Giant Magellan Telescope will sport the world's largest mirrors
August 28, 2013
Slated for completion by 2020, the Giant Magellan Telescope (GMT) will combine seven of the largest and most precisely built telescope mirrors, to offer image resolutions 10 times greater than Hubble at around one third of the cost. The telescope will be used to study the early universe and answer open questions on dark matter, supermassive black holes, and the nature of planets beyond our solar system.
The mirror of the Hubble Space Telescope is a relatively modest 7.9 feet (2.4 meters) in diameter; nevertheless, because it doesn't need to deal with atmospheric turbulence that impedes all ground telescopes, it has still managed to capture clear, remarkable pictures of our cosmic surroundings over the past several years.
Nowadays, the largest telescope in the world is Arizona's Large Binocular Telescope, which features two much larger, high-precision 27-foot (8.4-meter) mirrors. Scientists on the project say the system has worked so well that they have decided to combine seven of those mirrors in a one-of-a-kind configuration to build a new telescope. The result will be the GMT.
The telescope will be a marvel of optical engineering, sporting 10 times Hubble's resolution – a truly impressive feat for a ground telescope. It will have the advantage of lower costs (US$700 million to $1 billion versus Hubble's $2.5 billion) and, of course, it will be much easier to maintain as well (remember when the Hubble mirror turned out to be slightly off axis?).
The GMT is one of three so-called "extremely large telescopes" (ELT) which are set to start operations over the next few years and are expected to bring our imaging capabilities to new exciting levels, allowing us to capture celestial bodies that are far beyond Hubble's capabilities. The other two ELTs under construction are the soberly-named Thirty Meter Telescope in Hawaii, to be completed in 2018, and the European Extremely Large Telescope, which will see first light in 2022.
Because the design is a bit odd compared to previous telescopes, the system will work in a slightly unusual way. Six off-axis mirrors will surround a central on-axis mirror, forming a single, giant optical surface that will add up to a very respectable 80 feet (24.5 meters) in diameter.
Light will reflect off the seven primary mirrors to the seven smaller, adjustable secondary mirrors, and finally to a group of advanced imaging cameras that will measure individual photons to determine distance and composition of the captured objects.
Of particular interest are the secondary mirrors, which make use of so-called "adaptive optics." Each of these mirrors is directed by hundreds of tiny actuators that can precisely bend the mirror to compensate the images for atmospheric turbulence, resulting in a remarkable image quality.
"We will have 10 times the collecting area of our current telescopes in Chile and 10 times the angular resolution of the Hubble when working at infrared wavelengths," says project director Dr. Patrick McCarthy.
The seven 27-foot mirrors at the core of the telescope are being manufactured in the Steward Observatory Mirror Lab (SOML), a one-of-a-kind facility lying beneath the University of Arizona Stadium and that, to build the mirrors according to specs, had to develop a new production process from scratch.
"The challenge is that the shape of the GMT mirrors is asymmetric and strongly curved," Dr. McCarthy tells Gizmag. "This makes them roughly 10 times more difficult to polish than other large telescope mirrors."
The process that was devised is painstakingly slow, and it takes on average four years to build a single mirror. Even when production is fully geared up, only a maximum of four mirrors can be in the pipeline at any time, meaning that the lab's output is, in the very best scenario, only one mirror per year.
Glass is first poured into a rotating furnace that heats it up to 2120 °F (1160 °C) while spinning at 5 rpm to create an initial, rough parabolic shape on the front, reflective side of the mirror. On the back side, the furnace has an indented honeycomb pattern to it that makes the mirror hollow – meaning it is much lighter and easier to cool down.
But even with this trick, the mirror has to remain in the rotating furnace for about three months as it very slowly cools down to room temperature (a sudden temperature differential could create cracks). Once the glass has finally cooled down, the mirror is carefully lifted out of its hexagonal mold and, over the next several months, the front surface is patiently ground to its final shape with a series of diamond wheels within a twentieth of a wavelength of light.
In the finished product, the surface is so smooth that if the entire mirror was spread out from coast to coast of the United states, the tallest bump on it would be only half an inch tall.
"We have to make the optics precise enough so that light travels five or ten billion light-years and when it hits our telescope it doesn't scramble and lose that information," says McCarthy. "It's a challenge of one part in ten billion in terms of precision manufacturing."
Curiously enough, the main constraints on the size of the mirror aren't the technical capabilities of the lab or the size of the rotating furnace, but rather considerations on how to transport it to its destination (such as the width of the Chilean freeways).
The GMT will be housed on Las Campanas Peak in Chile's Atacama Desert, one of the highest and driest locations on Earth, where it is expected to enjoy good weather conditions for more than 300 nights a year.
"We expect that the telescope will operate for 50 years or more," Dr. McCarthy tells us. "Environmental conditions at observatory sites (light pollution) is usually the limiting factor and we see little chance of light pollution at our site in the Chilean desert."
The great collecting area of the telescope, a hundred times greater than Hubble's, is widely expected to take astronomical observation to new heights. The search for new exoplanets may get a significant boost because the glare from host stars challenges the current generation of telescopes, making detection harder, but will prove much easier with the GMT.
Scientists also expect significant results in the nature of dark matter and dark energy, the evolution of the first galaxies and the first stars, the evolution of the distribution of star and planetary formations throughout the history of the universe, and the growth of black holes.
The video below illustrates some of the challenges of building a telescope of this size, as well as some of its possible uses.
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