Big Mirror on Mount Hopkins Gets 'Perfect' Shining Coat

Jan. 3, 2002

MOUNT HOPKINS, Ariz. -- Months of preparation paid off in less than 60 seconds as University of Arizona and Smithsonian scientists successfully aluminized the mirror of the largest single-mirror telescope in North America.

Technicians at the MMT Observatory on Mount Hopkins bolted a vacuum chamber directly over the 6.5-meter (21 and one-half foot) mirror's surface and vaporized aluminum to create an incredibly thin reflective coating on the mirror.

"The coating is thinner than a human hair," said MMT Director Craig B. Foltz. "In fact, the amount of aluminum now on the face of this 20 square meter mirror would fit into the volume of an eraser on an ordinary pencil."

While aluminum is a common coating for telescope mirrors, the MMT staff faced a special challenge. Telescope mirrors are usually removed from the telescope for coating, but the cramped telescope site on the 8,550-foot mountain summit does not allow room for a separate coating chamber.

"Instead of taking the mirror to the vacuum chamber, we had to bring the vacuum chamber to the mirror," said operations manager J.T. Williams. The 36,000-pound vacuum chamber -- which observatory staff early noted resembles Mr. Bill of SNL fame -- has to be lifted by crane from its storage location to the telescope. A crane is also needed to remove a portion of the telescope structure to allow the chamber to be bolted to the mirror cell.

"Our two previous coating attempts were only partially successful," Williams said. "Our concept was correct, but we had component failures.

"This time, we got as perfect a coating as you could get. It's just beautiful. It's so flawlessly shiny that when you look at it you have a depth perception problem: it's hard to tell what's real and what's reflection," Williams said.

The MMT staff worked through the summer and early fall to devise a method that would overcome the previous problems. They rewired the interior of the vacuum chamber. They redesigned and extensively tested the power control circuitry.

"It was as close to a perfect shot as one can come, " Foltz said. "Those who have inspected the mirror call it 'awesome.'"

***Editors: Media are welcome to photograph the newly coated mirror on the mountain at 11 a.m. Tuesday, Dec. 18. Those who'd rather not drive themselves up the 4,300 vertical-foot, 13-mile narrow unpaved mountain road can catch a ride leaving 10 a.m. from Whipple Observatory base camp, contact Dan Brocious, 520-670-5706,

Directions to the Whipple Observatory Administrative Complex are on the web at

Dan Brocious or Lori Stiles, UA News Services, 520-621-1877,

**** MMTO Background***
The telescope of the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona, has been a pioneering large telescope throughout its career.

First dedicated in May 1979, the original telescope featured three ground-breaking departures from conventional telescope building.

MMT stood for Multiple Mirror Telescope because six individual, identical, 1.8-meter telescopes in a common mount were used as one. The combined light-collecting power was equal to that of a single 4.5-meter mirror, making the MMT the third largest optical telescope in the world at the time.

The six telescopes were supported on the first computer-controlled telescope mount. Computer control was needed because of the simpler, compact, naval gun type of altitude-azimuth mount used.

The compact mount allowed the MMT to fit into a four and one-half story building which was much smaller than those housing conventional telescopes of similar size. The entire 550-ton building turns with the enclosed telescope mount so as the telescope tracks the sky, the building stays out of the way.

The computer-controlled multiple optics, computer-controlled telescope mount, and co-rotating building are now standard features of most large optical telescopes.

By the mid-1990s, however, the 4.5-meter MMT was becoming a medium-sized telescope. The UA and Smithsonian decided to convert the telescope by installing a large single mirror which would double the light collecting power of the telescope.

On March 2, 1998, the 4.5-meter Multiple Mirror Telescope closed its chamber doors onto the night sky for the last time. The disassembly of the telescope began later that day.

The new 6.5-meter Telescope of the MMT Observatory uses a 6.5-meter diameter spin-cast borosilicate primary mirror, cast and polished in the Steward Observatory Mirror Lab.

The building was modified in 1995. The mirror cell was put in place on Aug. 6, 1998 and the primary mirror was successfully installed on March 25, 1999. The first secondary mirror was installed in the telescope in early May 2000. The "first light" was focused by the telescope on May 17, 2000. The image quality exceeded staff expectations. The telescope was dedicated on May 20, 2000