Few things in science seem as delicate or precarious as the giant mirrors at the heart of modern telescopes. These mirrors—meter-diameter glass donuts that weigh tons and cost millions of dollars—are polished within a fraction of a wavelength of visible light to the precise cavity needed to collect starlight from the other end of the universe and to focus.
When not working, they are sheltered in tall domes that protect them from the disturbances 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 Chile’s Las Campanas Observatory.
As my hosts showed off one of their prized telescope mirrors—20 feet of gleaming, impeccably curved aluminum-coated glass—I couldn’t help but notice a small, suspicious smudge. It looked like the kind of stain you might find on your windshield in the morning, especially if you parked under a tree.
“Birds,” grumbled an astronomer when asked what it was.
It happens all the time, say other astronomers. 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. were bird droppings all over the mirror,” he said. “It looked awful.”
It’s not just birds that can damage a mirror. Mike Brotherton, the current director of the Wyoming observatory, posted a photo to Facebook of ice that had built up on his transom while the dome was open for observation. “It’s hard to keep a mirror spotless,” he said. “It’s a balance between opening to take data and protecting the mirror.”
Bird remains hold a special place in astrophysical lore. In the early 1960s, radio astronomers Arno Penzias and Robert Wilson, both then at Bell Labs, attempted to calibrate an old horn antenna to study galaxies. In an effort to get rid of a persistent background hum, they scooped massive amounts of pigeon guano from their telescope, only to 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 clear beginning.
Fortunately, such biodegradable insults to the mirrors are temporary and don’t block much light. Observatories periodically wash their mirrors, removing the old aluminum coatings and applying a new coating, removing the mirror from the telescope.
That can be a tricky operation. Last fall, the 8-meter diameter main mirror of the Gemini North telescope on Hawaii’s Mauna Kea got a notch on its outer edge while being moved for cleaning and recoating. The damage was not to the part of the mirror that collects light, but the telescope’s managers chose to repair it anyway. On March 31, Jen Lotz, the observatory’s director, reported that repairs were complete and the telescope, she hoped, would be back in service sometime in May.
Some things are less easy to solve. 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 point blank several times at the primary transom of the new 9-foot reflecting telescope of the observatory. . Then he went at it with a hammer.
Preliminary reports indicated that the mirror had been destroyed; when the sheriff arrived he had noticed that it had a large hole in it. In fact, the mirror, of a common type called Cassegrain, is designed and built with central holes to allow light to pass through to instruments behind it.
No one was injured during the attack. And apart from seven small bullet holes, covering only about 1 percent of the mirror’s surface, the telescope was virtually unscathed.
“The telescope resumed its observing program the following night,” the observatory’s director, Harlan Smith of the University of Texas, reported to the International Astronomical Union shortly afterwards, “producing some of the best photographs (of quasar fields) yet. made with this telescope instrument in its first year of use.”
That is, telescope glass is stronger than you think. When I first visited the 200-inch Hale telescope on California’s Palomar Mountain—a rite of passage for a young science writer—I was shocked to discover, looking down the barrel of what was then the world’s largest and most famous telescope, a diner plate-sized gash left by a tool dropped by a worker years earlier.
Dr. Bolte described a close call at the Canada-France-Hawaii telescope on Mauna Kea. He and a colleague were working on a camera in the telescope at the top of the dome when they noticed that the covers that normally protected the mirror were open. They managed to radio down to the ground and get the covers closed.
“We did what we were going to do and got ready to come down,” wrote Dr. Bolte in a Facebook conversation. “You counted all the tools you brought into the primary focus cage and made sure the count on the way up matched the count on the way down. Just as I said to Bob, “I think we’re one tool short,” a large crescent wrench fell out of the cage and made an incredible noise, hitting the transom cap.
The most famous example of what can go wrong with a mirror happened in 1990, when the Hubble Space Telescope was launched with a deformed mirror that failed to focus.
Astronauts were able to fix it and Hubble is still doing it right. But the episode caused NASA to be extra careful about Hubble’s successor, the James Webb Space Telescope, planning extensive testing that vastly increased the telescope’s cost and time to build.
The Webb launched spectacularly and successfully on December 25, 2021, but space is also a shooting gallery. No sooner was the telescope installed than it was pelted 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 focuses on meteor streams.
And so it goes. The cosmos has a way of guarding its secrets.
