Work starts on world's largest ground-based telescope

Technicians blasted a Chilean mountaintop to rubble Thursday to level a perch for what will be the world's largest ground-based telescope.

Scheduled to take its first peek at the night sky in 2024, the Extremely Large Telescope will boast a main, light-gathering mirror 39 meters across – just under half the length of a football field.

The telescope's size, an ability to take the twinkle out of cosmic objects, plus superb "seeing" conditions high in Chile's Atacama Desert, will give astronomers an unprecedented look at the earliest galaxies more than 13 billion light-years away.

It will allow them to glean information about extra-solar planets in the sun's neighborhood as researchers hunt for other planets capable of hosting life. It will help reveal details about the behavior of supermassive black holes and the influence they wield over the evolution of the galaxies that host them. And it could uncover clues as to the nature and distribution of dark matter and dark energy.

The E-ELT, as the telescope project is known, is one of three extremely large telescopes in various stages of progress. It is overseen by the European Southern Observatory, headquartered in Garching, Germany.

The Giant Magellan Telescope project cleared its Chilean mountaintop two years ago, passed its final design reviews in February and is in a position to begin construction. And in April, the Thirty Meter Telescope project survived a legal challenge to the process for siting its eye on the sky. The telescope will join other observatories atop Hawaii's Mauna Kea.

The science case for these behemoths as been strong all along, knowing that the current generation of large telescopes would bump up against its observational limits, according to Robert Kirshner, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and a member of the Giant Magellan Telescope's board of directors.

This class of 30-meter to 40-meter telescopes has about 100 times the light-gathering ability of the venerable Hubble Space Telescope. And because all are designed to use adaptive optics to remove the image-distorting effects of the atmosphere, they will produce images at least 10 times sharper than Hubble's, Dr. Kirshner says.

"We knew they would be fantastic for studying the distant galaxies and tracing cosmic evolution" in conjunction with NASA's James Webb Space Telescope, he says of these new ground-based telescopes.

But during what he dubs "the larval stage" of these projects, an unanticipated field virtually exploded with the discovery of more than 1,000 extrasolar planets. Many of these objects are "crying out for an investigation of their properties," Kirshner says, a task for which "the extremely large telescopes will be superb."

Indeed, in addition to their own discoveries, these telescopic monsters will become astronomy's workhorses as they conduct systematic studies of objects uncovered by space-based observatories such as NASA's Transiting Exoplanet Survey Satellite and James Webb Space Telescopes.

The exoplanet-survey satellite is slated for launch in 2017, while the James Webb Space Telescope is slated for a 2018 launch.

Of the three, the E-ELT is perhaps the most technically ambitious. The ESO is building it atop Cerro Amazones, a summit about 13 miles east of the ESO's Very Large Telescope array, another premier observatory.

The E-ELT's 39-meter main mirror is built up using 798 hexagonal segments, each about 1.45 meters across and 5 centimeters thick. It's critical that the mirror hold its subtly concave shape, so each segment is designed to be tipped and tilted, as well as slightly distorted by pistons. These techniques prevent the mirror from shape shifting with temperatures changes or when astronomers change the telescope's elevation.

The Giant Magellan Telescope, by contrast, uses an array of seven mirrors, each 8.4 meters across, to achieve its large light-collecting area. Three of the mirrors already have been made, with a fourth slated for casting at the University of Arizona's Stewart Observatory Mirror Laboratory this year.

Many of today's telescopes use two mirrors – the large primary mirror to gather as much light as possible, and a secondary mirror centered above the primary. This secondary mirror collects the light from the primary and focuses it on cameras or spectrographs bolted to the back of the telescope.

The E-ELT uses five mirrors to transfer light from cosmic objects to its instruments.

One of these mirrors, 2.4 meters across, represents the adaptive-optics part of the light's path as it travels from a distant galaxy to the telescope's instruments. The mirror will host up to 8,000 tiny actuators behind it. These actuators will continuously massage the mirror to remove distorting effects of the atmosphere on incoming light.

In all, the housing for the telescope will be large enough to swallow the Arc de Triomphe in Paris.

All three ELT projects, which by some estimates will cost a collective $3.3 billion, still face technical and funding hurdles. And they all are said to be continuing their hunt for additional collaborating universities and research institutions.

For now, the ESO is happy just to have a road to the top of the mountain, a place to call home at the top, and a power line, noted Xavier Barcons, president of the ESO's governing council, during a ceremony that preceding the first blast atop Cerro Amazones.