The European Extremely Large Telescope to be built in Chile will be a giant light bucket with extremely sharp vision. How will that be achieved despite the Earth’s murky atmosphere?

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Duration: 2 minutes 37 seconds

Many hands make light work of building a full-size replica of the main mirror of the E-ELT. Visitors to the ESO Open Day in Munich, Germany on 15 October 2011 took only four hours to put 798 cardboard hexagons, each 1.4 metres wide, into place. Courtesy ESO/M. Kornmesser/L. Calçada. Music: Movetwo.

The ESO signed an agreement with Chile on Monday 17 October 2011 to build the E-ELT on the mountain Cerro Armazones with Chile donating almost 200 square km of land for the project. Only a final go ahead is needed and the telescope could have first light in a decade or so.

One of the aims of the giant telescope is to image Earth-like planets circling around other stars. This is a very challenging aim and to achieve it is necessary to minimise the image of the parent star. Although a distant star is just a point of light, the wave nature of light broadens the image in any telescope. The larger the telescope the smaller the star images that it records. Hence a large telescope is necessary to have any possibility of imaging a faint planet circling a star without the image being swamped by the light of the parent star.

A serious barrier to achieving diffraction-limited sharpness in images from any large telescope is the Earth’s atmosphere. As starlight travels through the Earth’s turbulent atmosphere it is bent slightly in constantly changing directions. This broadens the resulting images so that the image from a large telescope is no sharper than that from a small amateur telescope. So what’s the point of building a telescope with a mirror almost 40-metres in diameter?

In recent years astronomers have learnt to compensate for the broadening of images by the atmosphere using a technique called adaptive optics. In an article in the January-February 2011 issue of Australian Physics, Jason Spyromilio, who once worked at the Anglo-Australian Observatory in Sydney and is now the head of the E-ELT project office, explained how that is to be implemented on the giant telescope.

There are to be five mirrors altogether. There is to be the almost 40-metre wide main mirror made of 798 hexagons, a large convex secondary mirror and then a third concave mirror. The magic is in two flat mirrors that take the light from the three curved mirrors and redirect it to the instruments. The shape of the first flat mirror has 7000 push-pull devices on its back so it can change its shape one thousand times each second to compensate for the blurring in the light created by its passage through the Earth’s turbulent atmosphere. The changes in shape can also allow for any distortions due to the telescope optics. Finally, the last mirror can move up to ten times a second to compensate for any shifts in direction by one or more of the other mirrors.

With these five mirrors all shaped to very high accuracy, it is believed that the telescope will achieve the sharpness that is theoretically possible from its large width. According to my rough calculations, at that sharpness in infra-red light the telescope, if placed in Sydney, could easily see a five cent coin held up in Melbourne. Alternatively, it could see craters on the surface of the Moon as small as ten metres in width. Not quite enough to see the astronauts’ footprints, but impressive all the same.

Final note. There are two other extremely large telescopes being planned. Australia is a partner in one of these, the 27-metre wide Giant Magellan Telesscope.

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