Record-breaking telescope mirror cast in UA lab

Nicholas Valenzuela Arizona Daily Wildcat The Steward Observatory Mirror Lab furnace spins as the glass inside begins to soften. The laboratory is constructing an 8.4-meter mirror for the Large Binocular Telescope on Mount Graham.

The mirror will measure 8.4 meters (27.5 feet) in diameter and weigh about 17 tons. It will be one of two 8.4-meter mirrors installed side by side on a common mount at the Mount Graham International Observatory near Safford.

Lori Stiles, information specialist for University of Arizona News Services, said the telescope will have the light-gathering power of a single 11.8-meter telescope, a larger collecting area than any existing or planned single telescope.

Roger Angel, scientific director of the Steward Observatory Mirror Lab and UA regents professor of astronomy, pioneered the mirror's honeycomb spin casting.

He said mirrors need to be thick to have a small range of temperature change. Heat from the lens could distort any images being observed.

"At this size," Angel said, "a solid lump would be immensely heavy."

The honeycomb structure is lighter and allows the mirror to be air-conditioned by blowing air through the voids in the glass.

More than 20 tons of borosilicate glass, which has a relatively small rate of expansion and contraction through heating and cooling, was selected for the mirror. Polarized light was used to find any imperfections in the glass.

"Lumps in the glass could create stress and weaken the structure," Angel said.

Stiles said members of the casting team loaded about 6,000 pounds of glass per hour for two days.

The 12- to 20- pound chunks were loaded into a mirror mold that sits on a two-story, mushroom-shaped rotating furnace. The mirror mold holds 1,662 hollow, capped ceramic cores bolted to the floor of the mold.

Daniel Watson, senior research specialist for the mirror-casting team, said it is the highest quality mold the laboratory has ever made.

Stiles said that when the surface is heated, the glass will soften to the consistency of honey and flow around the cores, creating a honeycomb structure in glass.

The furnace reached a maximum temperature of about 1180 degrees Celsius early Sunday morning. The furnace, as estimated by the casting team, was to hold that temperature for about five hours.

Stiles said the furnace spins at 6.8 revolutions per minute to give the mirror its parabolic shape, much like water swirled in a spinning jar takes on a parabolic shape.

Watson said three cameras are keeping an eye on as much surface as possible in the furnace.

"There's always someone here and a whole bevy of computers, he said. "We're pretty confident. We've never had a failure," he added.

John Hill, casting specialist, said he is also confident the mirror will be a success. The UA team recently cast two 6.5-meter mirrors for the Multiple Mirror Telescope on Mount Hopkins and the Magellan 1 telescope in Chile.

"The LBT will be the largest and most powerful telescope in the world for optical and infrared astronomy," Stiles said. "It will provide unmatched sensitivity for the study of faint objects."

Hill said the LBT project originates as far back as August 1980, when scientists knew they would make mirrors this large.

"That was the whole point then," Hill said.

Partners sharing in the $60 million project are the UA, Arcetri Observatory on behalf of all Italian observatories, the Tucson-based Research Corp. and Ohio State University.

The telescope will cost two to four times less than other giant ground-based telescopes, according to a UA news release.