World's largest mirror cast by UA scientists

Leigh-Anne Brown Arizona Daily Wildcat Peter Strittmatter, director of Steward Observatory, explains Friday how the surface of the Large Binocular Telescope mirror was constructed. Roger Angel, director of the Steward Observatory Mirror Laboratory, elaborates on how the mirror will function.

"It looks fantastic. This glass is as clear and bubble-free as any mirror we have yet cast," said Roger Angel, University of Arizona regents professor of astronomy and mirror lab director.

Angel and his colleagues got their first glimpse of the mirror Thursday when, after three months of cooling from 2,156 degrees Fahrenheit, the glass finally reached room temperature and the lid of the mirror furnace was removed.

Inside, Angel and his team found "a perfect honeycomb structure," and a uniform mirror surface, 1.6-inches thick across its entire dish-shaped face, he said.

Made of more than 20 tons of specialized glass, the blank is 8.4 meters (almost 28 feet) in diameter. It is the first of two primary mirrors for the Large Binocular Telescope on Mount Graham. The blank will become an actual mirror after it is polished to a perfectly calibrated parabolic surface and covered with a thin layer of aluminum.

The mirror cost about $4 million, said Peter Strittmatter, director of Steward Observatory. The total cost of the LBT project will be nearly $80 million, he said.

The diameter of the 8.4-meter mirror is only 4 inches larger than the 8.3-meter blank now being polished for use in the Japanese National Observatory telescope in Hawaii, Angel said. But the UA blank is noteworthy for its revolutionary honeycomb structure.

"This is the largest by a little, but unquestionably the most complex piece of glass ever cast by a lot," Angel said.

The mirror blank is not a solid piece of glass like other telescope mirrors. Angel and his team at Steward have developed a process to make their mirrors semi-hollow. The mirror's surface is supported by a honeycomb-shaped network of supporting ribs that give the mirror its strength and rigidity. They are created by an intricate mold assembled in the furnace prior to casting.

The honeycomb structure of the mirror makes it much lighter than solid mirrors, and much less expensive, said LBT director and UA astronomer John Hill in an interview last spring.

It has one additional advantage over a solid mirror, Hill said: It can be "air-conditioned."

Solid mirrors react very slowly to changing temperatures. On mountain-tops, the night air cools rapidly leaving mirrors much warmer than the surrounding air.

Heat rising from the mirror actually distorts the images of stars, much as heat rising from a barbecue distorts images viewed through it, Hill said.

With the honeycomb structure, though, air can be circulated through the mirror's 1,662 hollow cells, keeping the mirror temperature very close to that of the night-time air.

Also unique to this mirror is the fact that it will be working in tandem with a second 8.4- meter mirror when the LBT is complete.

"There are a number of 8-meter class telescopes in the world, but this will be the first using two mirrors," Angel said.

"The result is we combine high resolution and high sensitivity to faint objects, so we will be able to see the farthest objects in the universe seen by the Hubble Space Telescope with much greater clarity and detail. It will be the only telescope to do that," Angel said.

When the telescope is finally online, sometime after late 2003, Angel and his colleagues will finally be able to relax and enjoy the fruits of more than 20 years work.

They will turn their telescope to the skies and see some of the clearest images of the oldest areas of the universe ever seen.

With the super light-collecting power of the LBT, Angel hopes to see the first images of small to mid-sized planets around other stars.

Looking at infrared and heat radiation of planets, he hopes to find out what they are made of, and what gasses, if any, are present in their atmospheres.