UA Mirror Lab works full tilt on huge scope

The 20 tons of glass melting in a spinning furnace under the bleachers of Arizona Stadium represent the second $20 million gamble for the Giant Magellan Telescope (GMT) project.

The first gamble was technological.

When the Steward Observatory Mirror Lab set out to make the first of six 8.4-meter (about 27.6 feet) off-axis mirrors to eventually be installed in a Chilean observatory, it was not certain such a thing could be done.

"It was a very bold move," said Matthew Colless, director of the Australian Astronomical Observatory. "This is really cutting-edge stuff. No one has done anything this difficult before."

That first mirror has been polished to more than 99 percent of perfection, according to three tests devised by the Mirror Lab and its partners in the UA College of Optical Sciences. On Saturday, its slightly rough outer edges were being polished by a 500-pound "stressed lap polisher" to a tolerance 2,000 times less than the average thickness of a human hair.

The technical "risk has been retired," said Peter Strittmatter, director of Steward Observatory, speaking to the project's partners at a mirror-casting event Saturday.

The remaining risk, for what will eventually be a 24.5-meter telescope consisting of six mirrors arrayed around a central mirror, is financial.

The GMT, with financial commitments from the governments of Korea and Australia and participation by seven U.S. institutions and universities, has nailed down 40 percent of the estimated $700 million cost of the telescope, said Patrick McCarthy, GMT director.

The National Science Foundation, which once contemplated taking a 25 percent share in one of two giant telescopes being built, now says it won't be able to contribute in this decade. Jim Ulvestad, director of the NSF Division of Astronomical Sciences, said part of the reason is that support of a "giant-segmented telescope" slipped in priority in the most recent ranking by the National Research Council's decadal survey from first to third.

"Given the budget outlook, it is extremely unlikely that NSF could make a construction investment into the third priority recommendation this decade," Ulvestad wrote in an email.

McCarthy said the GMT will be built without a federal commitment. It has already received "substantial contributions" from Korea and Australia, Texas A&M University and the UA. "It is 85-to-90 percent subscribed," he said.

He expects the U.S. government will ultimately join the consortium, to allow participation by the wider astronomical community. That role is usually given to the National Optical Astronomy Observatory (NOAO), based in Tucson.

NOAO Director David Silva said in an email that the organization "has no defined role in either the Giant Magellan or the Thirty Meter Telescope, a rival project headed by California universities. "That might change as NSF develops a partnership arrangement with either or both projects."

The UA's initial investment is already paying off.

Its mirror lab is "working three shifts, 24 hours a day, five days a week," said Jeffrey Kingsley, the lab's associate director. That pace will continue for the next decade as the GMT mirrors go into full production, he said.

The second mirror was scheduled to reach its melting point late Saturday. Materials have already been ordered for a third.

Strittmatter said the UA has so far contributed about $13 million in cash or services to the project. In return, it has snared $48 million in contracts for casting and polishing mirrors and devising the optical tests to measure them.

Arizona's goal is a 10 percent share in the project, Strittmatter said.

McCarthy said the corporation expects to spend about $120 million for the eight mirrors being produced at the Mirror Lab. The rest of the money will go for expenses like building the observatory, moving the mirrors and adding instruments.

The UA's Steward Observatory and its partners at Arcetri Observatory near Florence, Italy, are also working on the adaptive optics that will use lasers and a deformable secondary mirror to correct the blurring caused by the Earth's atmosphere. That will give the telescope images 10 times sharper than the space-based Hubble Telescope.

When the telescope is installed and in operation in the mountains of the Atacama Desert, it will combine its seven mirrors to create a single "light bucket" of 368 square meters.

Today, it would be the largest telescope in the world.

By the time it is built, nominally in 2020, it could have two larger rivals.

The Thirty Meter Telescope, being developed by a consortium of California universities, is to be built in Hawaii.

The Extremely Large Telescope, slated to be built in Chile by the European Southern Observatory, would be 42 meters in diameter.

Building without all the money in hand is standard practice for large, multi-decade astronomy projects, said Strittmatter.

He added that embarking on projects that experts deemed impossible has become standard operating procedure for the Mirror Lab and its director, Roger Angel.

When Angel set out to make lightweight but rigid honeycombed mirrors in a spinning furnace, the glass experts said it was impossible. When he proposed deeply concave mirrors to reduce the focal length and, consequently, the size of observatories, the opticians said the same.

Those challenges were met years ago, but the off-axis mirrors of the Giant Magellan Telescope, to be built on Las Campanas Peak in Chile, posed new ones.

The center mirror is easy to build because it sits directly beneath the focal point and can be cast and polished as a simple parabola, a section of a perfect sphere.

The six "petal" mirrors of the GMT are tilted and arrayed around that central mirror. Their shape must deviate from spherical and each one must be a perfect copy, said Angel.

That's why three tests were created to measure the mirror's perfection.

"You won't know till you get to the mountain whether you've got it right," Angel said, "so you'd better be really careful."