English: This diagram shows the novel 5-mirror optical system of ESO'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.