How do you build a 39-metre mirror?
The 39-metre primary mirror of the Extremely Large Telescope (ELT) will consist of hundreds of hexagonal segments. Four additional mirrors complete the novel design of the telescope. The ELT is going to be a showcase of European technology.
The 10-metre Keck telescope in Hawaii was the first to use a segmented primary mirror. But instead of Keck’s 36 segments, the 39-metre primary mirror of the Extremely Large Telescope will have 798 segments; together they have a collecting area of 978 m2. A novel design with four additional mirrors will correct the optical aberrations of the giant elliptical primary. One of these is the adaptive optics mirror with more than 5000 actuators which will correct for atmospheric turbulence up to 1000 times per second. The 3000-tonne telescope is a high-technology challenge for European industry.
You can’t make a 39-metre mirror from a single piece of glass. Instead, the main mirror of the ELT is like a giant jigsaw puzzle, with 798 hexagonal segments.