Jerry Johnston, CXC Program Manager &
Bob Hahn, Chief Engineer at Optical Coating Laboratories, Inc., Santa Rosa, CA
December 11, 1995
One of the crucial steps in the fabrication of the AXAF mirrors is the coating process. After application of a very thin base layer of chromium, the mirrors are coated with iridium, a rare metal that is more reflective than gold. The layers are required to be uniform to within a millionth of an inch. As program manager and chief engineer at OCLI, Jerry Johnston and Bob Hahn were in charge of coating the mirrors. We interviewed Johnston and Hahn at OCLI in December of 1995, when they were nearing the end of the process.
Q: What was the biggest challenge you encountered in coating the mirrors?
BOB HAHN: Keeping the glass clean. We had to develop a very strict protocol to keep the glass adequately clean of particulate and organic contaminants.
Q: Exactly how clean did you have to get them?
JERRY JOHNSTON: Less than 0.001 percent of the area could be covered with particles of any kind. Put another way, they had to be 99.999% clean.
Q: How did you clean the glass?
BOB HAHN: Just like you wash your car. You hose it down, wash it with a detergent and wash it again. And again. And again.
Q: And after it's clean?
JERRY JOHNSTON: It's a full time job to keep it clean. It is moved to the Clean Room and prepared for coating.
Q: How clean is the clean room?
JOHNSTON: It is better than Class 100, which means that it is 5,000 times fewer particles per cubic centimeter than a normal clean office. As soon as possible we move the glass into the coating facility. The mirrors are coated first with chromium, which is a binding layer, and then with iridium.
HAHN: The coatings are extremely thin. The chromium is about a millionth of a centimeter thick, and the iridium layer is about 3 millionths of a centimeter, or a millionth of an inch thick.
Q: How does this compare with a good bathroom mirror?
HAHN: The reflective layer on a bathroom mirror is about ten times thicker--and much rougher. Our surface must be smooth to within a few atoms.
Q: Why is iridium used?
HAHN: TRW(the prime contractor for AXAF) had the SAO mission support team do a sample program. They tested nickel, gold, platinum and iridium. Iridium proved to be the best choice for reflecting X-rays.
Q: Could you describe the coating process?
JOHNSTON: After we finish cleaning the glass, we put it into a vacuum chamber and take it down to less than a billionth of atmospheric pressure. We want to deposit the material onto the glass as uniformly as possible, so we don't want any turbulence or cloud formation in the chamber or condensation onto the glass. Once the high vacuum has been achieved, we lower a thin sheet of iridium into the chamber. Then we direct a beam of ions (charged particles) onto this sheet. The beam knocks the iridium atoms off the sheet and they drift onto the glass, which is rotating very slowly to ensure a uniform coating. The actual coating process takes a little less than an hour.
Q: What happens to the mirror after it is coated?
JOHNSTON: After the mirror is coated, it is never touched again by anyone here or anywhere else. In the Clean Room, it is packaged and stored for transport to Eastman Kodak in Rochester, New York. Inside the storage container, a steady flow of nitrogen gas at about one mile per hour is directed across the mirrors to keep them free from contaminants.
Q: How are the mirrors transported to Rochester?JOHNSTON: In an Air-Ride Van. A husband and wife team do all the driving. They go in a convoy with four vehicles--a lead vehicle, a motor home with equipment that monitors the acceleration, the temperature and so forth in the van, the moving van, and a following vehicle.
Q: When will the coating be complete?
JOHNSTON: We are scheduled to deliver the last pair of mirrors in February. Right now we are ahead of schedule.
NOTE: The last pair of mirrors were delivered to Eastman Kodak on February 12, 1996. The OCLI coating work exceeded specifications; the AXAF mirrors received the cleanest and smoothest mirror coating ever produced.[ Press Index ] [ Interviews ]