From lead to “bird droppings”: many tests go through telescopes

The New York Times – Life/Style – Few things in science seem as delicate or perilous as the giant mirrors at the heart of modern telescopes. these mirrors – Glass donuts a meter in diameter, weighing tons and costing millions of dollars – Polished to within a fraction of the wavelength of visible light, in order to obtain the precise concavity needed to collect and focus light from stars on the other side of the universe.

When they are not working, they are housed in high domes that protect them from the deformations of moisture, wind, and temperature changes. But this cannot protect them from all the vagaries of nature and humanity, as I mentioned on a recent visit to the Las Campanas Observatory in Chile.

When my host showed off one of their prized telescope mirrors—20 feet of aluminized glass, shiny and immaculately curved—I couldn’t help but notice a fishy little flaw. It looked like the kind of stain you’d find on a car’s windshield in the morning, especially if you’ve parked under a tree.

One astronomer grumbled when asked what the “birds” were.

Other astronomers say this happens all the time. Michael Bolt, now a professor emeritus at the University of California, Santa Cruz, remembers taking a tour with the governor of Wyoming of the Wyoming Infrared Observatory outside Laramie in 1981, and there was bird droppings all over the mirror,” he said. “It was horrible.”

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

Bird droppings have a special place in the astrophysical 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. Trying to get rid of the constant background hum, they removed loads of pigeon droppings from their telescope, only to finally discover that the hum was cosmic: it was The hissing of radiation leftover from the Big Bangand the persistent question was whether the universe had a distinct beginning.

Fortunately, these insults to biodegradable mirrors are temporary and don’t block much light. Observatories periodically wash their mirrors, removing old aluminum layers and applying a new one, which involves removing the mirror from the telescope.

This process can be delicate. last fall, The 8-meter primary mirror of the Gemini North telescope, on Mauna Kea, Hawaii, had its outer edge clipped while it was being removed for cleaning and painting. The damage wasn’t to the part of the mirror that collects the light, but the telescope’s managers chose to repair it anyway. On March 31, Jane Lutz, the observatory’s director, reported that repairs were complete and that, as she had hoped, the telescope would return to service in May.

Some things are not easy to fix. On February 5, 1970, a new employee of the McDonald Observatory in West Texas brought a handgun to work and shot, first at his boss and then several times at point-blank range into the main mirror of the new .7-meter reflecting telescope from the observatory. Then he attacked him with a hammer.

Initial reports indicated that the mirror had been destroyed; When the mayor arrived he noticed a large hole in it. Indeed, the mirror, of the common type called Cassegrain, was designed and built with central holes to allow light to pass through to the instruments behind.

No one was injured during the attack. Aside from seven small bullet holes, which only affected about 1% of the mirror’s surface area, the telescope was largely 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 thereafter.“producing some of the best images (of quasar fields) yet obtained with this instrument in its first year of use.”

In other words, telescope glass is stronger than you think. When I first visited Hale’s 16-foot telescope on Palomar Mountain, California—a rite of passage for a young science writer—I was surprised to find, peering into the cylinder of what was then the world’s largest and most famous telescope, a crack the size of a dinner plate left by an instrument dropped by a worker years before. .

Bolt described the Canada-France-Hawaii telescope emergency on Mauna Kea. He and a colleague were in the dome, working on a telescope camera, when they noticed that the covers that normally protect the mirror were open. They managed to get down and close the covers.

“We had what we needed to do, and we were preparing to go down,” Bolt wrote in a Facebook chat. We counted all the tools we took into 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 I’m missing a tool,” a large wrench fell out of the cage and made an incredible noise. , and hit the mirror cover.”

The most famous example of what can happen in the mirror occurred in 1990, when it was Hubble Space Telescope He is freed with a distorted mirror that cannot focus.

The astronauts were able to fix it and Hubble continued to work just fine. But the episode led to NASA To be more careful with Hubble’s successor, f James Webb Space TelescopeExtensive testing scheduling greatly increased the cost and time to build the telescope.

Webb had an amazing and successful launch on December 25, 2021, but space is also a shooting gallery. The telescope had barely begun to function when a small, larger-than-expected meteor hit it, leaving a small hole in one of the telescope’s mirror segments. Since then, NASA has modified its protocols to reduce the amount of time the telescope is pointed at the meteor streams.

And so on and so on. The universe has a way of keeping its secrets. / Translated by LÍVIA BUELONI GONAALVES

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