Most advanced giant telescope mirror completed

The most challenging astronomical mirror ever made – 10 times more powerful than any other large mirror – has been completed for a giant telescope that will explore star formation, black holes and planets in the early universe.

Engineers at the University of Arizona and in California have been working for the past several years at the Steward Observatory Mirror Laboratory – located underneath the university’s football stadium – to polish an 8.4-m-diameter mirror with an unusual, highly asymmetric shape.

This mirror, and six more like it, will form the core of the 25-m Giant Magellan Telescope (GMT), providing more than 380 sq m of light-collecting area. The GMT – to be located on a remote mountaintop in the Chilean Andes where the skies are clear and dark – will lead the next generation of giant telescopes in addressing critical questions in cosmology, planetary science and astrophysics.

The first of seven mirrors from the Giant Magellan Telescope is being polished by the Large Polishing Machine, which was developed at the UA Steward Observatory Mirror Laboratory for more efficient and accurate polishing of deeply curved optical surfaces.

“The Giant Magellan Telescope has the potential to transform how we see the cosmos and our place in it,” said Matthew Colless, director of the Australian Astronomical Observatory.

The mirror was cast at the lab from 20 tons of glass, melted in a rotating furnace until it flowed into a honeycomb mold. Once the glass had cooled and the mold was removed, it was polished using fine abrasives. The mirror’s figure was frequently checked using a series of precision optical tests.

It has an unconventional shape because it is part of a single 25-m optical surface composed of seven circular segments, each 8.4 m in diameter.

“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. “Only then will the seven large mirrors form a single, exquisitely sharp image when they all come together in the telescope in Chile. We have now demonstrated that we can fabricate the mirrors to the required accuracy for the telescope to work as designed.”

The mirror surface matches the desired prescription to a precision of 19 nm – so smooth that if it were the size of the continental US, the highest mountains would be barely more than a half-inch high.

“Making this first GMT mirror required all the expertise and experience that the university has built up over 25 years of making telescope mirrors and a great deal of innovation to push beyond previous limits in optical fabrication and testing,” said astronomy professor Buell Jannuzi, director of the UA Steward Observatory. “In achieving this remarkable milestone, the team built and demonstrated all the equipment and techniques that will lead to efficient production of the remaining mirrors for the GMT.”

The second of the seven giant off-axis mirrors was cast at the mirror lab in January 2012; the third will be cast this August. The telescope is slated to begin operations late in the decade.

“The technical achievements at the UA’s mirror lab and the dedication and commitment of our national and international partners will allow us to open a new window on the universe,” said GMT board chair Wendy Freedman. “An exciting future of discovery awaits us.”