UA mirror lab gets one mirror closer to observing faint objects

Image credit: image provided by GMTO (Giant Magellan Telescope Org)
Generated image of what the Great Magellan Telescope will look like once assembled. Image provided by GMTO (Giant Magellan Telescope Org)

Astronomical observations are about to move one step closer to reaching new heights as the Richard F. Caris Mirror Lab, at the University of Arizona, prepares to remove another segment of the Giant Magellan Telescope from the furnace.

The segment is the fourth of eight mirrors being constructed at the lab located underneath Arizona Stadium and will be removed from the furnace on Monday, Dec. 21. When completed and assembled, the mirrors, costing $23 million each, will make the telescope the most powerful optic available for observing the universe.

The ability to gather as much light as possible is the key to seeing faint objects when making deep-space observations. The GMT, once operational at its location at Las Campanas Observatory in Chile, will be able to gather more light than any other telescope currently available to astronomers.

According to the Giant Magellan Telescope Website, the projected target for the completion of the telescope is 2021.

Being able to observe faint objects will not only increase the distance of observation, but it will also provide the opportunity to see smaller objects closer to Earth with more sharpness than previously available.

While the GMT will provide a new advancement in observational capabilities to different fields of astronomy, it promises a new capability in the study of planets outside the solar system known as exoplanets.

According to the GMT Science Book, the telescope will provide the ability to expand the sensitivity and speed of different methods used in studying the exoplanets.

Steward Observatory Professor and Deputy Director Dennis Zaritsky talks about the current knowledge we have of exoplanets and what GMT can potentially provide.

“There are thousands of planets now known, but we know them sort of indirectly,” Zaritsky said. “So we know there’s a planet there, but we don’t know what it looks like or anything, we haven’t taken a picture of it.

“We’re starting to, now, there are a few planets where we’ve actually taken a picture of the planet, but that’s very hard because you have this very bright star and a very faint planet,” he said.

The planets however, are not like Earth. They are large planets, bigger than Jupiter and very bright. GMT should be a step in allowing astronomers to take pictures of smaller and fainter planets, Zaritsky said.

Oven is rotating at 5 rpm when the Giant Magellan Telescope Fourth Primary mirror was cast on September 18, 2015. Photo by Ray Bertram, Richard F. Caris Mirror Lab, University of Arizona.
Oven is rotating at 5 rpm when the Giant Magellan Telescope Fourth Primary mirror was cast on September 18, 2015. Photo by Ray Bertram, Richard F. Caris Mirror Lab, University of Arizona.

These large mirrors being produced by the lab are actually not reflective when they leave Tucson. That process is done elsewhere and what the University produces is the large structures supporting the reflective surfaces.

“We call them mirrors, really they’re pieces of glass,” Zaritsky said. 

“We’re building, really the substrate, the thing that holds a very thin aluminum layer that is the reflective,” he said.

These large glass bases will eventually be given their reflective aluminum coating in Chile. Once there, they will be placed in a vacuum and have an evaporated mist of aluminum released that will settle evenly on the glass surface.

The coating process is the best way available to evenly coat the aluminum without leaving uneven patterns from spraying it directly on the surface. Once coated, the aluminum will only be a few atoms thick, Zaritsky said.

The benefit of coating the front of the glass as opposed to the back of the glass, like normal mirrors, is the light will not have to travel through the extra layer of glass before the astronomers can make their observation. Even a small portion of glass can change the light enough to degrade the sharpness of the image.

Although the aluminum is on top and performs the reflective duties to gather the light, the glass is still necessary to hold the aluminum steady. The layer of aluminum would not be able to support itself given the size of the reflective surface unless it was much thicker than it is.

“Well metal is very temperature sensitive, so if there’s a little bit of temperature difference within the metal it will change shape,” Zaritsky said. “But these mirrors have to keep their shape very precisely to form sharp images.”

While glass is also effected by temperature changes, it does not deform to the extent metal does. It is also a stiff material for stability, easy to mold and bonds well with aluminum, Zaritsky said.

To make these supports for the aluminum surface, 20 tons of glass will be used to make each 28-foot (8.4 meters) segment. This is despite the honeycomb structure used to mold the glass that makes it lighter than it would be if completely solid.

The GMT will use seven mirrors once completed, six off-axis mirrors surrounding a central mirror. These seven segments will be assembled in Chile to make the primary mirror of the GMT with a light gathering surface of 83.5-feet (24.5 meters) in diameter.

It will be the central mirror coming out of the furnace next, but there are currently two mirrors ahead of it waiting to finish the polishing process.

This polishing process is the reason the mirrors take as long as they do to be produced, Zaritsky said.

While the process of molding the glass can take several months, or more when accounting for the preparation and mold construction, there is no need to speed up the process since the polishing can take two years to complete.

The delicate process involves using liquid grit to finely sand the glass to the exact shape. The precision is so exact it will be polished to 20 nanometers; a human hair is approximately 80-100 thousand nanometers wide.

Part of the process involves constant checking and measuring throughout the polishing.

“We polish a little bit, we test it, we polish a little bit, we test it,” Zaritsky said. “The problem with this process is if we take too much off, we can’t add it back on.

“So that means you have to sort of start over and re-polish the whole thing again,” he said.

While there are other places capable of making large primary mirrors for telescopes, the University’s mirror lab is the only facility in the world that can produce large segments of this size.

Other facilities rely on assembling smaller segments to build the large diameters required to gather large amounts of light, but the fewer the segments, the sharper the image can be.

Currently the lab has one furnace capable of making these mirrors and will be busy working on the next four mirrors. The telescope will use seven mirrors leaving the eighth mirror to be used as a spare to circulate while a new mirror is constructed.

The mirrors will have to be circulated out eventually due to the downside of having the aluminum on the outside of the glass. Normally the glass would protect the aluminum from the effects of the atmosphere, but since it would obstruct the viewing it is left exposed and vulnerable.

The mirrors being constructed at the University, for the GMT, will be an integral step in advancing the study of the Universe.

“We’ll have the sharpest images, ten times sharper than the Hubble Space Telescope just because of its size,” Zaritsky said. “And since it’s so much bigger than anything that exists currently, we’ll be able to look at much fainter things.

“And so it means that in any part of astronomy, whatever people are interested in astronomy, from planets to the farthest galaxies, any science will benefit,” Zaritsky said.

Jorge Encinas is a reporter for Arizona Sonora News, a service from the School of Journalism with the University of Arizona. Contact him at jencinas9@email.arizona.edu.

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