Making the mirror for the world's biggest telescope

Flora Graham, deputy editor, newscientist.com

(Image: Ray Bertram, Steward Observatory, The University of Arizona)

Despite its huge 8.4-metre diameter, this mirror being cast inside the University of Arizona's giant red furnace is only the second of seven such reflectors being made for the Giant Magellan Telescope (GMT).

The mirror was cast this week in the underground mirror-making lab that sits beneath the university's football stadium. To form the challenging off-centre shape of the reflector, 21 tonnes of borosilicate glass is heated to 1170 ?C and the furnace spins it into a parabolic honeycomb pattern. The cells of glass in the pattern are hollow, allowing the mirror to be lighter and more easy cooled to night-time temperatures, avoiding heat distortion.

With a total optical area of 24.5 metres, the GMT promises images 10 times sharper than the Hubble Space Telescope, but don't hold your breath: the telescope is scheduled to be finished in "about 10 years", and the first mirror has taken seven years to polish to perfection. Thanks to this practice, the mirror makers estimate that it will only take a couple of years to polish off this one.

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