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US to spin out giant mirror

01 Jul 2005

Progress continues on the Giant Magellan Telescope with the rotary casting of an 8.4 m mirror.

This month, the University of Arizona, US, will fire up its huge spinning furnace to produce an 8.4 m diameter mirror for the Giant Magellan Telescope (GMT), the world’s first extremely large ground-based telescope to begin construction. The GMT’s primary mirror design features of a ring of six 8.4 m off-axis mirrors and a central on-axis mirror to give astronomers 10 times the resolution of the Hubble Space Telescope.

Back in April, the casting team from the University’s Steward Observatory Mirror laboratory spent seven weeks installing an array of 1,681 ceramic cores that form the mold. The 14 day pre-firing process, which is necessary to centre core glue joints, burn out any impurities and stress the mould, was successfully completed at the end of June.

During the final cast, it will take six days and 1.1 MW of electricity for the rotating furnace to heat the glass to its peak temperature of 1178 degC. At this point the glass starts to melt around the cores to give a honeycomb structure weighing just one fifth of its solid equivalent. The speed of rotation, which will be set at 5 rpm for the GMT mirror, is used to control the depth of curvature.

Once the glass has taken position, the team will then spend between 11 and 12 weeks cooling the mold. The final stages of manufacture involve washing the ceramic cores out of the glass and grinding and polishing the mirror to an accuracy of plus or minus 15 – 20 nm. A 100 nm thick coating of aluminium gives the single-piece mirror its reflective surface.

“The fact that we are already in production is directly related to the successful technology developed for the twin 6.5 m Magellan telescopes at Carnegie’s Las Campanas observatory in Chile,” said Matt Johns, GMT project manager. “The Magellan telescopes have proved to be the best natural imaging telescopes on the ground.”

The GMT, which will be located in northern Chile, is due to be ready for operation in 2016. Astronomers hope to use its enormous collecting area to unravel the birth of stars and planetary systems in our Milky Way and solve the mystery of black holes.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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