Image caption The E-ELT would take advantage of the very arid conditions in Chile's Atacama desert

Construction of world's biggest optical telescope has been approved.

The European Extremely Large Telescope (E-ELT) will have a primary mirror some 40m in diameter, and will be built on top of a mountain in Chile.

Member states of the European Southern Observatory (Eso) organisation declared their support for the project on Monday at a meeting in Garching, Germany.

They have not yet, however, put all of the 1bn euros (£0.8bn) of financing in place.

That may be possible by December, at the organisation's next council meeting.

By then, Brazil should also have become the 15th full member of Eso, further spreading the E-ELT's cost and making it more affordable for all nations.

The telescope should be ready for use by about 2022, and will be one of the key astronomical facilities of 21st Century, complementing other huge observatories that will view the sky at different wavelengths of light.

Voting positions

The E-ELT will detect objects in the visible and near-infrared. Its 39.3m main mirror will be more than four times the width of today's best optical telescopes (antennas for radio telescopes are still very much bigger).

Its sensitivity and resolution should make it possible to image directly rocky planets beyond our Solar System.

To see this technology being brought together in the E-ELT, with European leadership, is simply wonderful Prof Gerry Gilmore, Cambridge University

The observatory should also be able to provide major insights into the nature of black holes, galaxy formation, the mysterious "dark matter" that pervades the Universe, and the even more mysterious "dark energy" which appears to be pushing the cosmos apart at an accelerating rate.

At the Garching meeting, six nations (Austria, the Czech Republic, Germany, the Netherlands, Sweden and Switzerland) were in a position to declare their full support to proceed with the project; four nations (Belgium, Finland, Italy, and the United Kingdom) declared their support pending approval from their governments; and the remaining four (Denmark, France, Portugal, Spain) said they continued to work towards full approval.

The commitments from two-thirds of the members are sufficient at this stage to permit Eso to proceed with the project.

Prof Isobel Hook is the UK E-ELT project scientist from the University of Oxford. She said Monday's decision was hugely exciting.

"We've all been working towards this moment for a long time, and this decision means we're now just a few years away from using this telescope," she told BBC News.

"The E-ELT's great size will give us much sharper images, provided we can correct for atmospheric turbulence [which makes stars twinkle], and that will be part of the telescope's design. The E-ELT will also have a much larger collecting area than any telescope we have now. That combination of sharpness and collecting area is what will make it so powerful."

Image caption Eso member states took the historic decision at their council meeting in Garching, Germany

The telescope will be sited on Cerro Armazones, a mountain that is just 20km from Cerro Paranal where Eso currently operates its Very Large Telescope (VLT) facility - a suite of interconnected optical telescopes that includes four units with primary mirrors measuring 8.2m.

Like Paranal, Armazones will enjoy near-perfect observing conditions - at least 320 nights a year when the sky is cloudless.

The famous aridity found in Chile's Atacama desert means the amount of water vapour in the skies above the observatory will be very limited, reducing further the perturbation starlight experiences as it passes through the Earth's atmosphere.

One of the first tasks will be to remove the top of the mountain to make a flat base for the telescope and its housing, which will be the size of a football stadium.

Even before that work is undertaken, a road will need to be constructed on the slopes of Cerro Armazones to get heavy earth-moving equipment on site.

The funding for the road and some design work on the E-ELT's Number Four mirror was approved at last December's council meeting.

Europe at the front

Eso's principles require 90% of the funding to be in place before spending goes beyond the initial civil works.

The total cost of the venture is currently projected to be 1,083 million euros (at 2012 prices).

Prof Gerry Gilmore at Cambridge University played a key role in the early definition phases of the E-ELT, bringing together various competing ideas into a single project.

He told the BBC: "We all know the grand questions we want to ask - 'What is time? What is existence? What is reality? Is there life out there?'. And we know that we need technology to answer those questions. So, to see this technology being brought together in the E-ELT, with European leadership, is simply wonderful."

39.3m-wide primary mirror (M1) is made up of almost 800 segments M2 is 4.2m wide and hangs upside down. It will weigh less than 12t 3.8m-wide M3 sits in a hole in M1. M3 moves with M2 and M4 to focus 2.4m-wide M4 can deform its shape to remove twinkling in stars M5 is 2.6m x 2.1m. It stabilises the light on to the instrument detectors Lasers make artificial stars on the sky to help correct the imaging E-ELT will have two instrument decks; each deck to hold three units The telescope will be sensitive to visible and near-infrared light

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter