Why does the E-ELT have a mirror diameter of 42 metres?
The size of a telescope is important for two reasons: one is the amount of light it can collect and the other is the level of detail it can see. With its 42-metre diameter, the E-ELT will gather 15 times more light than the largest optical telescopes operating today. It will also provide images 15 times sharper than those from the Hubble Space Telescope. The E-ELT performance is thus orders of magnitude better than the currently existing facilities. Such a telescope may, eventually, revolutionise our perception of the Universe, much as Galileo’s telescope did, 400 years ago. The 42-metre diameter turned out to be the minimum diameter needed to achieve some of the driving science cases: to image rocky exoplanets to characterise their atmospheres and to directly measure the acceleration of the expansion of the Universe.
