focus on the stringent reliability requirements which the mirror blank must meet during fabrication, transportation, installation and operation atop high mountains with extreme environmental fluctuations. In particular, the paper will present strength and fatigue data for ULE glass as a function of surface finish. Such data are critical for selecting the appropriate surface finish to ensure mechanical reliability of the mirror blank at various stages of fabrication and during transportation. The use of Weibull statistical distribution for surface flaws combined with Power Law fatigue model helps arrive at a safe stress level which should not be exceeded to ensure the mechanical reliability of the mirror blanks. The safe stress level is verified through independent static fatigue tests on ULE discs with surface finish identical to that of the mirror blank. In this manner the mechanical reliability of large ULE mirror blanks can be ascertained at extremely low failure probabilities. The successful application of reliability model to both Subaru and Gemini mirror blanks will be illustrated.

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Suresh T. Gulati and Mary J. Edwards "Long-term reliability of large ULE mirror blanks", Proc. SPIE 3352, Advanced Technology Optical/IR Telescopes VI, (25 August 1998); https://doi.org/10.1117/12.319294

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