When the Hooker Telescope first looked skyward in 1917, no one knew what wonders it might reveal. Within a decade, astronomer Edwin Hubble used it — then the largest telescope in the world, at 100 inches across — to discover that galaxies exist beyond the Milky Way, and that the universe is expanding.
History repeated itself starting in 1949, when the 200-inch Hale Telescope took its first photograph of the night sky. In the early 1960s, astronomer Maarten Schmidt used the instrument to analyze unusual, “quasi-stellar radio sources”— quasars for short. These turned out to be supermassive black holes accreting matter in the centers of galaxies, a science-fiction fantasy when the Hale Telescope was built.
By the 1990s, technology advanced far enough to usher in an era of telescopes 8 to 10 meters across (26 to 33 feet), and the same story played out once more. With an essential assist from the 2.4-meter Hubble Space Telescope orbiting above Earth’s image-distorting atmosphere, these instruments could analyze a few dozen distant Type Ia supernovas — the cataclysmic explosions of white dwarf stars. Shockingly, researchers discovered that the expansion of the universe is accelerating. Again, this was only possible with the increased firepower of the latest telescopes.
Now, astronomers stand on the threshold of a new telescope revolution. During the next several years, researchers expect three instruments that are more than twice the size of their closest competitors to start scanning the skies. And a fourth telescope, one “only” 8 meters in diameter, will use advanced technology to image the entire night sky every three days.
This quartet of new instruments promises to deliver stunning science on the hot-button issues. But, as with the previous great leaps forward in size, the new scopes likely will also make discoveries that no one can yet envision. As Pat McCarthy, vice president of the Giant Magellan Telescope (GMT) Organization, puts it: “We expect to learn things we don’t know.”
Astronomers are always looking to stretch boundaries — to see fainter objects in greater detail. A bigger telescope collects more light, and so allows a deeper view of the cosmos. Double the diameter of the main mirror gathering light for the telescope and you’ve quadrupled its surface area, and thus the amount of light it gets. An observation that once took four hours can now be accomplished in one, and this same mirror will let you see roughly twice as far away.
But you might wonder where the law of diminishing returns sets in. There’s only so far you can see, after all. Perhaps the Hubble Space Telescope recently approached those limits when it wrapped up its Frontier Fields program, which allowed researchers to observe galaxies as they existed only a few hundred million years after the Big Bang. And for closer objects, Hubble delivers images beyond compare despite a relatively small size. What else can people want?
Well, professional astronomers don’t live by imaging alone. More often than not, they need breakdowns of light, called spectra, of the things they observe, to tease out information about an object’s temperature, velocity, rotation and composition. Indeed, a spectrum is the only way to distinguish starlight from a glowing gas cloud, or a faint star in the Milky Way’s vicinity from a fuzzy galaxy in a distant corner of the universe. And to get enough light to do even a minimal amount of spectral analysis takes about 100 times longer than getting an image does. Luckily, bigger scopes allow that processing time to come down significantly.
Resolution also increases with a telescope’s diameter. Make a mirror twice as wide and it delivers twice as much detail. And thanks to a quirk of physics, you can reap the same benefit by placing smaller telescopes farther apart and then combining their light, through a process known as interferometry. (Radio astronomers using this technique produced the first image of a black hole earlier this year: A global network of radio telescopes saw across about 54 million light-years to capture the supermassive black hole at the center of the giant galaxy M87.)
Ground-based telescopes face an additional challenge: Earth’s detail-destroying atmosphere. As light from a celestial object passes through air at different temperatures, it gets jostled about and loses clarity. That’s a big reason why designers place large telescopes on high mountaintops — there’s far less air above them to interfere. Even temperature differences between the air outside and inside a telescope’s dome can generate air currents that adversely affect an image’s sharpness.
That’s where adaptive optics comes in. In the past few decades, astronomers have honed this technique, which mechanically compensates for any atmospheric shenanigans and
delivers images nearly as sharp as the mirror can theoretically produce. The heart of an adaptive optics system is a thin, flexible, computer-controlled mirror. Astronomers target a fairly bright reference star close to the object they want to study. The computer analyzes the incoming light to measure how the atmosphere blurs it, then tells the control system how to adjust the mirror’s shape to correct the image in real-time. Because atmospheric turbulence changes constantly, such systems can alter the mirror’s shape up to 1,000 times each second. And if no bright reference star lies nearby — as often happens — astronomers can simply shine powerful laser beams into Earth’s upper atmosphere and create their own reference light.
Before they can take advantage of the next generation of telescopes, of course, engineers have to craft the parts — namely, those essential and enormous mirrors. Astronomers have developed two designs for them.
In the first, they cast a single, monolithic mirror. University of Arizona astronomer Roger Angel pioneered this method after conducting a backyard experiment around 1980. Technicians start the process by loading chunks of glass into a furnace mold. They then raise the furnace’s temperature to 2,100 degrees Fahrenheit, and spin the entire assembly at a rate of five revolutions per minute. Once the chunks melt to the consistency of thick honey, the glass flows into a bowl-like or parabolic shape — perfect for focusing incoming starlight — as a result of the rotation. The mirrors are no more than 1 inch thick and have a honeycomb structure to keep their weight down. Technicians then grind and polish the mirror’s surface to the exact shape needed.
Arizona’s Richard F. Caris Mirror Lab has cast mirrors for many of the world’s largest telescopes, including the 6.5-meter MMT Observatory and the twin 8.4-meter monsters of the Large Binocular Telescope, both in Arizona.
The second design technique, developed in 1977 by the late astronomer Jerry Nelson of the University of California, Santa Cruz, combines many hexagonal mirror segments into a single structure. Although the segments themselves are not huge, joining them together can result in a world-class telescope. Both of the 10-meter Keck telescopes on Hawaii’s Mauna Kea feature 36 segments, each about 6 feet across and weighing 880 pounds. The 10.4-meter Gran Telescopio Canarias on La Palma in the Canary Islands has the same number of hexagonal segments as the slightly smaller Kecks.
Superfast Sky Survey
So what will these new instruments actually be, and what will they do? Of the four next-generation scopes preparing to revolutionize astronomy, the Vera C. Rubin Observatory should be the first to land on the scene. What sets the Rubin Observatory's Simonyi Survey Telescope apart is not its size — its 8.4-meter primary mirror would fit in comfortably at several current mountaintop observatories — but its ability to image wide swaths of sky quickly.
Situated atop Cerro Pachón in north-central Chile, the Rubin Observatory should take just 15 seconds to deliver sharp images covering 9.6 square degrees of sky — equivalent to the area of more than 40 full moons, and nearly 5,000 times the field of Hubble’s Wide Field Camera 3.
“The [Rubin Observatory] will get the big picture in space-time by taking over 800 images [nightly] of every visible patch of sky in six color filters,” says Rubin Observatory chief scientist Tony Tyson of the University of California, Davis. “This will be a digital color movie of the universe, probing nature in new ways.”
Equally important to the Rubin Observatory’s success is its 3.2-gigapixel imaging camera. The largest digital camera in the world is not one you would want to lug along on your next vacation: It spans 5.5 by 9.8 feet and weighs about 6,200 pounds. With it, the Rubin Observatory will take two consecutive 15-second images of a single patch of sky, and then quickly compare them to reject any stray radiation hitting the detectors. (It’s similar to taking multiple photos of a famous building to digitally remove the tourists.) The scope then whips to the next area of sky — a movement that takes just 10 seconds, on average — and repeats the process. Such rapid-fire imaging means the Rubin Observatory can cover the entire sky visible from Cerro Pachón every three days.
Computer software will initially process the images in 60 seconds, looking for anything that has changed brightness or position compared with previous images of the same area. When it finds something, it’ll immediately send out an alert to researchers for quick follow-up. Astronomers expect the Rubin Observatory to deliver up to 10 million alerts per night — an average of 278 per second during a typical 10-hour observing session.
This will be a boon to scientists studying transient events, such as the stellar explosions that produce novas and supernovas. The Rubin Observatory's efforts should also develop a detailed census of small solar system objects, discovering 10 to 100 times more near-Earth objects and distant Kuiper Belt objects beyond Neptune’s orbit.
The Simonyi Survey Telescope’s mirror, cast in the Caris Mirror Lab starting in March 2008, made it to the mountaintop May 11, 2019. Astronomers expect it to come online in 2020, with full science operations for its planned 10-year survey starting in 2022 after it’s fully calibrated.
Seven Times the Charm
If one huge mirror can deliver so much science, why not try seven? That’s the idea behind the GMT, under construction at Chile’s Las Campanas Observatory. The GMT comprises seven 8.4-meter mirrors in a single structure, arranged in a daisylike pattern with one central mirror surrounded by six “petals.” The Caris Mirror Lab has been busy working on this project, and just completed the second mirror in July; the next three have all been cast and are at various stages of grinding, polishing or testing. At Las Campanas, a 40-person crew finished excavating the telescope’s foundation last spring.
“We can operate with four mirrors in place,” says McCarthy. “That still makes it the largest telescope in the world by far.” The GMT should reach that milestone in 2026, and all seven should be in place by 2028. Collectively, the mirrors will give the instrument an effective aperture of 24.5 meters, about 10 times that of Hubble, so it should achieve resolutions 10 times better than the orbiting observatory. And its location some 8,248 feet above sea level in the arid Atacama Desert will give it superb views in visible light as well as the near-infrared spectrum. But it won’t be the only one with those new and improved views.
A Hex Upon Your Scope
The other two giant telescopes of the next decade have gone a different route. Both the Extremely Large Telescope (ELT) and the Thirty Meter Telescope (TMT) will consist of hundreds of hexagonal segments joined together to create mammoth collecting areas.
Europe’s ELT boasts 798 segments in its primary mirror — each measuring 55 inches across — giving the telescope’s primary mirror an aperture of 39 meters. The German optical company Schott cast the first of these segments in early 2018, and has been churning them out since. Groundbreaking for the mammoth telescope took place in June 2014 on Cerro Armazones, a 9,993-foot mountain in Chile. If all goes according to plan, the ELT should see first light in 2025, around the same time as the GMT.
As its name suggests, the TMT’s 492 segments will give the telescope’s primary mirror an aperture of 30 meters. The project’s Japanese partners are producing the rough mirrors, which are about the same size as the ELT’s, while groups in Japan, China, India and the United States will polish, cut and mount them. The TMT will join its Keck cousins on the summit of Mauna Kea at an altitude of 13,287 feet. The site gives the TMT access to the entire northern sky, something none of the other three can get from their sites in Chile. It is also the highest of the big new scopes, placing it above more of Earth’s atmosphere.
But the site also comes with a major drawback. Mauna Kea is sacred to Native Hawaiians, and the telescope’s construction has drawn various protests. It wasn’t clear whether the new observatory would ever be built, but Hawaii’s Supreme Court ruled in October 2018 that construction could proceed.
The TMT’s enclosure — which will house the scope itself and related electronics — is already finished and awaiting shipment to the island from Canada. With the legal challenges presumably settled, scientists are looking toward first light in 2026.
Science by the Boatload
With their unprecedented light-gathering power and resolution, the GMT, ELT and TMT promise astronomers the best views yet of faint objects and crowded regions. Scientists expect these behemoths to shed light on a variety of vexing problems. Close to home, hunting for Earth-like planets in Earth-like orbits around nearby stars will be a priority. Even more exciting will be the new ability to scrutinize these worlds. “Most of these exoplanets are in too close to their parent stars to study today,” says McCarthy. But with the GMT and other large scopes, “We’ll separate the light of hundreds of planets from their host stars. We’ll be able to track weather through color changes and look at the chemistry of planetary atmospheres.”
Star birth and star death should also be fertile fields of study. High-resolution spectra will help researchers understand why stars come in such a wide range of masses, and probe deeper than ever into the lower-mass failed stars known as brown dwarfs. At the opposite end of a star’s life, these monster instruments will search for supernovas in the farthest reaches of the universe and study closer ones in extraordinary detail, looking at the cosmic alchemy happening in these exploding stars. The scopes’ high resolution will also let astronomers study the crowded central regions of the Milky Way Galaxy and star clusters such as R136 in the Large Magellanic Cloud.
These giant telescopes should also answer even bigger questions about the basic structure of the universe. With these large-aperture scopes and infrared capabilities, McCarthy says, “we’ll [be able to] look back to the early universe, to galaxies only 100 to 500 million years old.” This will be a vital first link to providing a grand view of how galaxies evolve over time, and their relation to the supermassive black holes at their centers. The scopes should even illuminate how the Milky Way has grown by swallowing nearby dwarf companions, and potentially solve the riddle of what came first: galaxies or their black holes.
On the biggest stage, the cosmos still baffles scientists seeking explanations of the dark matter that holds galaxies together and the dark energy that causes the expansion of the universe to accelerate. These new telescopes will provide vital new data to help solve these mysteries, and may help resolve the discrepancy between different ways of measuring the universe’s expansion rate.
In most of these endeavors, the big new scopes will work together with the orbiting 6.5-meter James Webb Space Telescope, which is scheduled to launch in 2021. With any luck, we may know a lot more about the intricacies of our cosmos in the next 10 to 15 years. But as the Hooker and Hale telescopes showed, we may also have a new batch of mysteries to try to figure out.
Editor's Note 12/7/2020: Following its renaming after the original text of this story was published, references to the Large Synoptic Survey Telescope (LSST) have been updated to the Vera C. Rubin Observatory and the Simonyi Survey Telescope.
Richard Talcott is a senior editor at Astronomy magazine. This story originally appeared in print as "Go Big."