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Almost two decades ago, astronomers were looking for the next really big telescope. The so-called Overwhelmingly Large Telescope (OWL) rendered a primary-mirror diameter of 100 m. The OWL was a desktop concept study initiated by the European Southern Observatory (ESO) in 2001 and completed in 2005. SCHOTT participated in the study as manufacturer of the low-thermal-expansion glass ceramic ZERODUR which successfully had been provided to the Very Large Telescope a few years earlier. However, the VLT blanks, with a diameter of 8.2 m, manifested the size limit of monolithic mirror substrate production. Larger-sized primary telescope mirrors were proposed by combining hexagon segments to complete the large mirror optical surface. This approach had been established in the twin KECK telescopes, combining 36 hexagonal segments to form a primary mirror of 10 m in diameter. The OWL study, incorporating the segmented-mirror approach, was targeting at the next level of telescope size: the Extremely Large Telescopes. Three different options on segment size were investigated in the study, yielding segment counts between 1500 and 5000 combined for primary and secondary mirror together. SCHOTT evaluated the technical feasibility of product specification and the process technology required to yield segments at a production frequency large enough to finish the scope of work within a reasonable timescale and budget. Today’s market demands for industrial applications, as well as large telescope projects, triggered SCHOTT to increase capacity and capability via investments enhancing ZERODUR production along the entire production sequence. This contribution comments on the ZERODUR production capacity driven by industrial applications with regard to design options of the OWL study. Today’s installed capacity will enable SCHOTT to deliver the OWL mirror substrates within a timeline comparable to the ELT M1 segment-blank contract. Further capacity expansion required by continuously increase industrial demand will enable even faster timelines.
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