The world's biggest telescope is getting smaller—but more affordable. The designers of the future European Extremely Large Telescope (E-ELT) have decided to shrink the telescope's primary mirror from a diameter of 42 meters to 39.3 meters. The resulting 13% decrease in sensitivity is likely to reduce its scientific payoff. But the 18% savings in its overall cost gives the telescope a better chance to remain on schedule for first light in 2022.
The decrease in mirror diameter has not yet been officially announced. "But it's part of the new design study that will be presented to the ESO Council," says Tim de Zeeuw, director general of the European Southern Observatory. ESO plans to build the telescope at Cerro Armazones, a 3064-meter-high peak close to its existing Very Large Telescope, which consists of four identical 8.2-meter instruments. ESO's governing council is expected to make a final decision on the project in December.
Apart from a smaller primary mirror (which will consist of hundreds of hexagonal segments), the new E-ELT will also sport a significantly smaller secondary mirror (4.2 meters instead of 5.9 meters), and a smaller and more compact overall structure. The new dimensions will make it harder for the E-ELT to accomplish one of its primary goals, to image Earthlike planets orbiting other stars than the sun. "It hurts," says de Zeeuw, "but we've been able to bring the projected costs down from €1275 to €1055 million [US $1.5 billion]. This will enable us to build the instrument in 10 or 11 years."
Astrophysicist Isobel Hook of the University of Oxford in the United Kingdom, who chairs the E-ELT Science Working Group, says that the downscaling "is not disastrous" but that there will be implications for the science. "The ultimate goal of imaging an exoplanet similar to our own Earth might still be feasible," she says, "but it's gonna be extremely difficult, and it will only be possible for nearby stars. The smaller size is disappointing from a scientific point of view, but we need to get on with it now. A further delay would also compromise the science."
ESO is in a race with two other consortia to be the first in the next generation of jumbo telescopes. A U.S.-led international collaboration is preparing the 24.5-meter Giant Magellan Telescope on Cerro Las Campanas in Chile, while an international consortium led by Californian institutions is planning the Thirty-Meter Telescope (TMT) on Mauna Kea, Hawaii. Astronomer Richard Ellis of the California Institute of Technology in Pasadena, who is a member of the Board of Directors for the TMT, says the E-ELT's smaller mirror size will lead to a "quite significant loss" in its ability to study remote galaxies as well as directly image exoplanets. For observations that use adaptive optics—a complicated but essential technology to compensate for air turbulence—a small change in mirror diameter can have big consequences, he says. "Still, the E-ELT will of course be a huge gain over the Thirty-Meter Telescope," says Ellis.
The E-ELT received a financial boost in December when Brazil decided to become the 15th (and first non-European) member state of ESO. But even with Brazil's entrance fee of €130 million (spread out over 10 years), de Zeeuw says that construction of the original 42-meter E-ELT would have taken at least 16 years at its previous price. The decision to go with a smaller mirror, he says, represents a "tremendous opportunity" for the E-ELT to see first light before the TMT.
Ellis says the TMT could be completed in 2018 if commitments for the entire price tag of just over $1 billion are secured by next year. So far, however, only $300 million plus funds for the completion of a detailed design study have been raised, he says. The TMT consortium is led by the California Institute of Technology, the University of California, and the Association of Canadian Universities for Research in Astronomy. Collaborating national institutes in Japan, China, and India might find it easier to raise money if the U.S. National Science Foundation decides to become a partner in the TMT, says Ellis, at a proposed 20% share.
*This item has been updated to add information about the Giant Magellan Telescope.