The Gemini Observatory
recently reached an important milestone - successful coating of the secondary
mirror on Gemini South with silver. Previously, with aluminum coatings on
both primary and secondary mirrors, Gemini's combined emissivity was 3.8%.
Reaching this new milestone, we are increasingly confident that achieving
our ambitious goal of 2% total telescope emissivity is attainable.
Reflectivity
and emissivity are two measures of a mirror's performance. A highly reflective
mirror is desirable in any optical telescope in order to maximize the amount
of light reaching the science instrument. Ideally, a coating should reflect
as close to 100% of the light that strikes it. In the thermal infrared region
of the spectrum, however, a mirror coating emits a great deal of infrared
radiation, and the statistical "noisiness" of this emission reduces an infrared
instrument's sensitivity to emission from astronomical sources. The emissivity
of an optical surface is defined as the ratio between its level of emission
and that of a perfect "blackbody" emitter, and it is roughly the inverse of
the reflectivity. In order to maximize infrared sensitivity, the mirror emissivity
must be as low as possible. Gemini's design requirement is for emissivity
not exceed 4% at 10-microns for both primary and secondary mirrors combined,
with a goal of 2% at 10 microns for both mirrors.
"Low emissivity
translates into higher sensitivity for the telescope," said Tom Hayward, Gemini's
instrument scientist for T-ReCS, "and enhances Gemini's scientific capabilities."
Until recently,
all of Gemini's mirrors were coated with aluminum, which is the material used
by most telescopes (Subaru is the only other telescope in the 4-meter to
10-meter class with a silver coating on a secondary mirror, and it's bare
rather than protected.) No large telescopes at present have silver-coated
primary mirrors. The challenge with using silver as a coating material is
that, unlike aluminum, it tarnishes with exposure to air, specifically to
sulfur.
"Like the family
silver set," explains Tom Geballe, Gemini Senior Astronomer, "which slowly
develops brown tarnish spots over time and must be regularly polished, the
shiny silver on a telescope mirror also tarnishes rapidly reducing reflectivity
and increasing emissivity. The observatory's engineers, however, can't just
grab a cloth and some polish when the tarnish spots appear."
Mirrors must
be recoated, which does not occur more than once per year for logistical and
economic reasons, so a coating has to last at least that long.
The solution
to the tarnish problem is a multi-layer coating process. A three-layer process
was used recently to coat Gemini's secondary mirror with silver. The first
layer is an adhesive mixture of nickel and chromium spread evenly over the
mirror's surface with a thickness of 50 angstroms. The second layer is the
reflective layer of pure silver spread about 1000-2000 angstroms thick. The
third layer is an extremely thin coating (5 angstroms or 1-2 atoms thick)
of more nickel and chromium, which mildly protects the silver from tarnishing
and acts as an adhesive in a four-layer coating process.
"Because the
secondary mirror faces downward," explains Maxime Boccas, Opto-Mechanical
Engineer for Gemini, "it's not exposed to as much dust and debris as the
primary mirror, and a three-layer coating process is adequate to maintain
the required reflectivity and emissivity over a one-year period." Maxime
Broccas led the coating team during this complex procedure.
The primary mirror,
however, requires more anti-tarnish protection than the secondary mirror.
Thus, a four-step process will be necessary and will include the three layers
described above plus a final layer of anti-tarnish silicon spread very thinly
at 50-100 angstroms.
The three-step
process applied recently to Gemini South's secondary mirror was a success:
the mirror's emissivity was measured in laboratory tests to be 0.9% at 3.8
microns wavelength and is expected to be even lower near 10 microns. The Gemini
facility instrument T-ReCS measured the combined emissivity of the aluminum-coated
secondary and the aluminum-coated primary to be 3.8% at 9 microns. Breaking
these results down, it seems likely to achieve emissivity of 0.9% for each
mirror when coated with silver, or 1.8% total, which brings mirror performance
in-line with Gemini's ambitious goal of 2% total emissivity. Baseline silver
samples were measured during the feasibility studies and are still being
used for comparison, lending greater confidence to numbers measured now.
"It is encouraging
to see us making significant progress toward our goal," said Scott Fisher,
Gemini Science Fellow.
Durability is
equally important as performance. Gemini requires that after one year, emissivity
does not increase by more than 1%. Experiments that are currently underway
are measuring emissivity increase. Gemini engineers expect to have enough
data on the four-layer process by January 2004 to extrapolate over one year
and render a decision whether or not to coat the primary mirror with silver.
Gemini will likely be the first observatory ever to coat a primary mirror
with silver.
Because Gemini
was designed and built for infrared optimization, which includes silver-coated
mirrors, each telescope has an on-site coating facility. The coating facility
consists of a unique coating chamber and multiple magnetrons (the machines
that apply the coating atoms and molecules to the mirror's surface), which
are distinctive for each layer in the coating process. This onsite capability
makes the coating process efficient and fairly quick, taking only about an
hour for the secondary mirror, plus the 6 to 12 hours of vacuum cycling required
to remove all of the air from the chamber.
