Free Astronomy Magazine September-October 2024

38 ASTRO PUBLISHING requirements. The outstanding qual- ity of the blanks was maintained throughout the mass production of more than 230 tonnes of this super- performing material. ESO is thus very thankful for the professionalism of the skilled teams at SCHOTT, our trusted partner.” Thomas Werner, ELT Project Lead at SCHOTT, says: “Our en- tire team is thrilled to conclude what has been the largest single order of ZERODUR® in the history of our com- pany. For this project, we successfully concluded the serial production of hundreds of ZERODUR® mirror sub- strates, when we usually have a sin- gle-piece operation. It’s been an honour for all of us to play a part in shaping the future of astronomy.” Once cast, all segments follow a multi-step, international journey. After a slow cooling and heat treat- ment sequence, the surface of each blank is shaped by ultra-precision grinding at SCHOTT. The blanks are then transported to French company Safran Reosc, where each of them is cut into an hexagon shape and pol- ished to a precision of 10 nanometres across the entire optical surface — meaning the surface irregularities of the mirror will be less than one thou- sandth of the width of a human hair. by ESO − Bárbara Ferreira T oo large to be made from a sin- gle piece of glass, M1 will consist of 798 hexagonal segments, each about five centimetres thick and 1.5 metres across, working together to collect tens of millions of times as much light as the human eye. An ad- ditional 133 segments have been pro- duced to facilitate the maintenance and recoating of the segments once the telescope is operational. ESO has also procured 18 spare segments, bringing the total number to 949. The M1 blanks, shaped pieces of ma- terial that are later polished to be- come the mirror segments, are made from ZERODUR®, a low-expansion glass-ceramic material developed by SCHOTT and optimised for the ex- treme temperature ranges at the ELT’s site in the Atacama Desert. This company has also manufactured the blanks of three other ELT mirrors — M2, M3, and M4 — at their facilities in Mainz, Germany. “What ESO ordered from SCHOTT is more than just ZERODUR®,” says Marc Cayrel, Head of ELT Optome- chanics at ESO. “In close collabora- tion with ESO, SCHOTT fine-tuned every single production step, tailor- ing the product to meet and often exceed the ELT’s very demanding Last segment blank of the ELT successfully delivered