4-meter mirror blank cast for the ELT project

The ELT team of SCHOTT Advanced Optics cast the 4.25-meter secondary mirror substrate (M2) for the Extremely Large Telescope (ELT) mid of May. Due to the very narrow specification for the quality of the glass, the casting had to meet extremely high demands. After the grinding, a defect-free, extremely curved mirror substrate only 10 centimeters thick that weighs three tons is to be created from the blank that still weighs several tons. “This represents a major challenge because the M2 will be the largest convex mirror ever produced,” says ELT Project Manager Thomas Werner.

After it was transported from the main plant to the Rhine shipyard on Rheinallee, the mirror substrate will now rest under a hood for several weeks. The originally 1.400° C hot material will first be cooled down to room temperature for an initial quality test. The blank will then be heated up to around 700° C. During this temperature process, the raw glass will be transformed into ZERODUR® glass-ceramic by way of crystallization. The material will have a thermal expansion coefficient close to zero and practically not expand at all during temperature fluctuations.
The flattened mountaintop of Cerro Armazones
In the next step, the mirror substrate will be transported back to the main plant. The workpiece will then be ground precisely into an extremely curved shape that comes close to the final geometry on a new CNC machining machine. The secondary mirror is scheduled for delivery in January 2019, in order to then continue its journey to near Paris for fine polishing at REOSC, the optical department of the SAFRAN Group.

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