November 1997

Washing a Mirror of the Universe

By Laura van Dam

Cleaning this giant "eye" is anything but a pedestrian task. The job was the last one performed by Corning technicians in New York state before they shipped the mirror blank to Paris for grinding and polishing, in preparation for the eight-meter-diameter mirror's use in one of the world's largest telescopes.

The work is part of the international $176 million Gemini Project, which plans to finish constructing two identical telescopes on mountaintops in the Northern and Southern hemispheres-Mauna Kea, Hawaii, and Cerro Pachon, Chile-by 1998 and 2000, respectively. Designed to detect infrared wavelengths of light, a method that produces clear extraterrestrial images, the scopes will let researchers try to discover planetary systems and more distant stellar birthplaces and galaxies, says Matt Mountain, director of the Gemini Project. The latter work should enable astronomers to track the history of stars' and galaxies' formation.

In part, the telescopes will be the product of "superb mechanical engineering," Mountain says. Making the 22-ton mirror required producing and fusing 55 hexagons of glass that remain virtually constant in size during temperature changes. Technicians then reheated the giant blank to slump it to precise specifications before applying an acid-etch treatment to the underside that rounded off corners of microcracks and reduced the chance of their propogating. Now French technicians at REOSC Optique are shining the top surface so smoothly that were the mirror to stretch across the Atlantic Ocean, no blemish would protrude more than an inch from the surface. To clean the mirror, Gemini experts are considering using a carbon-dioxide gas that sprays on like snow to blast off contaminants weekly.