Making a Giant Mirror to Scour the Skies

Workers at the Steward Observatory Mirror Lab

Workers at the Steward Observatory Mirror Lab load glass into the 27-foot-diameter mold used to cast the first mirror for the Giant Magellan Telescope. The team loaded a total of 40,000 pounds of the glass into the mold. Lori Stiles hide caption

itoggle caption Lori Stiles
This 40-foot diameter spinning furnace at the University of Arizona

This 40-foot diameter spinning furnace at the University of Arizona is used in casting the glass "honeycomb" mirrors. Lori Stiles hide caption

itoggle caption Lori Stiles
Spincasting

When the surface of a mirror is clear and bubble-free, and furnace temperature has cooled to about 700 degrees Celsius, the furnace lid is raised temporarily to allow rapid cooling. Lori Stiles hide caption

itoggle caption Lori Stiles
An artists' conception of the Giant Magellan Telescope

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. hide caption

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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.

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