June 6, 2023

Few things in science are as fragile or unstable as the giant mirrors at the center of modern telescopes. These mirrors — multimillion-dollar glass donuts measuring meters in diameter and weighing tons — are polished to precise concavities in a fraction of the wavelengths of visible light to collect and focus starlight from the other side of the universe .

When not working, they hide in towering domes that protect them from humidity, wind, and temperature changes. But that doesn’t make them immune to all the vicissitudes of nature and man, as I was reminded on a recent visit to the Las Campanas Observatory in Chile.

As my hosts showed off one of their prized telescope mirrors—20 feet of shiny, perfectly curved aluminized glass—I couldn’t help but notice a small, suspicious blemish. It looks like the kind of smudge you find on your windshield in the morning, especially if you’ve parked the car under a tree.

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

Other astronomers say it happens all the time. Michael Bolt, now a professor emeritus at the University of California, Santa Cruz, recalls a 1981 visit by the governor of Wyoming to the Wyoming Infrared Observatory outside Laramie. There was bird droppings all over the mirror,” he said. “It looked terrible. “

It’s not just birds that can damage mirrors. Mike Brotherton, the current director of the Wyoming Observatory, posted a picture on Facebook of frost on his mirror when the dome was opened for viewing. “It’s hard to keep the mirrors where they are,” he said. “It’s a balance between open access to data and protecting mirrors.”

Guano has a special place in the knowledge of astrophysics. In the early 1960s, radio astronomers Arno Penzias and Robert Wilson, both at Bell Labs at the time, were trying to calibrate an old horn dish to study galaxies. To smooth out the constant background hum, they shoveled copious amounts of pigeon guano out of the telescope, only to learn that the hum was cosmic: a hissing radiation remnant from the Big Bang that settled firmly Down to the question of whether the universe had a unique beginning.

Fortunately, this biodegradable insult to the mirror is temporary and doesn’t block much light.The observatory regularly cleans the mirrors, strips the old aluminum coating and applies a new one, which involves remove the mirror from the telescope.

This can be a tricky operation. Last fall, the outer edge of the 8-meter-diameter primary mirror of the Gemini North Telescope on Mauna Kea in Hawaii was scratched while it was being moved for cleaning and recoating. It wasn’t the light-gathering part of the mirror that was damaged, but the telescope’s managers chose to fix it anyway. On March 31, Observatory Director Jen Lotz reported that the restoration work had been completed, She hopes the telescope will return to work Sometime in May.

Some things are not so easy to fix. On February 5, 1970, a new employee at McDonald Observatory in West Texas came to work with a gun, first firing at his boss and then several times at the primary mirror of the observatory’s new 2.7-meter reflecting telescope. He then struck the knock it.

Initial reports indicated that the mirror had been destroyed; when the sheriff arrived, he noticed a large hole in it. In fact, the mirror is a common type called a Cassegrain, designed and manufactured with a central hole to allow light to pass through the instrument behind it.

No one was injured in the attack. Aside from seven small bullet holes, which affected only about 1 percent of the mirror’s surface area, the telescope remained virtually unscathed.

“The telescope resumed its observing program the following night,” said Harlan Smith, director of the observatory at the University of Texas. report to the International Astronomical Union Soon after, “in the first year of using the instrument, some of the best pictures (of quasar fields) ever taken with this instrument were taken.”

That said, telescope glass is tougher than you might think. When I first visited the 200-inch Hale Telescope in Mount Palomar, California—a rite of passage for a young science writer—I was amazed to find that as I looked down at what was then the largest and most famous telescope in the world When looking at the lens barrel, a plate-sized gash left by a tool dropped by a worker years ago during one dinner.

Dr. Bolte describes a close call with the Canada-France-Hawaii Telescope on Mauna Kea. He and a colleague were on the dome, working on the telescope’s camera, when they noticed that the cover that normally protects the mirror was open. They managed to get the radio to the floor and close the cover.

“We did what we wanted to do and were ready to go down the mountain,” Dr. Bolt wrote in the conversation on Facebook. “You counted all the tools you brought to the prime cage and made sure the count up matched the count down. Like I said to Bob, ‘I think we’re missing one tool’, a big The crescent wrench fell out of the cage with an unbelievable sound and hit the mirror cover.”

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

Astronauts were able to fix it, and Hubble is still going strong. But the incident led NASA to be extra cautious with Hubble’s successor, the James Webb Space Telescope, scheduling extensive testing that dramatically increased the telescope’s cost and construction time.

Webb launched spectacularly and successfully on December 25, 2021, but space is also a shooting range. The telescope was barely set up when it was hit by a larger-than-expected micrometeorite, which left a small crater in one of the telescope’s mirrored sections. NASA has since revised the protocol to minimize the amount of time the telescope spends aiming at the meteor stream.

That’s the way it is. The universe has a way of keeping secrets.

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