European Extremely Large Telescope secondary mirror. This image shows an artist's impression of the secondary (M2) mirror that will be used on ESO's Extremely Large Telescope (ELT). Designed and manufactured by Sener, the secondary mirror is 4.2 metres in diameter with a convex shape. The total weight for the M2 system will be less than 12 tonnes. When completed, this optical/infrared telescope will have a 39.3-metre diameter primary mirror, the largest on Earth, and will be used to study extrasolar planets, super-massive black holes and dark matter.