How do you move a 6,700-pound telescope mirror that’s 14 feet across but only four inches thick?

Leave it to University of Arizona optical scientists and technicians. They plan to move the mirror Wednesday, Aug. 1, to a polishing table in the College of Optical Sciences’ Large Optics Shop. Polishing is the next step in finishing the 4.3-meter (14 foot) primary mirror for the Discovery Channel Telescope.

The telescope is a partnership between Flagstaff’s Lowell Observatory and Discovery Communications. It is being constructed at Happy Jack, Ariz., 40 miles southeast of Flagstaff. The primary mirror, which was cast and fused by Corning, Inc., in Canton, N.Y., was delivered to UA’s College of Optical Sciences in August 2006. 

Martin J. Valente, director of Optical Sciences’ Fabrication and Engineering Facility, is principal investigator on the project to polish and figure the mirror. He and his team have bonded 36 pucks at precise locations the mirror’s convex backside. The team also made a support structure that holds the mirror just as it will be held in the telescope, so the mirror doesn’t flex under the force of grinding and polishing.

A 40-ton crane will move the mirror using a specially designed handling fixture at 2 p.m. Wednesday, Aug.1. Researchers in optical sciences and the University Research Instrumentation Center, directed by Bob Kingsley, designed and built the handling fixture. It will be used in moving the fragile mirror about 150 feet across the lab onto the polishing support structure.

Valente said that his crew plans to set up the handling fixture on Tuesday night, then lift and balance the mirror in the handling fixture on Wednesday morning. They will balance the mirror by adjusting lengths of straps to make the mirror as level as possible. Once the mirror has been moved over the polishing table, technicians begin making a series of adjustments needed to safely lower the mirror onto the platform. The lowering steps may take hours, Valente noted.

After months of grinding to get the mirror closer to its ideal shape, the team will polish the mirror to an accuracy of a fraction of a wavelength of light. If the finished mirror were the size of the United States, all imperfections would be polished to under an inch high.

Optical sciences Professor Jim Burge and his students are developing several different systems to test the mirror. Test systems will include a laser tracker and infrared and visible wavelength interferometers.

UANews.org