The Giant Magellan Telescope will be the world’s biggest telescope — and it will look deeper into the past than we’ve ever seen before.
The universe is a big place.
Let’s rephrase that. The universe is a gargantuan place. At this moment in time, astronomers believe that it measures some 93 billion light-years across.
When we look out across space, though, we’re not just looking through distance. We’re looking back in time. All of the light we see here on Earth is from the past, so observing something a billion light-years away means we see it as it existed a billion years ago.
Since astronomers estimate the age of the universe at 13.8 billion years, all we have to be able to do is see something 13.8 billion light-years away, and we can see the beginning of time. The beginning of the universe. The beginning of everything we know.
And we’re almost there. Just this month, researchers officially announced the discovery of the most distant galaxy we’ve ever seen, from over 13 billion light-years away: EGS-zs8-1, located in the constellation of Boötes. Data about EGS-zs8-1 from the W.M Keck Observatory in Hawaii revealed some interesting clues about galaxy formation in the early universe; but the farther we can see, the more we will know.
Cue the Giant Magellan Telescope — an Earth-based telescope planned for first light in 2021, located in Chile’s Las Campanas Observatory. What will make the Giant Magellan Telescope so spectacular is its size: its reflectors will consist of an array of seven mirrors 8.4 metres in diameter each (27 feet), resulting in a single reflective surface 24.5 metres (80 feet) across. Its total collecting area will be 368 square metres.
This will give the GMT the biggest reflecting surface ever created — over four times the size of the world’s current biggest single-aperture optical telescope, the Gran Telescopio Canarias.
“The Giant Magellan Telescope will enable us to look back through time to see the birth of stars, galaxies, and events that occurred shortly after the Big Bang. Through this we will achieve a better understanding of our origins,” explained Dr Patrick McCarthy, director of the Giant Magellan Telescope.
“We’ll also be able to characterise planets orbiting other stars to try and, perhaps, determine whether or not they are suited to supporting life. The GMT will also help us to better understand fundamental phenomena such as dark energy and dark matter.”
The reason we don’t already have a telescope this size on Earth is due to the technical challenges. As demonstrated by the Hubble telescope, even an error of just a few micrometres off the curve of what the glass of the reflector is supposed to be can result in images too blurred to be of use. These pieces of glass need to be cast in a rotating mould, and shaped according to calculation, ground and polished to exactitude.
“Prior to the late 1990s we did not understand how to make mirrors larger than the Mt. Palomar mirror made in the 1930s. In the late 1990s, several six- to eight-metre-diameter mirrors, or arrays of mirrors, were deployed around the world and astronomers worked to understand their performance,” Dr McCarthy explained.
“Following this, starting around 2002, scientists and engineers began designing giant telescopes with multiple large mirrors, like the GMT, or telescopes with hundreds of smaller mirrors, to make a single aperture larger than can be formed from a single piece of glass.”
Making the GMT’s mirrors is a painstaking process, conducted by experts at the University of Arizona Steward Observatory Mirror Lab.
Each mirror is made from 18 tons of low expansion (borosilicate) glass, carefully inspected, then loaded piece by piece into a mould. This is made of silicon carbide cement, lined with ceramic fibre and filled with 1,700 alumina-silica fibre hexagonal boxes, the top of which must follow the shape of the final mirror.
The mould then becomes the bottom part of a furnace that melts the glass. As the furnace’s temperature is brought to 1,160 degrees Celsius (2,120 degrees Fahrenheit), the furnace spins at a rate of about five rotations per minute, which gives the mirror its parabolic curved shape.
This temperature needs to be maintained for four hours to allow the glass to melt, after which it is dropped rapidly to 900 degrees Celsius. Then the glass has to cool a lot more slowly, over a period of three months, to avoid imperfections.
Once it is cooled, it is removed from the mould and the hexagonal boxes removed. This gives the piece of glass a hollowed-out honeycomb on the back, keeping it its weight down. Then the polishing process begins, using a special polishing tool that actively conforms to the mirror’s aspherical curve, while the mirror’s accuracy is tested.
It will then be coated in a vacuum chamber with a layer of aluminium just a few atoms thick for ultra-high reflectivity. Because it is so fine, this will degrade, and will need to be reapplied to the glass every two years.
“The biggest technical challenge we foresaw was the ability to fabricate and polish the off-axis primary mirror segments. These segments will be positioned on the outside of the mirror array and act as the outer edge of this large, segmented mirror,” Dr McCarthy said.
“Because of this position, they cannot be symmetrical and instead are shaped more like a saddle. This was very hard to do precisely, but thanks to the team at the University of Arizona’s Steward Observatory Mirror Lab, we have been able to retire this challenge and are on our way to producing all of the mirrors for the GMT.”
To date, four of the seven mirrors have been cast — enough for the telescope to commence operations, should the remaining three mirrors remain unfinished by the time construction of the telescope facility is complete.
It’s expected that the quality of the images produced by the telescope will exceed even the James Webb Space Telescope, the successor to the Hubble, planned to be launched into low-Earth orbit in 2018. This is in spite of distortion. Unlike space telescopes, ground-based telescopes need to be able to correct for a distortion effect in the atmosphere.
This effect is called atmospheric refraction, and the GMT will use lasers and adaptive optics — to help correct it.
“We will use six laser beams to make artificial stars on the sky to provide us with beacons that probe the temperature and density profiles of the atmosphere above the telescope. The distortions due to temperature and density variations blur the images of the stars just as heat rising from a roadway on a summer day makes images of distant objects seem to shimmer in the heat. The GMT adaptive optics system and the lasers will allow us to correct for these distortions and create sharp images.
It will be erected on a peak in the Chilean Andes, near other existing telescope facilities. A suite of instruments will help scan the skies, including three spectrographs, and a near-infrared and adaptive optics imager.
Hundreds of actuators underneath each secondary mirror shift the mirror’s position in very slight, precise increments, constantly adjusting the mirrors to counteract atmospheric turbulence. These motors are what will allow the telescope to photograph clean, crisp images 10 times sharper than those taken by the Hubble.
It won’t be long until the GMT has assistance. The European Southern Observatory is planning an even more massive telescope — the European Extremely Large Telescope, due for first light in 2024, with a 39-metre mirror; and the TMT International Observatory is planning the Thirty Metre Telescope, due for first light in 2022, with a 30-metre mirror.
But, just as there is room for more than one telescope in the world today, there’s a lot out there to see, and the Giant Magellan Telescope will be looking for something very specific.
And, hopefully, the GMT will find it. The GMT’s primary goal is to photograph the Big Bang, to help us understand the origins of the universe. But that’s not its sole purpose. It will also help study dark matter and dark energy, the invisible forces acting on the universe, the growth of black holes over billions of years and look for extraterrestrial life.
“One of the most interesting unresolved questions about our universe that we hope to answer is: are we truly alone?” Dr McCarthy said. “The GMT will enable us to identify potentially habitable planets and to characterise the chemical composition of their atmospheres. This could lead to detection of biomarkers.”
But, he added, the beginning of everything — that will be the most wonderful discovery.
“The prospect that most excites me is looking at the most distant objects in the universe to a time when galaxies were young and still forming and acquiring their distinctive spiral shapes that we see today. To see the ‘first light’ in the Universe would be a profound experience.”
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