Magellan super-scope gets green light for construction

By Jonathan AmosBBC Science Correspondent

The overall cost of the GMT is projected to be just over one billion dollars

Construction of the Giant Magellan Telescope has been given the go-ahead.

One of the largest optical observing systems ever conceived, the GMT will sit atop Cerro Las Campanas in Chile.

With its 24.5m-wide primary mirror system, astronomers should be able to see the first objects to emit light in the Universe, investigate dark energy and dark matter, and identify potentially habitable planets.

The GMT's international partners have all approved the $500m assembly phase.

Contracts against this money can now be awarded to suppliers.

Flattened mountain

The mountain ridge of Las Campanas itself, which is in the Atacama Desert, is ready to receive the observatory's components.

Two-and-a-half-thousand cubic metres of rock have been removed from its southern end to create a flat surface the size of four football fields. A road is in place to take all the elements to the summit when they become available.

Chief among these, of course, will be the seven 8.4m mirrors that comprise the GMT's primary reflecting surface.

Three are already at various stages of production (one is actually finished); the other four will begin their manufacture very soon.

"We expect in late 2021, possibly in early 2022, we will put three or four primary mirrors in the telescope, start doing some engineering, start doing some astronomy, and by that point we will have the largest (optical) telescope on the planet by a good margin," said GMT director, Pat McCarthy.

"We'll then slowly integrate the rest of the mirrors as they come along so that by 2024 or 2025, we should have all seven mirrors in the telescope," he told BBC News.

The GMT is one of three ground-based optical super-scopes planned for the next decade.

The other two are the European Extremely Large Telescope (E-ELT), also in Chile, and the Thirty Meter Telescope (TMT), to be sited in Hawaii.

Construction of the latter has been in the news of late because of a dispute with Native Hawaiian activists, who say the installation on Mauna Kea volcano is a desecration of sacred land.

With primary diameters of 39m and 30m respectively, the E-ELT and the TMT will be bigger than the GMT at completion. They will also have a very different architecture in that their big reflecting surfaces will be made up of many hundreds of smaller mirror segments.

The designers of the GMT, on the other hand, think their decision to go for just seven large units will pay dividends in certain types of observations.

Sharper than Hubble

"We think there is great advantage in having as much of your collecting area as possible in a contiguous, uninterrupted optical surface," explained Dr McCarthy, a researcher from Carnegie Observatories.

"This will limit the number of phase jumps that you have. So, for high-contrast applications, where you're using adaptive optics, imaging planets around nearby stars - we feel that having this much of the telescope's aperture as coherent pieces of glass will help us when we try to achieve that very high contrast."

The "adaptive optics" of which Dr McCarthy speaks is really now a must-have in modern astronomy.

Such systems counter the "twinkling" of stars and other astrophysical phenomena caused by their light passing through Earth's turbulent atmosphere.

By manipulating flexible secondary mirrors, it is possible to subtract this effect based on information gleaned from artificial stars projected on to the sky by lasers.

This approach, together with its great aperture size, should enable the GMT to capture images that are 10 times sharper than those from the Hubble Space Telescope.

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