The ELT has a main mirror 39 metres in diameter, covering a field of view on the sky about a tenth of the size of the full Moon. The telescope's "eye" will be almost half the length of a soccer pitch in diameter and will gather 15 times more light than the largest optical telescopes operating today. The mirror design itself is revolutionary, and is based on a novel five-mirror scheme which results in exceptional image quality. The primary mirror consists of 798 segments, each 1.4 metres wide and only 50 mm thick. The optical design calls for a secondary mirror four metres in diameter — the largest secondary mirror ever employed on a telescope, and the largest convex mirror ever produced.
ESOcast 37: Full-size Mock-up of World's Largest Telescope Mirror Built at ESO's Open House Day. Download this video in other formats from the video archive.
Adaptive mirrors are incorporated into the optics of the telescope to compensate for the twinkling of astronomical sources caused by atmospheric turbulence. One of these mirrors, M4, is supported by more than 5000 actuators that can correct its shape a thousand times per second.
The telescope will have several science instruments, and it will be possible to switch quickly from one instrument to another. The telescope and dome — which together will weigh more than 8000 tonnes — will also be able to change positions on the sky and start a new observation within minutes.
The ability to observe over a wide range of wavelengths from the optical to the mid-infrared will allow scientists to exploit the telescope's size to the fullest extent, allowing astronomers to tackle the biggest scientific challenges of our time.