SCHOTT Delivers World’s Largest Convex Mirror Substrate for the European Southern Observatory’s Extremely Large Telescope
In the ELT, M2 will reflect the light from the 39-m main mirror extremely precisely onto the downstream mirror.
SCHOTT recently announced that it has manufactured a curved secondary mirror substrate made of ZERODUR® glass-ceramic in Mainz, Germany, for the European Southern Observatory’s (ESO) Extremely Large Telescope (ELT). The substrate is 10 cm thin, with an extreme curvature, a diameter of 4.25 m and a weight of 3 tons. The substrate was manufactured for the ELT’s secondary mirror (Mirror 2, or M2).
After 16 months of production time, the substrate is journeying to France from the main plant in Mainz in a huge, extra-wide transport box. It will receive its “final touch” there, comprising fine polishing at REOSC, a SAFRAN Group company. The actual mirror layer made of silver with a wafer-thin layer of silicon oxide as a protective film will be applied at the coating facility of the Paranal Observatory in Chile before it is mounted in the telescope.
The requirements for the casting of the 4.25 m mirror substrate were extremely high due to very narrow specifications for the glass quality. Last year, a highly convex mirror substrate 10 cm thin was ground from the blank weighing more than 10 tons on state-of-the-art CNC machines.
“It was a great challenge for our melting team and the experts running the CNC machines,” said Thomas Westerhoff, Ph.D., head of the strategic business field for ZERODUR at SCHOTT. “A workpiece with this geometry had never been manufactured before: The M2 will be the largest convex mirror ever made.”
In the ELT, M2 will reflect the light from the 39-m main mirror extremely precisely onto the downstream mirror. After the “First Light,” the commissioning of the ELT in 2025 on the mountain Cerro Armazones in Chile, its performance will provide the sharpest view into space compared to previous telescopes.
SCHOTT is manufacturing the substrate material for four of the five mirror components of the ELT optics: for the ESO’s ELT 39-m primary mirror with 798 hexagons (plus 131 replacement segments), the 4.25-m secondary mirror that is now being delivered, the 4-m tertiary mirror, and the segmented fourth mirror (M4). Additional details are available at www.schott.com.