The present core concept is for a telescope with a mirror 39 metres in diameter, covering a field on the sky about a tenth the size of the full Moon. The mirror design itself is revolutionary and is based on a novel five-mirror scheme that results in an exceptional image quality. The primary mirror consists of almost 800 segments, each 1.4 metres wide, but only 50 mm thick. The optical design calls for an immense secondary mirror 4.2 metres in diameter, bigger than the primary mirrors of any of ESO's telescopes at La Silla.

Adaptive mirrors are incorporated into the optics of the telescope to compensate for the fuzziness in the stellar images introduced by atmospheric turbulence. One of these mirrors is supported by more than 6000 actuators that can distort its shape a thousand times per second.

The telescope will have several science instruments. It will be possible to switch from one instrument to another within minutes. The telescope and dome will also be able to change positions on the sky and start a new observation in a very short time.

The ability to observe over a wide range of wavelengths from the optical to mid-infrared will allow scientists to exploit the telescope's size to the fullest extent.