Last segment of the world's largest telescope mirror successfully cast

Last segment of the world's largest telescope mirror successfully cast
by Robert Schreiber
Berlin, Germany (SPX) Jul 01, 2024
The European Southern Observatory's Extremely Large Telescope (ESO's ELT), currently being constructed in the Chilean Atacama Desert, has reached a significant milestone. German company SCHOTT has successfully cast the final blank for the last of the 949 segments required for the telescope's primary mirror (M1). Once completed, M1 will have a diameter exceeding 39 meters, making it the largest mirror ever created for a telescope.

Due to its immense size, M1 cannot be made from a single piece of glass. Instead, it will be composed of 798 hexagonal segments, each approximately five centimeters thick and 1.5 meters across, working collectively to gather tens of millions of times more light than the human eye. Additionally, 133 segments have been produced for maintenance and recoating purposes, and 18 spare segments have been procured, bringing the total to 949.

The M1 blanks, which are shaped pieces of material later polished into mirror segments, are crafted from ZERODUR, a low-expansion glass-ceramic material developed by SCHOTT, designed to withstand the extreme temperature variations at the ELT's site in the Atacama Desert. SCHOTT has also manufactured the blanks for 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 Optomechanics at ESO. "In close collaboration with ESO, SCHOTT fine-tuned every single production step, tailoring the product to meet and often exceed the ELT's very demanding requirements. The outstanding quality 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, adds: "Our entire team is thrilled to conclude what has been the largest single order of ZERODUR in the history of our company. For this project, we successfully concluded the serial production of hundreds of ZERODUR mirror substrates, when we usually have a single-piece operation. It's been an honor for all of us to play a part in shaping the future of astronomy."

After casting, each segment undergoes a slow cooling and heat treatment sequence. The surface of each blank is then shaped by ultra-precision grinding at SCHOTT. The blanks are transported to the French company Safran Reosc, where each is cut into a hexagon and polished to a precision of 10 nanometers across the entire optical surface, ensuring surface irregularities are less than one-thousandth of the width of a human hair. Additional contributions to the M1 segment assemblies come from Dutch company VDL ETG Projects BV, producing segment supports; the German-French FAMES consortium, developing 4500 nanometric-accuracy sensors for monitoring segment positions; German company Physik Instrumente, designing and manufacturing 2500 actuators for segment positioning; and Danish company DSV, handling segment transportation to Chile.

Once polished and assembled, each M1 segment is shipped to the ELT Technical Facility at ESO's Paranal Observatory in the Atacama Desert, a journey of 10,000 kilometers. Over 70 M1 segments have already made this trip. At Paranal, each segment is coated with a reflective silver layer and stored until the telescope's main structure is ready.

When operational later this decade, ESO's ELT will be the world's largest eye on the sky, addressing the most significant astronomical challenges of our time and making unprecedented discoveries.