European Extremely Large Telescope mirror system. This diagram shows the novel 5-mirror optical system of the European Southern Observatory's Extremely Large Telescope (ELT). Before reaching the science instruments the light is first reflected from the telescope's giant concave 39-metre segmented primary mirror. It then bounces off two further 4-metre-class mirrors, one convex and one concave. The final two mirrors form a built-in adaptive optics system to allow extremely sharp images to be formed at the final focal plane. The telescope is under construction on top of the Cerro Amazones mountain in the Atacama Desert, Chile.