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Putting a shine on Palomar.

Every two or three years the primary mirror of the 5-meter Hale Telescope is removed, cleaned, and given a fresh coat of aluminum.

"I'M SWEATING BLOOD when I approach taking the mirror out of the telescope," said Robert P. Thicksten just before an aluminizing run last year. I was surprised. After all, recoating the magnificent 5-meter (200-inch) instrument atop Palomar Mountain has to be done every two or three years, and one would think that after five decades it would be a routine operation. But Thicksten's furrowed, perspiring brow suggested otherwise. "I realize," admitted the facility's meticulous superintendent, "just how incredibly valuable this piece of glass is."

Later he recalls, with considerable pain in his voice, a moment some years ago when he thought, for an instant, that he had destroyed the mirror. After a new reflective aluminum coating had been deposited on its polished surface (a critical three-day process), air rushed into the vacuum chamber too quickly. An aluminum pan was flung up and out of the mirror's center hole and into the heating filaments suspended above the glass.

"When we removed the bell jar, it looked like the mirror had been shattered," he remembers, "with fragments of filament all over it. I felt like I had just watched my son being run over by a freight train." Fortunately no damage was done, except possibly to Thicksten's blood pressure. "I just about had my first heart attack."

"There are two ways to be famous in astronomy," concludes Robert J. Brucato, the observatory's assistant director. "One is to discover something incredible, and the other is to break the 200-inch mirror!"

Brucato sings praise for Thicksten, who as a child learned respect for optics while grinding his own telescope mirrors. "He's extremely conscientious," Brucato explains. "He's put the place on a preventative-maintenance program, and as a result the machinery functions better than it ever did."

During aluminizing I watched Thicksten and his crew solve one of the telescope's thorniest problems since its completion in 1948. From time to time, but especially after aluminization, star images suffered from astigmatism, doubling from 0.45 to 0.90 arc second in diameter. Historically, the problem had been remedied by "exercising" the mirror -- bobbing it up and down on its 36 support mechanisms like someone on a waterbed. This loosened the system's levers, weights, and bearings.

But on this occasion, for the first time in 45 years, chief engineer Hal L. Petrie decided to remove one of the complex support devices. He found that its bearings had been damaged by repeated trips through the vacuum chamber. "The whole mirror cell goes into the chamber during mirror coating," explains Petrie. "The bearing grease had oxidized, hardened, and essentially turned into a very hard plastic. Some of the bearings were absolutely frozen."

Having just visited NASA in Houston, I offered Petrie the phone number for one of the space agency's big vacuum chambers. Soon Palomar was using a NASA-qualified space lubricant in place of the half-century-old grease. As of January all 39 bearings in each of the 36 mechanisms had been replaced. "We've reduced the astigmatism by a factor of five," says Petrie proudly. "The telescope works better now than it did when it was new."

"It's a truly awesome scientific instrument," says Thicksten at the beginning of the third, final day of the aluminizing run. "To me Palomar is the most efficient of all the telescopes I know about. For example, when the Moon comes up, it takes only five minutes to swap from the CCD to a spectrograph.

"If there's a superstar among telescopes, Palomar is it. It's legendary," he continues. "In the '30s and '40s, Palomar was America's entire 'space' program. It was like all the NASA projects of the '60s rolled up into one big effort."
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Title Annotation:the Hale Telescope on Palomar Mountain
Author:Ressmeyer, Roger H.
Publication:Sky & Telescope
Date:Dec 1, 1994
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