Engineers complete mirror for Giant Magellan Telescope

Researchers have completed what they call ‘the most challenging large astronomical mirror ever made’ ready for the next generation of ground-based telescopes.

A team of scientists and engineers from the US recently completed the 8.4m-diameter mirror built for the Chile-based Giant Magellan Telescope (GMT), which will study planets orbiting other stars and the formation of stars, galaxies and black holes.

The mirror has an unusual, highly asymmetric shape — the scientists say it was 10 times more difficult to build than any previous large telescope mirror — because it will form part of a single 25m optical surface composed of seven circular segments and providing more than 380m2 of light-collecting area.

‘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 University of Arizona Steward Observatory, where the mirror was made in a lab beneath the university’s football stadium.

‘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 was cast at the mirror lab from 20 tons of glass, melted in a rotating furnace until it flowed into a honeycomb mould.

Once the glass had cooled and the mould material was removed, scientists at the lab used a series of fine abrasives to polish the mirror, checking its figure regularly using a number of precision optical tests.

The GMT will be located on a remote mountaintop in the Chilean Andes, above much of the atmosphere and away from any sources of light pollution, and is scheduled to begin operating towards the end of the decade.