Few things in science appear to be as delicate or precarious as the giant mirrors at the heart of modern telescopes. Glass donuts meters in diameter, weighing tons and costing millions of dollars, these mirrors are polished within a fraction of a wavelength of visible light into the precise concavity required to gather and focus starlight from the other end of the universe

When they are not working, they shelter in high domes that protect them from the distortions of humidity, wind and temperature changes. But this cannot protect them from all the vicissitudes of nature and humanity, as I was reminded on a recent visit to the Las Campanas Observatory in Chile.

As my hosts displayed one of their prized telescope mirrors—20 feet of immaculately curved, shiny aluminum-clad glass—I couldn’t help but notice a suspicious little blemish. It looked like the kind of stain you might find on your windshield in the morning, especially if you’d parked under a tree.

“Birds,” one astronomer complained when asked what it was.

It happens all the time, other astronomers say. Michael Bolte, now a professor emeritus at the University of California, Santa Cruz, recalled giving the Wyoming governor a tour of the Wyoming Infrared Observatory outside Laramie in 1981. downstairs, and there was bird droppings all over the mirror,” he said. “It looked awful.”

Not only birds can disfigure a mirror. Mike Brotherton, the current director of the Wyoming observatory, posted a photo on Facebook of the frost that had accumulated on his mirror while the dome was open for observation. “It’s hard to keep a mirror spotless,” he said. “It’s a balance between opening up to take data and protecting the mirror.”

Bird waste has a special place in astrophysics lore. In the early 1960s, radio astronomers Arno Penzias and Robert Wilson, both at Bell Labs, were trying to calibrate an old horn antenna to study galaxies. In an effort to get rid of a persistent background hum, they removed copious amounts of pigeon guano from their telescope, only to finally discover that the hum was cosmic: it was the hissing remnants of radiation from the Big Bang, and it settled firmly. the question of whether the universe had a different beginning.

Fortunately, these biodegradable insults to mirrors are temporary and don’t block much light. The observatories periodically wash their mirrors, remove the old aluminum coatings and apply a new coating, which means remove telescope mirror.

That can be a delicate operation. Last fall, the 8-meter-diameter main mirror of the Gemini North telescope, on Mauna Kea in Hawaii, was sheared off its outer edge as it was moved to be cleaned and resurfaced. The damage was not to the light-collecting part of the mirror, but telescope administrators chose to repair it anyway. On March 31, Jen Lotz, director of the observatory, reported that the repairs were complete and that the telescope, he hoped, would be up and running again sometime in May.

Some things are less easy to fix. On February 5, 1970, a new employee at the McDonald Observatory in West Texas brought a gun to work and opened fire, first at his boss and then several times at point blank range at the main mirror of the new 9-foot reflecting telescope. of the observatory. . He then hit him with a hammer.

Preliminary reports indicated that the mirror had been destroyed; when the sheriff arrived, he noticed that it had a big hole. In fact, the mirror, of a common type called a Cassegrain, was designed and built with central holes to allow light to pass through to the instruments behind it.

No one was injured during the assault. And aside from seven small bullet holes, which affected only about 1 percent of the mirror’s surface area, the telescope was largely unscathed.

“The telescope resumed its observing program the following night,” said observatory director Harlan Smith of the University of Texas. reported to the International Astronomical Union soon after, “producing some of the best pictures (of quasar fields) obtained so far with this instrument in its first year of use.”

That is, the glass of the telescope is stronger than you think. When I first visited the 200-inch Hale Telescope on Palomar Mountain in California, a rite of passage for a young science writer, I was surprised to discover, looking down the barrel of what was then the world’s largest and most famous telescope, a dinner. —a plate-sized cut left by a tool a worker had dropped years before.

Dr. Bolte described a close call at the Canada-France-Hawaii Telescope on Mauna Kea. He and a colleague were in the dome, working on a camera on the telescope, when they noticed that the covers that normally protect the mirror were open. They managed to radio the ground and close the covers.

“We did what we were going to do and were preparing to go down,” Dr. Bolte wrote in a Facebook conversation. “You counted all the tools you brought to the main focus cage and made sure that the count on the way up matched the count on the way down. Just as he was saying to Bob, ‘I think we’re missing a tool,’ a big wrench fell out of the cage and made an incredible noise, hitting the mirror cover.”

The most famous example of what can go wrong with a mirror occurred in 1990, when the Hubble Space Telescope was launched with a misshapen mirror that couldn’t focus.

The astronauts were able to fix it, and Hubble is still going strong. But the episode led NASA to be more cautious with Hubble’s successor, the James Webb Space Telescope, scheduling extensive tests that vastly increased the telescope’s cost and construction time.

The Webb launched dramatically and successfully on December 25, 2021, but the space is also a shooting gallery. The telescope had barely been set up when it was thrown off by a larger-than-expected micrometeorite, which left a small crater in one of the telescope’s mirror segments. NASA has since modified its protocols to minimize the amount of time the telescope is pointed at meteor streams.

And so it goes on. The cosmos has a way of keeping its secrets.

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