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Face-to-Face with
Jerry Johnston, CXC Program Manager &
Bob Hahn, Chief Engineer at Optical Coating Laboratories, Inc.,
Santa Rosa, CA
December 11, 1995
Background:
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.
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*In December 2002, TRW was aquired by Northrop Grumman and is now a part of Northrop Grumman Science Technology.
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