"Potato chip" mirror a thing of perfection

An artist's impression of the completed Giant Magellan Telescope (GMT). Image: Todd Mason/Mason Productions and GMTO Inc. The first GMT mirror after its removal from the furnace. Photo: Ray Bertram/UA The first GMT mirror being polished at the University of Arizona's Steward Observatory Mirror Laboratory. Photo: Ray Bertram/UA A comparison of the shape of the first GMT mirror with a potato chip. Image: GMTO Inc.

 After almost seven years of work scientists have completed a giant telescope mirror that slightly resembles - a potato chip.
The 8.4-m mirror is the first of seven to be completed for the Giant Magellan Telescope, an US$790-million international project in which Australia has a 10 per cent share.
"This is the most challenging astronomical mirror ever made," said Professor Matthew Colless, Director of the Australian Astronomical Observatory and Vice-Chair of the Giant Magellan Telescope Board.
"It is subtly shaped, with two curves, like a potato chip or a saddle. This is the first time anyone has made a mirror of this shape so big."
The telescope will have six such mirrors, arranged around a seventh central mirror like the petals on a daisy.
This first mirror is bigger than an average living room floor. It has been polished smooth to within a thousandth of the thickness of a human hair ~19 nanometres Its surface is so smooth that, if the mirror were the size of Australia, any bumps on it would be less than 2 centimetres high.
The mirror was cast and polished at the University of Arizona's Steward Observatory Mirror Laboratory, which is located under the university's football stadium in Tucson, Arizona.
Casting the mirror's 20 tonnes of glass took several weeks and was completed in November 2005.
"This mirror took a long time to polish because the team had to learn how to make this first-of-a-kind mirror, and to design and build the equipment to test its performance," explained Professor Colless.
"Now they know how, the other mirrors will be quicker and will roll off the assembly line at the rate of one every 12 to 18 months. One mirror will be being cast, one rough-polished and one fine-polished, all at the same time."
The second mirror was cast in January 2012 and the third will be cast in August 2013.
The Giant Magellan Telescope (GMT) is one of the "Extremely Large Telescopes" or ELTs now being developed - optical telescopes with light-collecting mirrors more than 20 metres in diameter. Today's largest optical telescopes have mirrors eight to ten metres in diameter.
The enclosure covering the GMT will be over 60 metres high, and the moving part of the telescope will have a mass of more than 1000 tonnes.
The telescope, due for completion late this decade, will allow astronomers to address critical questions in cosmology, astrophysics and planetary science.
It will be located in Chile at Las Campanas Observatory, about 100 kilometres northeast of the town of La Serena in the foothills of the Andes. Groundbreaking for the site took place in February 2012.
Australia's share of the GMT project is funded by an $88-million grant from the Australian Government's Education Investment Fund. ($65 million goes to the GMT Organization for design and construction of the telescope, and $23 million is for support activities in Australia.)
"We expect that Australian industry and institutions will receive obtain an excellent return on this investment in terms of contracts from the GMT Organization over the course of the project," Professor Colless said, "Australia is already heavily engaged in design and building some of the key optical systems and two of the first generation of instruments."
The GMT partner institutions are the Australian National University, Astronomy Australia Limited, the Carnegie Institution for Science, Harvard University, the Korea Astronomy and Space Science Institute, The Smithsonian Institution, Texas A&M University, the University of Arizona, the University of Chicago and the University of Texas at Austin.

More information

Professor Matthew Colless is travelling to Europe on 24 October 2012. After that date:
M: +61 431 898 345

Associate Professor Andrew Hopkins, Head of AAT Science, Australian Astronomical Observatory
M: +61 432 855 049
T: +61 2 9372 4849

Helen Sim (AAO - media assistance)
T: +61 2 9372 4251
M: +61 419 635 905

Wendy Freedman
Chair, Board of Directors, Giant Magellan Telescope Organization

Buell T. Jannuzi
Director, Steward Observatory,
The University of Arizona

Patrick McCarthy
Director, Giant Magellan Telescope Organization