Scientists must recast thin telescope mirror

The 8.4-meter mirror was cast for the Large Binocular Telescope on Mount Graham.

The lid to the mirror's furnace was removed around 4 p.m. Wednesday at the Steward Observatory Mirror Laboratory, underneath Arizona Stadium. University of Arizona scientists confirmed there had been some glass leakage.

"In terms of the general quality, it (the mirror) looks good," said Roger Angel, director of the mirror laboratory.

John Hill, project director for the Large Binocular Telescope, said, "We're still measuring the thickness using an ultrasound gauge."

He said scientists already suspected there had been some glass leakage in January.

"We saw the level going down on the monitors," Hill said.

Scientists had several cameras on-board the furnace, relaying images to monitors in a control room where they could observe the heating process.

The mirror lab began casting the 17-ton mirror in January. The casting process consists of melting chunks of glass in a rotating furnace to a temperature of 1,180 degrees Celsius.

The furnace is rotated at 6.8 rotations per minute to give the lens its parabolic shape, much like swirling water in a bottle. Scientists had to wait 10 weeks for the mirror to cool and to determine if the casting had been a success.

Hill said there will be another week of measuring before more chunks of glass are loaded on the mirror's surface and the furnace is heated to 700 degrees Celsius for another month. It will take the furnace three more months to cool down.

"We've never done this before," Hill said. "Others have repaired mirrors this way, but not by adding a new layer."

Hill said the lab had been conducting smaller scale experiments that seem to indicate this repair method will work.

He said the same risks present in any casting process will exist for this repair process as well. He said the power must stay on and the furnace temperature must be controlled for the repair to be a success.

A computer in the control room divides the mirror into 20 temperature zones that can be individually monitored, Hill explained.

For the repair process, only the top zone will be heated to melt the additional glass. The bottom zone's temperature will be maintained at a lower temperature that is yet to be determined.

"The worst part is that we have to watch the furnace for 24 hours a day, for the next four months," Hill said.

He said despite the glass leakage, this mirror casting has been the best at the lab.

He said the mirror lab will definitely do something differ-ent with the next casting, fol-lowing an engineering study.

Dan Watson, a research specialist for Steward Observatory, said, "We're still in the early stages of the investigation. We'll take whatever we learn from this investigation and apply it to future castings."

Hill said contracts for future mirrors are still being negotiated. He said it could be a couple of years before the second 8.4-meter mirror is cast.

The two mirrors will be installed on a side-by-side common mount at the Mount Graham International Observatory near Safford. The binocular telescope will have the light-gathering power of a single 11.8-meter mirror.