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Making a Giant Mirror to Scour the Skies

An artists' conception of the Giant Magellan Telescope, scheduled to see first light in 2016. The telescope will have a central mirror surrounded by six 27-foot-diameter mirrors.
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An artists' conception of the Giant Magellan Telescope, scheduled to see first light in 2016. The telescope will have a central mirror surrounded by six 27-foot-diameter mirrors.

Over the past 20 years, a new generation of big telescopes has allowed astronomers to discover distant planets and galaxies. But those telescopes are runts compared to what's coming. Mirrors for what will be the world's largest telescope are now being made at the University of Arizona.

The Steward Observatory Mirror Lab recently began casting the first of seven mirrors -- each 27 feet in diameter -- that will become part of the Giant Magellan Telescope, creating an enormous reflective surface of glass.

The facility uses a unique process to cast single-piece mirrors so large. Twenty tons of glass are spun inside a two-story rotating furnace to create a naturally concave mirror. The bowl shape is highly effective at gathering and focusing light. As the glass melts, it flows around ceramic forms inside the furnace, creating a mirror a few inches thick on top of a glass honeycomb. That makes it just one-fifth the weight of a solid mirror.

Cooling the furnace will take almost three months. After that, the mirror will be moved to another room for polishing, a process that takes about a year. The finished product will be accurate to within one-billionth of a meter.

Eventually, the giant mirror will end up on a mountaintop in Chile, where it will join six others surrounding a seventh central mirror to become part of the Giant Magellan Telescope.

The Giant Magellan will have 10 times the resolution of the Hubble. Astronomers hope those powerful optics will help them look directly at some of the dozens of planets spotted orbiting other stars.

Copyright 2022 NPR. To see more, visit https://www.npr.org.

As supervising editor for Arts and Culture at NPR based at NPR West in Culver City, Ted Robbins plans coverage across NPR shows and online, focusing on TV at a time when there's never been so much content. He thinks "arts and culture" encompasses a lot of human creativity — from traditional museum offerings to popular culture, and out-of-the-way people and events.