How do you build a 39-metre mirror?

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.