“Extremely” precise

New CNC machine enables highly accurate processing of mirror components for the Extremely Large Telescope (ELT)

Before a telescope mirror can perform its actual function in a telescope, a long and challenging path lies before it. This applies not only to the transport to the place of use, but especially to the many steps during the manufacturing process. After all, high-precision optical components are crucial for the quality of images from distant galaxies – and thus the success of the entire project.

The ESO (European Southern Observatory) Extremely Large Telescope has once again raised the bar for specifications of individual components with its 39-meter main mirror and a sophisticated mirror system. The SCHOTT technology group, which recently received the order for the secondary (M2) and tertiary mirror (M3) of the ELT, is ideally prepared for this super project with its state-of-the-art equipment: A new 5-axis CNC machining center, which has now been completed, will enable ZERODUR® glass-ceramic mirrors to be produced up to five meters in diameter dimension with a not achieved precision.
Thomas Werner, project leader ELT at SCHOTT
“The machining is so precise that ZERODUR® mirror substrates with a surface shape tolerance of less than 100 micrometers, or 0.1 millimeters, can be ground in the future,” explains Thomas Werner, who is responsible for the ELT project at SCHOTT. “The precision is thus twice as much than with the previous machine, which is at 0.2 millimeter envelope curve accuracy, the technical term used,” the expert adds.

The swivel head of the “Computerized Numerical Control” (CNC) machine can grind in all directions and the speed of movement of the grinding tools is 60 meters per minute when changing tools – the spectrum ranges from rough grinding to so-called heavy cutting of the required final geometry during fine grinding. Thomas Werner: “During the complex grinding process, glass-ceramic substrates can also reduce the weight by 10 tons. This applies to the M2 mirror substrate, for example. It is only 100 millimeters thick, weighs 3.5 tons, is 4.25 meters in diameter and will have an extreme radius of curvature.”
Other special features: The CNC machining center can automatically change the spindle head with a work spindle. This process takes two minutes. Manually, this has taken eight to sixteen hours before because of its very elaborate orientation. The 140-ton facility at the SCHOTT plant in Mainz is based on a new foundation, which was designed by an expert company and required 700 tons of concrete. It will be running around the clock in triple shifts. For the first time, the processing table is not exclusively made of steel but includes a special ceramic cover that protects the table against corrosion and replaces expensive stainless steel. Also new is a so-called turner, with which the mirror substrates can be turned automatically within only two minutes. In addition, since a 32-ton crane has been purchased for the loading of the machine and the turner, the company will be independent of external service providers in the future, which greatly increases our own flexibility and shortens delivery times.
Due to the numerous optimizations, the CNC machining center at SCHOTT will offer greater process flexibility in future mirror substrate manufacturing for the ELT. Thomas Werner concludes: “With a second melting tank and the fully-simultaneous CNC machining center for our ZERODUR® glass-ceramic, we are ‘extremely’ well prepared for the ELT project.” After the M4 mirror substrates left the plant in Mainz in 2016, the secondary mirror will be delivered in January 2019 and the M3 mirror in June 2019. ZERODUR® glass-ceramic has a thermal expansion coefficient close to zero and is now used in all major telescope projects.
The optical system of the E-ELT and the role of ZERODUR®
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