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Last Updated: Wednesday, 13 December 2006, 14:12 GMT
Final stage for telescope design
E-ELT artist's impression   Image: Eso
The E-ELT will have a 42m-wide primary mirror
A project to build the world's biggest telescope has been given the go-ahead to move into its final design stage.

European Southern Observatory (Eso) officials have given approval for a detailed design study of the European Extremely Large Telescope to proceed.

The 57m euro (38m) design study, will make it possible to start construction of the telescope in three years time.

The telescope's mirror will be 42m in diameter - four times bigger than any other currently in existence.

Eso operates the 8.2m Very Large Telescope (VLT) at Cerro Paranal in Chile, which has been described as a "discovery machine".

An average of 1.5 scientific papers published each day are based on observations carried out at the VLT.

Revolutionary observatory

Eso is now scoping out a new facility that it hopes will revolutionise ground-based astronomy.

It presented a detailed concept for the E-ELT - as the huge telescope has been dubbed - two weeks ago to more than 250 European astronomers at a conference in Marseille.

Officials say the concept received an enthusiastic response, allowing the detailed design phase to move ahead.

"At the end of the three-year final design study, we will know exactly how everything is going to be built including a detailed costing," said Catherine Cesarsky, Eso's director general.

"We then hope to start construction and have it ready by 2017, when we can install instruments and use it."

The present concept is estimated to cost around 800m euros (537m).

Big rethink

Dr Cesarsky says the design will require "a complete rethinking of the way we make telescopes".

The primary 42m mirror is to be composed of 906 hexagonal segments, each 1.45m in size, while the secondary mirror could be as large as 6m in diameter.

In order to compensate for the fuzziness of stellar images due to atmospheric turbulence, the telescope needs to incorporate "adaptive" mirrors.

A tertiary mirror, 4.2m in diameter, relays the light to the adaptive optics system, composed of two further mirrors.

One of these is a 2.5m mirror able to distort its own shape a thousand times per second with the help of many actuators, the other is 2.7m in diameter and makes final image corrections possible.

This five-mirror approach will, Eso hopes, result in an exceptional image quality.

The site of the E-ELT is not yet fixed; Eso plans to make a decision by 2008. Sites that have been considered include South Africa, Tibet, Morocco, Greenland and Antarctica.

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