A craftsman finishes the mould for an 8.4-metre mirror at Steward Observatory, the world’s only facility for making giant, single-piece glass mirrors (Image: Lori Stiles, UA News Services) When completed, the Giant Magellan Telescope’s seven 8.4-metre mirrors will more than quadruple the power of today’s best observatories

Expert mirror makers are gearing up to cast the first component of what is planned be the world’s largest telescope. When completed, its seven 8.4-metre mirrors will more than quadruple the power of today’s best observatories.

Like other multiple-dish instruments, such as the four 8-metre dishes of the Very Large Telescope in Chile, the planned Giant Magellan Telescope will take advantage of its expansive surface area to boost its sensitivity to light.


But unlike the VLT, the GMT will arrange its multiple mirrors into a single “super dish” – with one mirror at the centre and the other six curved around it like petals. Such “off-axis” mirrors have never been made as large as this.

The design makes the telescope’s vision keener than it would be if all seven mirrors remained separate. It will have 10 times the resolving power of the Hubble Space Telescope.

“The double advantage is that by making them into a single surface, you get a sharper image as well as collecting more light,” says Roger Angel, director of the mirror lab at Steward Observatory in Tucson, Arizona, US.

Grind and polish

Steward Observatory is home to the world’s only spinning glassmaking furnace, which is now going through final warm-up before casting the first of the GMT’s seven mirrors. The spinning oven uses centrifugal force to give the mirror its initial concave shape, saving greatly on the cost of glass and the time and work needed to grind and polish the mirror into its final form.

Construction of the mirror’s mould has just been completed, and the oven is now undergoing a pre-firing before technicians carefully load it with 18,000 kilograms of borosilicate glass made from sand gathered on Florida’s Gulf coast.

The glass will be heated for about a week, starting around 18 July. When it reaches 1200°C, the glass will flow like honey into the intricate mould as the 90-tonne apparatus slowly spins. Then it will gradually cool down for about 12 weeks, spinning all the while.

The huge lab, built beneath a part of the university’s football stadium, has previously cast two 8.4-metre mirrors for the Large Binocular Telescope now being commissioned on Arizona’s Mount Graham. Angel says the GMT used the same size because a larger mirror could not be transported on ordinary roads.

The telescope, planned to be built in northern Chile, is expected to take a decade to complete if it receives the necessary funding.