MARSEILLE, France (AFP) -- European astronomers gathered in a meeting that ended here agreed on the technical specifications for what will be the world's biggest optical telescope, an official said.

The so-called Extremely Large Telescope (ELT), budgeted at around a billion euros ($1.3 billion), will have a 42-metre (136.5-feet) -diameter composite mirror.

The "consensus" by astronomers at their four-day meeting will be put to the board of the European Southern Observatory next week, ESO Director General Catherine Cesarsky said.

Several years of more studies are needed before the project breaks ground, at a location which also is yet to be determined. If all goes well, the ELT will start operations in 2018.

The initial design sketched for the telescope was an OWL ("OverWhelmingly Large") design with a 100-metre (325-feet) mirror.

But this was scaled back in 2005 because it was too costly and too complex to build on ESO's budget timescale, said Cesarsky.