UA scientists to fix telescope surface after glass leakage

The 17-ton mirror is one of two designed for installation in the $60 million Large Binocular Telescope project on Mount Graham, about 80 miles northeast of Tucson.

After measuring the mirror earlier this month, scientists confirmed that some glass leaked during the casting process. They determined that one-tenth of the mirror's surface was below optimum thickness.

Roger Angel, director of the Steward Observatory Mirror Laboratory, said adding two tons of fresh boroxilate glass to the mirror's surface will solve the problem.

He said the additional glass and the recasting process will cost an additional $70,000. Angel said that is only a small percentage of the mirror's total.

At present, the mirror is 90 percent complete with a sound honeycomb structure and almost no bubbles, University of Arizona officials said.

Angel designed the honeycomb structure, which consists of 1,662 ceramic cores and gives the mirror its unique lightweight design. The ceramic cores are made of the same material used in the shield tiles on NASA's space shuttles.

Based on a series of experimental castings, scientists are confident the recasting will be a success, UA officials said.

"There's always the potential for something to go wrong," Angel said. "We have the new aspects pretty well tested."

Engineers prepared for the recasting by inspecting the honeycomb structure and insulating around the edge of the mirror's surface so only the top of the mirror will be reheated. The existing surface was also cleaned and an extra video camera was installed to monitor the edge of the mirror while the glass is melted.

In the reheating process, the rotating furnace will heat the glass to a temperature of 650 degrees Celsius. Scientists say they can prevent thermal stress by reheating the mirror's surface slowly.

Around June 10, with the furnace rotating at 6.8 rotations per minute, the top of the mirror will reach a maximum temperature of 1,180 degrees Celsius.

The glass is then allowed to slowly cool for 11 weeks to room temperature. At that point the structure's honeycomb cores will be washed out and the mirror surface will be ready to polish.

Lab officials said the recasting will not delay the Large Binocular Telescope project.