Challenging 8.4 m mirror completed for Giant Magellan Telescope

Tucson, AZ--Scientists at the University of Arizona (UA) and in California have completed what they believe is the most challenging large astronomical mirror ever made. For the past several years, a group of optical scientists and engineers working at the UA Steward Observatory Mirror Laboratory underneath the UA’s football stadium have been polishing the 8.4-m-diameter mirror, which has an unusual, highly asymmetric shape. The mirror has an unconventional shape because it is part of what ultimately will be a single 25 m optical surface composed of seven circular segments, each 8.4 m in diameter.

By the standards used by optical scientists, the degree of fabrication difficulty for this mirror is 10 times that of any previous large telescope mirror. The mirror surface matches the desired prescription to a precision of 19 nm.

This mirror, and six more like it, will form the heart of the 25 m Giant Magellan Telescope (GMT), providing more than 380 square meters of light-collecting area. The Giant Magellan Telescope will lead a next generation of giant telescopes that will will find and examine exoplanets and explore the formation of stars, galaxies, and black holes in the early universe.

The mirror was cast at the mirror lab from 20 tons of glass and melted in a rotating furnace until it flowed into a honeycomb mold. Once the glass had cooled and the mold material was removed, the mirror was polished. “We need to be certain the off-axis shape of this mirror, as well as the other six that will be made for GMT, is precisely right to an accuracy of 1/20 of a wavelength of light,” said Buddy Martin, polishing scientist at the Mirror Lab.

The GMT will be located on a remote mountaintop in the Chilean Andes where the skies are clear and dark, far from any sources of light pollution. At the Carnegie Institution for Science’s Las Campanas Observatory in northern Chile, earthmovers are completing the removal of 4 million cubic feet of rock to produce a flat platform for the telescope and its supporting buildings. The telescope is slated to begin operations late in the decade.