Polishing off a scope

Chris Richards Arizona Daily Wildcat With a new 10-ton mirror hanging in the balance, project engineer J.T. Williams directs a team moving the behemoth into its mounting, today in the UA's Steward Observatory Mirror Lab. The 6.5-meter mirror is the largest of its kind, and is destined for the observatory on Mount Hopkins in the Santa Rita Mountains.

"This represents a great milestone in the life of this mirror," said Buddy Martin of the University of Arizona Mirror Lab, where the mirror was cast for the mountain observatory, about 40 miles south of Tucson.

About 25 astronomers, workers and engineers gathered in the Steward Observatory Mirror Lab yesterday to hoist the 10-ton semi-hollow mirror into its 20-ton steel case, known as a mirror cell.

It was a delicate mating.

During the last two years, the mirror's surface has been polished to within a millionth of an inch of an optimal surface, said Martin, a Mirror Lab project scientist careful not to overstate the mating's significance.

"It's good that we're one step closer to finishing up, but there are a thousand steps to this process," he said.

The glass disc swayed gently about 15 feet above the ground, held aloft by 36 vacuum-sealed suction cups hanging from a 40-ton crane.

The white mirror cell, 8 feet tall and more than 30 feet in diameter, waited beneath to receive the reflector as project engineer J.T. Williams directed the mirror down, inch by inch.

"It worked quite well," Williams said after the three-hour procedure. "It was well executed by all these guys and really there were no surprises."

This spring, the new 6.5-meter mirror will replace six smaller mirrors inhabiting the Multiple Mirror Telescope (MMT).

"We hope to get scientific first light at the end of 1998," said Craig Folz, director of the Smithsonian Institution-University of Arizona MMT.

Williams added there were about 1,000 interfaces between the mirror and its cell that had to connect, including load-spreaders to support the mirror, air conditioning tubes, air-flow jets and wires.

In the end, the mirror landed beautifully.

During the next several months, scientists and engineers will adjust the mirror cell's support system, consisting of 104 computer-controlled actuators lifting the mirror to evenly distribute its weight and thus keep its surface from distorting.

"We make the stiffest mirrors in the world, but they're not rigid and they'll flex," Martin said. "If we don't control the forces correctly, all our efforts polishing are wasted."

Scientists at the Mirror Lab cast the Mount Hopkins reflector in 1991, using a spinning furnace the size of a carnival carousel and an elaborate mold to make the semi-hollow mirror.

UA telescope mirrors are not solid. Instead they are made of a thin faceplate supported by a backing honeycomb grid, a process pioneered by UA astronomer Roger Angel and the lab team.

This allows UA mirrors to be five times lighter than solid mirrors of a similar size.

The mirror has been polished for the past two years using another revolutionary technique called stressed lap polishing, developed at the Mirror Lab and used nowhere else in the world.

The process makes it possible to make deeply curved mirrors with a short focal length.

The deeper the curvature of a mirror, the shorter its focal length, and the shorter the telescope needs to be, Martin said.

Such polishing uses a computer-controlled tool, a rotating disk called a "lap," that can apply different pressure at different points as it turns.

The lap actually changes its shape to fit the profile of the mirror as it spins around the glass surface, updating its shape about 1,000 times per second, said polishing manager Steve Miller.