Pasadena-Based Giant Magellan Telescope Organization is Casting Mirrors for the World’s Largest Telescope

Published : Monday, November 6, 2017 | 5:48 AM


Technicians at the University of Arizona’s Richard F. Caris Mirror Laboratory have begun casting the fifth of seven mirrors that will form the heart of the Giant Magellan Telescope (GMT), the Giant Magellan Telescope Organization in Pasadena announced Friday.

Once completed, the 25-meter diameter Giant Magellan Telescope will be installed in the Chilean Andes and will be used to study planets around other stars and to look back to the time when the first galaxies formed.

The Giant Magellan Telescope will combine the light from seven of these 8.4-meter mirrors to create a telescope with an effective aperture 24.5 meters in diameter (80 feet). With its unique design, the GMT will produce images that are ten times sharper than those from the Hubble Space Telescope in the infrared region of the spectrum.

“We are thrilled to be casting the Giant Magellan Telescope’s fifth mirror,” Giant Magellan Telescope Organization President Robert N. Shelton said. “The Giant Magellan Telescope project will enable breakthrough discoveries in astronomy, and perhaps entirely new fields of study. With the talents of the team at the University of Arizona and across our entire community, we are taking the next step towards completing the seven-mirror GMT.”

GMTO said the casting process involves melting nearly 20 tons of glass in a spinning furnace. The mirrors begin as pristine blocks of custom manufactured low-expansion E6 glass from the Ohara Corporation of Japan. Precisely 17,481 kg of these glass blocks have been placed by hand into a custom-built furnace pre-loaded with a hexagonal mold.

At the peak of the lengthy casting process, in which the giant furnace spins at up to five revolutions per minute, the glass is heated to 2129°F for about four hours until it liquefies and flows into the mold. The casting process continues as the glass is carefully cooled for three months while the furnace spins at a slower rate.

Once cooled, the glass undergoes an extended period of shaping and polishing. The result of this high-precision process is a mirror that is polished to an accuracy of one-20th of a wavelength of light, or less than 1,000th the width of a strand of human hair.

“Casting the mirrors for the Giant Magellan Telescope is a huge undertaking, and we are very proud of the UA’s leading role creating this new resource for scientific discovery,” UA President Robert C. Robbins said. “The University of Arizona has such an amazing tradition of excellence in space exploration, and I have been constantly impressed by the things our faculty, staff, and students in astronomy and space sciences can accomplish.”

With its casting this weekend, the fifth GMT mirror joins three additional GMT mirrors at various stages of production in the Mirror Lab. Polishing of Mirror 2’s front surface is well underway; coarse grinding will begin on the front of Mirror 3 shortly, and Mirror 4, the central mirror, will soon be ready for coarse grinding.

The first GMT mirror was completed several years ago and was moved to a storage location in Tucson this September, awaiting the next stage of its journey to Chile. The glass for Mirror 6 has been delivered to Tucson, and Mirror 7’s glass is on order from the Ohara factory in Japan, GMTO said.

The giant mirrors will then be transported to the giant telescope’s future home in the Chilean Andes at the Carnegie Institution for Science’s Las Campanas Observatory. The site is known for being one of the best astronomical sites on the planet, with its clear, dark skies and stable airflow producing exceptionally sharp images. GMTO has broken ground in Chile and has developed the infrastructure on the site needed to support construction activities.

Taft Armandroff, Vice-Chair of the GMTO’s Board of Directors, said creating the GMT is a monumental endeavor.

“GMT will be among the largest privately-funded scientific initiatives to date,” Armandroff said. “With this next milestone, and with the leadership, the technical, financial and scientific prowess of the members of the GMTO partnership, we continue on the path to the completion of this great observatory.”