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See a slideshow of images from the Mirror Lab here.


The Steward Observatory Mirror Lab has never been busier.

Five mirrors are currently in some phase of production at the University of Arizona facility. Six, if you consider that some of the raw materials for another mirror are already being stockpiled.

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Photo: Mark Duggan

The first Giant Magellan Telescope mirror, GMT1, was cast in 2005 and is now fully polished. It sits in a room in the mirror lab, covered with a blue plastic coating to protect it from scratches.

And they're eventually headed to three different telescopes, which, like the mirrors, are also various stages of production.

Three of the mirrors are being polished; another is being cast in a giant centrifugal oven; and a fifth sits ready for its six siblings to catch up.

It was the first of what will eventually be seven 8.4-meter mirrors the lab will create for the Giant Magellan Telescope.

Cast in 2005, it sits in a room at the mirror lab, coated in a protective blue plastic. Nearby sits GMT2, the second of the seven-mirror array. Just over a year old, it's being prepared for polishing.

Two rooms away, GMT3 is taking shape.

Last week, mirror lab workers finished the painstaking process of loading a mirror mold with hundreds of glass chunks. Then they put the mold into a giant, spinning oven and began what's known as the 'casting' process.

For about three days, the oven will cook the glass chunks at more than 2,000 degrees Fahrenheit. The rotation, at a rate of about 5-6 revolutions per minute, will spread the liquified glass throughout the mold using centrifugal force.

Then, the oven thermostat and the spinning rate will be very slowly decreased.

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Photo: Mark Duggan

Optics Professor Jim Burge describes the polishing process at the Mirror Lab

“We cool it for about three months,” said Jim Burge, Professor at the UA's College of Optical Sciences and at Steward Observatory.

“At that point the glass is solid enough that it's not going to move. We have to slow cool it so it doesn't build stress into it. If we cooled it quickly, then the glass would break.”

Sometime in November, a fully formed GMT3 mirror will emerge, ready to be milled and polished.

Then, workers will begin sorting and loading more glass chunks for GMT4. Three more mirrors will follow.

When complete, the mirrors will be shipped to their new home on a Chilean mountaintop. The Giant Magellan Telescope (not to be confused with the twin Magellan Telescopes at Chile's Las Campanas Observatory) will be operational in the year 2020.

The seven mirrors will be installed in a honeycomb layout, slightly off-axis from each other. The effect will be to create a single optical surface of 24.5-meters, or more than 80-feet in diameter.

And it will take some pretty high resolution images of the night sky. Astronomers will be able to see heretofore unknown amounts of detail in distant bodies.

Roger Angel, the founder and Scientific Director of the Mirror Lab, says that, for example, today's telescopes can see only the very brightest planets around distant stars.

“But with the Giant Magellan Telescope, we should really be able to see lots of them,” he said.

“And that's completely made possible by the bigger aperture. It's simply not possible with what we have now.”

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Photo: Mark Duggan

A polishing machine makes its way across the surface of the Large Synoptic Survey Telescope mirror. Two levels of concavity are visible in the mirror.

Two other mirrors are being produced at the lab right now.

The 8.4-meter mirror for the Large Synoptic Survey Telescope was cast in 2007. It's taken more than six years to finish the mirror because of its unique design. It has two different slopes of concavity, not unlike a deep, wide-rimmed dinner plate.

This creates both a primary and tertiary mirror on one piece of glass, and it took years to grind and mill the dual curvature.

The LSST will be a wide-field survey telescope, taking high-resolution images of large areas of the night sky. It's slated to become operational from another summit in Chile sometime in the next decade.

The other mirror is a 6.5-meter, eventually bound for Mexico's San Pedro Màtir Observatory. It's taking currently taking turns with GMT2 undergoing pre-polish processing.

Optical Professor Jim Burge explained that the final polishing is perhaps the mirror labs most exacting process.

“We have a polishing tool that runs under computer control,” he said. “We put rouge on this tool, and as the tool moves across the surface it wears the surface down. The key to making an accurate shape is to measure the shape you have and compare that to the ideal shape. And then, run the polishing tool to wear it down, until it gets closer and closer to perfect.”

Burge compared it to sanding a table. Only the tolerances are in the millionths of an inch.

Anything more, and it alters the mirror's shape and astronomical observations.

“For the telescope's performance, it's all about the surface,” Bruge said.

“The light reflects off of a thin metal film that's on top of a polished glass surface. The shape of that film has to be almost mathematically perfect.”

So are the mirrors being created at the mirror lab almost technically perfect?

Based on some of the observations that the telescopes they're in have made, and continue to make, they're close enough.

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Photo: Mark Duggan

The casting oven at the Mirror Lab heats and spins the glass chunks into solid form, making an unpolished mirror in about three months.