Corning 7972 ULE material for segmented and large monolithic mirror blanks

Ultra-Low Expansion (ULE®) glass has been and continues to be a significant material for astronomical applications. With a nominal composition of 7 wt. %TiO2 in SiO2, Corning Code 7972 ULE® has a mean room temperature coefficient of thermal expansion (CTE) of 0 ± 30 ppb/°C with a typical CTE range of less than 15 ppb/°C, properties vital to the manufacture of high resolution optics requiring extreme thermal stability. Combined with lightweighting techniques developed at Corning during the past 30 years, ULE® has been successfully employed for numerous monolithic and lightweight mirror applications including the 2.4 meter Hubble Space Telescope lightweight primary mirror, the Airborne Laser (ABL) primary mirrors, and most recently the Discovery Channel Telescope 4 meter mirror blank. ULE® maintains its strong candidacy for future ELT applications. Recent challenges in mirror surface specifications and the development of alternative material choices calls for a comparison with ULE®. The objective of this article is to review ULE® properties and manufacturing capabilities, and to compare relevant material properties to those of alternative material options, thus allowing designers to properly execute material selection. Finally, recent development efforts directed toward improving ULE® will be discussed.

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