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Palomar Observatory crews completerealuminizing of 200-inch Hale mirror

[Note: Palomar Observatory will host a conference on mirror-cleaning initiated by Bob Thicksten and supported by AURA Inc. The conference, which will be attended by about 40 telescope specialists from around the world, will be held at the observatory April 25-27. Media representatives are invited to attend if they make prior arrangements. For more information, contact the Palomar campus office at (626) 395-4033]

Slowly and cautiously, Palomar Observatory superintendent Bob Thicksten leans over on a clean paper cloth to dab soapy water on the 14-ton Pyrex mirror that serves as the heart of the Hale Telescope. He has outfitted his assistants with high-quality sea sponges and has cautioned them to dab rather than rub so that the priceless 200-inch glass surface will not be scratched by a grain of dust.

Thicksten, a 22-year employee of the observatory, is preparing the mirror surface so the thin aluminum coating can be stripped and another coating put on. The aluminum film, scarcely a hundredth the width of a human hair, has degraded due to exposure to the elements and no longer reflects as it did when deposited just a year ago.

"We realuminize the mirror about every two years on average, though we'd like to increase that to once a year," says Thicksten, while his crew runs water continuously over the surface of the mirror to keep water marks from forming.

Once he's satisfied that all loose dirt and grime have been removed, Thicksten and employees Pam Thompson, Dane Cuney, and Tammy Allen don protective clothing and masks so they can wash down the mirror surface with an acid to remove the old aluminum. Again, Thicksten is literally in the middle of the work—standing on a plate rising from the floor and jutting through the 41-inch center hole to avoid walking on the mirror surface itself. Because a dropped tool or misstep could damage the mirror's surface, Thicksten does all the tricky work itself.

"I've got the ultimate responsibility, so the things that worry me the most are the things you'll find me doing," he says.

One thing everyone is certain about is that the 200-inch mirror is virtually irreplaceable. No one—not even Corning, the original caster of the mirror blank—is making such huge monolithic pieces of glass the old way these days. In fact, when the California Institute of Technology checked a few years back to see how much it would cost to commission a replacement for the mirror in the event of a disaster, the Institute was told that a $1 million study would be necessary to even answer the question.

This is not to say that the 200-inch mirror is the largest ever cast—it's not. And in fact, there is a newer spin-casting technology that could be employed in making a replacement. Though it would not be an exact replica, a spin-cast mirror would cost about $9 million, according to a previous estimate. But the days are over when tons of Pyrex were melted in a giant mold and the entire mass was left undisturbed to slowly cool over a period of many months.

When Palomar Observatory was dedicated in the summer of 1948, the Caltech facility instantly became the pre-eminent astronomical observatory in the world and its 200-inch Hale Telescope the best existing instrument by far. The 200-inch, named after famed astronomer George Ellery Hale, maintained its distinction as the world's leading astronomical instrument until the Keck Observatory opened about a decade ago.

Almost immediately after it was commissioned, the Hale Telescope was responsible for revolutionizing the cosmological distance scale, and in later years was used to better describe the evolution of stars and to establish the optical basis of quasars. It was the first telescope to be used to study infrared sources—still a major emphasis of Palomar research and still a furiously active area in both space- and ground-based research programs—and currently is used to aid in the studies of galactic structure and gamma-ray bursts.

But as Thicksten points out, the entire usefulness of the telescope ultimately depends on an extremely thin surface of aluminum weighing less than an ounce.

After about four hours of washing, gently rubbing with an acid bath to remove the aluminum, rewashing, rubbing the surface with potassium hydroxide, again washing with distilled water, and finally patting down the surface with 200-proof ethyl alcohol until the glass literally squeaks with cleanliness, the crew is ready to seal the mirror in a giant "bell jar," or vacuum chamber, for the actual aluminizing process to begin.

Also involved is Palomar Observatory's chief engineer Hal Petrie. A Caltech alumnus and 13-year employee of the observatory, Petrie has been working with Thicksten on improvements to the aluminum deposition process. Too much aluminum, he explains, and the mirror surface takes on a wavy pattern of haze that the researchers can't live with. Too little and the reflectivity suffers. In fact, way too little and the mirror becomes transparent, allowing the 114 honeycombed cells inside the disk to become visible.

"It's really remarkable how much difference a new coating of aluminum makes," says Petrie. "Aluminum gets corroded easily, and even though we take the mirror down every six months to wash it, we can't restore it completely through washing."

Particularly vulnerable to an aging aluminum coating is any research study that depends upon good data at the blue end of the visible spectrum. A bit of oxidation doesn't hamper infrared studies too much, but an observer can discover that 50 percent of the reflectivity of blue light has dropped off if the aluminum coating is showing some age.

"If we did only infrared work, we'd probably realuminize the mirror less often and rely more on the weekly carbon dioxide 'snow-cleaning' and twice-yearly wet washes," says Petrie.

Thicksten and his crew have become so adept at mirror-cleaning that they've scheduled an April conference at Palomar to discuss issues shared by about 40 other observatory technicians from around the world.

After the crew has finished preparing the mirror surface, the bell jar is lowered into place and all the air is pumped out. Then, Palomar employees Bruce Baker and Steve Einer fire off 350 tungsten filaments coated with pure aluminum in the roof of the bell jar one by one. This process, dating to the original design of the telescope in the 1930s, acts something like a perfume atomizer to deposit an extremely smooth coating over the entire surface.

A vacuum-pump malfunction slows down progress for a time, but four days after the 28-ton mirror and metal holding cell were originally unbolted from the back of the Hale Telescope, the crew does a final inspection of the new aluminum surface and decide they are happy with the results.

Indeed, Thicksten and Petrie determine with a special laser gadget called a portable reflectometer that the new surface is reflecting in the visible wavelengths at about 90 percent. This is about the best that can be hoped for with an aluminum mirror surface, so the giant mirror is bolted back into place and the Hale Telescope is ready for another year of scientific work.

Thicksten, a relieved man to say the least, isn't yet sure if he and his crew will do anything to celebrate their success. Because the observatory is in a remote part of rural northeastern San Diego County, the venues are somewhat limited, he says.

"Sometimes we'll maybe have ice cream," he says. "It's not as if we're downtown where we can go to the local bistro!"

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Contact: Robert Tindol (626) 395-3631

Caltech Media Relations