By Dr David
BBC News Online science
The main mirror for what will be the most powerful telescope on Earth has been installed at its Arizona site.
Two mirrors will stare at the Universe
The Large Binocular Telescope will have two identical mirrors each 8.4m across, which should enable it to see planets around other stars, scientists believe.
"First light", the time it starts its work, is expected later this year.
When both mirrors are installed, the LBT will be the world's most advanced optical telescope with images sharper than the Hubble Space Telescope's.
The mirror was cast and figured at the University of Arizona's Steward Observatory Mirror Lab and transported 220km (135 miles) to the top of Mount Graham where the telescope is being constructed.
Now that the mirror has been installed, engineers can test the telescope's support and control systems.
Two identical mirrors, each costing $22m, will be placed in the binocular telescope.
The first of the two mirrors is now in place
Work on the project began in 1996 and will be completed next year.
The telescope's twin mirrors will combine their images, providing the light-gathering power of a much larger, single mirror, equivalent to one 11.8m across.
"It will be the first of a new generation of extremely large telescopes and will signal a new golden era in this type of space exploration," says Peter Strittmatter, president of the Large Binocular Telescope Corporation.
"This is a huge step in what has been a very long and challenging process," added John Hill, the project's director.
The second mirror is still being polished and will be transported to Mount Graham at a later date.