From bullets to “bird waste”, the many telescope tests

From bullets to “bird waste”, the many telescope tests

From bullets to "bird waste", the many telescope tests

Few things in science seem as delicate or precarious as the giant mirrors at the heart of modern telescopes. These mirrors – glass donuts several meters in diameter, weighing tons and costing millions of dollars – are polished to a fraction of a wavelength of visible light in the precise concavity required to collect and focus starlight. from the other side of the universe.

When they are not at work, they are sheltered under high domes which protect them from the distortions of humidity, wind and temperature changes. But that cannot shield them from all the vicissitudes of nature and humanity, as I was reminded during a recent visit to the Las Campanas Observatory in Chile.

As my hosts showed off one of their prized telescope mirrors – 20 feet of perfectly curved, gleaming aluminum-coated glass – I couldn’t help but notice a suspicious little speck. It looked like the kind of smear 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 professor emeritus at the University of California, Santa Cruz, remembers showing the Governor of Wyoming the Wyoming Infrared Observatory outside Laramie in 1981. service platform and looked down, and there was bird droppings all over the mirror,” he said. “It looked awful.”

It’s not just birds that can mar 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 intact,” he said. “It’s a balance between being open to take data and protecting the mirror.”

Bird remains have a special place in astrophysical traditions. In the early 1960s, radio astronomers Arno Penzias and Robert Wilson, both then 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 shoveled copious amounts of pigeon guano out of their telescope, eventually learning that the hum was cosmic: it was the hissing remnants of Big Bang radiation, and he settled firmly. the question of whether the universe had a distinct beginning.

Luckily, these biodegradable insults to mirrors are temporary and don’t block out much light. Observatories periodically wash their mirrors, remove old aluminum coatings, and apply a new coat, which involves removing the mirror from the telescope.

This can be a tricky operation. Last fall, the 8-meter-diameter primary mirror of the Gemini North Telescope on Mauna Kea in Hawaii was nicked on its outer edge as it was moved for cleaning and re-covering. The damage was not to the part of the mirror that collects light, but telescope officials opted to repair it anyway. On March 31, Jen Lotz, the observatory’s director, announced that repairs were complete and that the telescope, she hoped, would be operational again in May.

Some things are harder to fix. On February 5, 1970, a new employee at the McDonald Observatory in West Texas took a gun to work and opened fire, first at his boss, then several times at close range at the main mirror. of the observatory’s new 2.7-meter reflecting telescope. . Then he went there with a hammer.

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

No one was injured in the attack. And aside from seven small bullet holes, which only affected about 1% of the mirror’s surface, the telescope was virtually unscathed.

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

Simply put, telescope glass is tougher 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 find, looking down the barrel of what was then the The world’s largest and most famous telescope, a supper – a plaque-sized gash left by a tool a worker had dropped years earlier.

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

“We did everything we were going to do and were preparing to go down,” Dr. Bolte wrote in a Facebook conversation. “You counted all the tools you took to the main focus cage and made sure the count on the way up matched the count on the way down. Just as I was saying to Bob, “I think we’re missing a tool,” a large adjustable 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 happened in 1990, when the Hubble Space Telescope launched with a distorted mirror that couldn’t focus.

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

The Webb launched spectacularly and successfully on December 25, 2021, but space is also a shooting range. The telescope had just settled down when it was bombarded by a larger-than-expected micrometeorite, which left a tiny crater in one of the telescope’s mirror segments. NASA has since changed its protocols to minimize the time the telescope is pointed in meteor streams.

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


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