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News & Updates - November 2000

Mirror Coating Facility Being Built in Louisiana

On an inland waterway on the Louisiana coast a couple hundred miles west of New Orleans lies a 25-acre site where a large stainless steel vacuum chamber designed to coat the SOFIA telescope mirror is now being fabricated.

Located just outside the town of New Iberia next to thousands of acres of sugar cane fields, Chart Inc.'s Coastal Fabricators has been subcontracted to build a special vacuum chamber roughly 14 feet in diameter and 16 feet tall.

The project is managed by Chart's Westborough, Massachusetts-based Process Systems Division (PSD), which specializes in vacuum chambers both to test space satellites and to coat many of the world's largest telescope mirrors. Recent projects include a coating system for the 8.3- meter (27-foot) Subaru telescope on the top of Mauna Kea on the Big Island, Hawaii.

As needed, the top of the stainless steel vacuum chamber will be lifted off and the SOFIA mirror will be lowered into it. Then the top will go back on, and soon the mirror will have a new, delicate coating of aluminum.

The intriguing process involves vaporizing aluminum.Within the chamber is a filament array system containing more than 60 tungsten filaments, similar in principle to the filaments inside many lightbulbs, but much thicker, connected to high-current, low-voltage power. Typically, before each coating task, a worker enters the chamber and hangs four or five aluminum strips on each filament. Then the mirror is lowered into the chamber, and everything but the mirror is shielded with special materials. The simple version of what happens next (not to be confused with the technical version): the filaments are fired up with current, the resulting heat generated within the filaments vaporizes the aluminum, and the vapor travels essentially everywhere in the vessel, landing on anything that is not shielded - that is, the mirror.

USRA's Patrick Waddell reviewing SOFIA mirror coating facility blueprints.

The facility will be used to provide the mirror with its first-ever coating at the end of 2002. After that, how often will the SOFIA mirror be re-coated? The real answer will only become clear once operations begin, but for the moment the answer appears to be one to two times per year.

Predicting the need for re-coating is complex because so many factors come into play, including levels of condensation on the mirror during take-off and landing. The more the condensation, the greater the need for re-coating, so many steps are being taken to minimize condensation, including installation of a complex drying system for the telescope assembly cavity.

Patrick Waddell, who recently joined the staff of NASA prime contractor Universities Space Research Association as associate director of the SOFIA Mission Operations and Support Group, points out that many intermediate steps can be taken between re-coatings to maximize mirror reflectivity and minimize the build-up of dust.

"When dust falls on the mirror surface," he explains, "those particles become a place where the aluminum can be damaged, either because they become condensation nuclei or because they bond to the aluminum and start pinholes."

"So, once a week, we'll `CO₂ snow-clean' the mirror. That involves opening a special nozzle on a bottle of CO₂ (carbon dioxide) gas, which condenses into snow just as it flows on to the mirror. Basically, you just take a wand-type nozzle and move back and forth, catching the surface of the mirror at a glancing angle. The CO₂ snowflakes carry the dust away."

Although this cleaning slows down the need for coating dramatically, the mirror will continue to degrade, so an additional intermediate cleaning step is what is known as a "water wash." As Waddell explains, "We'll take the telescope to zenith to put it in a birdbath configuration and then very gently wash it. We're studying the approaches taken by a number of large ground-based observatories to determine which types of cleaning materials to use in this procedure. A well-executed water wash can bring you back to your original reflectance."

Fabrication of the SOFIA mirror coating chamber and many of the major system components has begun and is scheduled to be completed and tested by the end of January, 2001. The vessel assembly is due to be delivered to Moffett Field (via truck) in February. At that time, on-site integration of the chamber into the full facility begins, to be completed by the late spring.

Interesting factoid of the day: New Iberia was a familiar town to Tabasco lovers for more than 100 years, because McIlhenny Tabasco Sauce listed the town as its home base on all its labels from 1868 until around 1970. Eventually that changed to neighboring Avery Island, which is actually not an island at all but rather a salt dome surrounded on three sides by bayous and marshes. The reason for the label change: Avery Island, the real home of McIlhenny Tabasco Sauce all along, finally got its very own post office in the early 1970s.

November 10, 2000