Lab gives Keck a protective coat
Two Lab scientists, Jesse Wolfe of Electronics Engineering Technology
Division, and Norman Thomas of Chemistry & Material Sciences, together
with their team of supporters, have patented a new ultra-thin silver coating
for mirrors, which is proving to be far more durable than any previously
used.
"Mirrors used in telescopes and lasers around the world, and in space,
suffer significant efficiency losses due to the deterioration of the mirror
coatings," said Thomas, principal investigator on the project. "On
earth, numerous air-borne factors, from smog to automobile pollutants
to ocean salt, can rapidly affect the coatings.
"In space, the mirrors often deteriorate due to the impact of atomic
oxygen and micrometeorites, and exposure to solar radiation, temperature
extremes and other contaminants. Our new coating is showing signs of being
virtually indestructible in terrestrial conditions, and we’re very
excited that it will be exposure-tested on the International Space Station
as of this spring."
The coating was recently installed on a 22-inch mirror for use at the
Keck Observatory in Hawaii, where, Thomas reports, "the astronomers
are just ecstatic." The mirror coating is scheduled for use in several
of the world’s major telescopes within the next few months, including
Kitt Peak in Tucson, Ariz., and possibly the South African Large Telescope
(SALT) and, in the more distant future, the California Extremely Large
Telescope (CELT).
Thomas explained, "These large telescopes typically involve five
or six reflections from coated mirrors, which affect the collection efficiency.
Our coating is going to give each mirror a consistent 97 percent reflectivity.
Previous coatings provided about 90 percent reflectivity. Combine the
effect from several mirrors, and we may have up to 35 percent increased
collection efficiency for many years from each of these large telescopes."
This coating is also being installed to protect the thousands of mirrors
to be used in the NIF flash lamps, according to Wolfe. "Many intense
hours, many of them volunteer, have paid off in a revolutionary protection
for NIF, the world’s top astronomical observation equipment and components
of the international space station."
"In fact," added Thomas, "this project began in support
of NIF, then funding was secured from NASA, the Air Force and Lockheed-Martin
due to the great potential for use in both the Hubble telescope and the
Spaced-Based Laser program."
When Wolfe and Thomas’ samples of their new silver coating go with
the shuttle to the International Space Station this June, they will travel
in a small suitcase with samples of other materials being tested for space
applications. An astronaut will have to take the suitcase out by hand
in an extra-vehicular activity (EVA) mission, and secure it in the initial
test area.
In this experiment known as MISSE (Materials on International Space Station
Experiment), the various samples will be tested in one- and three-year
periods, exposed in both the ram and leeward directions, in full sunlight
and in the earth’s shadow.
Other control samples will be tested during the given times for degradation
during storage on earth. They must show sufficiently low rates of degradation
over a useful period to qualify for service on the space station and other
space-based applications.
Wolfe and Thomas have been pursuing this project for over three years,
but have had their work involved with space missions before, as scientists
for Optical Coating Laboratory, Inc. in Santa Rosa. In 1984, shuttle astronauts
placed some of their samples in the Long Duration Exposure Facility, an
orbiting framework used for the testing of various materials in space.
The samples were retrieved in 1990, and results helped lead to this latest
invention.