The Extremely Large Telescope: Our Biggest Eye on the Sky

It’s a weird sight. On a 3,046-meter-high (9,993 feet) mountaintop in the desolate Atacama Desert sits what looks like a shiny steel marble, large enough to be visible from tens of kilometers away. I’ve seen it many times before, but always in artist’s impressions and computer renderings. This time, it’s for real. I’m approaching the giant spherical enclosure of the largest optical telescope in the history of mankind.

When Dutch spectacle maker Hans Lipperhey invented the telescope in the early 17th century, he realized that bigger optics would yield better results. But back then, no one could have imagined something as monstrous as the Extremely Large Telescope, under construction by the European Southern Observatory (ESO). With a primary mirror 39.2 meters (129 feet) wide, the ELT will have more light-collecting power than all previous professional telescopes in history put together. Yes, it sounds incredible, but I checked the math, and it’s really true.

I’m joining a Chilean TV crew on a two-hour visit of Cerro Armazones, the location of the ELT. Armazones lies just 20 kilometers (12 miles) east of ESO’s Paranal Observatory, home to the 25-year-old Very Large Telescope (VLT). In an administration building a few hundred meters below the summit, we are provided with hard hats, safety shoes, and reflective vests. Next, our host, ESO’s Sofía Otero, takes us to the observatory site, which is buzzing with building activity.

Safety coordinator Luigi Pinto, of the Italian consortium that is building the telescope, takes us on a tour around the 90-meter-diameter dome, which is about half-finished. Huge cranes have put the first elements of the dome’s two giant doors in place. “Here’s part of the second door,” Pinto says, while walking around a complicated curved steel structure as large as a bridge. “In fact, one of the previous jobs that I worked on was a large bridge. To me, there’s not that much difference.”

After entering the building, we climb a number of stairways to a high gallery, which runs all the way around the inside of the dome. From here, the view is even more impressive. The white telescope structure, consisting of dozens of interconnected steel tubes, and still surrounded by scaffolding, is nearing completion. It is so large that I have difficulty in perceiving the right scale. Trucks and forklifts that drive around on the concrete floor down below look as small as toy cars.

Pinto points to a technician in an orange overall, who is working inside the telescope, almost too small to discern. “He sits at the level where the primary mirror will be,” he says. “But of course, the mirror segments will only be installed when everything else is ready.” The giant primary will consist of a staggering 798 hexagonal segments, manufactured in Germany. “I believe some 200 of them have already been delivered,” says Pinto.

That evening, at neighboring Paranal Observatory, I meet Guillaume Blanchard, the head of ESO’s optical group, who confirms Pinto’s guess. “If you want, I can take you into the ELT Technical Facility tonight,” he says. “That’s where the mirror segments are tested, coated, and stored.” In fact, there will be 931 segments in the end — 133 spares of various curvatures are needed to keep the mirror complete during the continuous recoating process. “If we want to re-aluminize the whole mirror once every two years, we need to take out and recoat two segments per day,” explains Blanchard.

After dinner, we walk toward a large, rectangular building. Inside is the testing and coating facility, which is relatively small, since the mirror segments are just some 1.5 meters wide. The whole lab is a clean room, so I’m not allowed to enter, but through a large window, I can see one shiny hexagon on a test bench, ready to have its quality checked.

Next, Blanchard takes me into another, much larger room. When he turns on the light, I’m flabbergasted. I’m in a giant warehouse, with meters-high storage racks. Large, flat metal crates fill about one-fifth of the total storage capacity. Every crate, uniquely identified by a QR code, contains a single mirror segment. Around me is the largest telescope mirror in the world, in pieces. About once per month, a new truckload arrives.

When we walk back to Paranal’s Residencia, it’s utterly dark outside. Blanchard looks up to the night sky, with an upside-down Orion high above the northern horizon. Even after working with the largest telescopes in the world for many years, the naked-eye view never ceases to amaze and inspire him, he says. “This is really one of the best places on the planet to do astronomy.”

Unfortunately, that discerning quality is soon to be challenged. Earlier that day, Ortego told me about the plans of AES Andes — a subsidiary of an American energy company — to build a “green hydrogen plant” as large as a small city, just 11 kilometers south of the VLT and 20 kilometers southwest of Cerro Armazones. According to a recent ESO study, the facility, with huge numbers of wind turbines and solar panels, would produce air turbulence, vibrations, and dust contamination, and lead to a 35% increase in light pollution.

“Nothing is definitive yet,” says Ortego (the proposed site still has to be approved by the Chilean government), “but we really need to have our voice heard. I just don’t understand why they couldn’t choose another location.” Three weeks after my visit, an ESO press release states that the impact of the facility would be “devastating and irreversible.”

When I drive away from Cerro Armazones, eventually heading north to Arica, where my six-week Chile road trip comes to an end, I keep seeing the dome of the Extremely Large Telescope in my rearview mirror, surrounded by tower cranes. Some four years from now, humankind’s biggest eye on the sky will start observing the universe, hopefully unhindered by industrial activity. I’m looking forward to returning to this astronomical paradise to attend the telescope’s inauguration.