Press Release

Gemini Mirror is First with Silver Lining

By SpaceRef Editor
June 10, 2004
Filed under , ,
Gemini Mirror is First with Silver Lining

To investors looking for the next sure thing, the silver
coating on the Gemini South 8-meter telescope
mirror might seem like an insider’s secret tip-off to
invest in this valuable metal for a huge profit.
However, it turns out that this immense mirror
required less than two ounces (50 grams) of silver,
not nearly enough to register on the precious metals
markets. The real return on Gemini’s shiny
investment is the way it provides unprecedented
sensitivity from the ground when studying warm
objects in space.

The new coating –the first of its kind ever to line the
surface of a very large astronomical mirror– is
among the final steps in making Gemini the most
powerful infrared telescope on our planet. “There is
no question that with this coating, the Gemini South
telescope will be able to explore regions of star and
planet formation, black holes at the centers of
galaxies and other objects that have eluded other
telescopes until now,” said Charlie Telesco of the
University of Florida who specializes in studying star-
and planet-formation regions in the mid-infrared.

Covering the Gemini mirror with silver utilizes a
process developed over several years of testing and
experimentation to produce a coating that meets the
stringent requirements of astronomical research.
Gemini’s lead optical engineer, Maxime Boccas who
oversaw the mirror-coating development said, “I
guess you could say that after several years of hard
work to identify and tune the best coating, we have
found our silver lining!”

Most astronomical mirrors are coated with
aluminum using an evaporation process, and
require recoating every 12-18 months. Since the twin
Gemini mirrors are optimized for viewing objects in
both optical and infrared wavelengths, a different
coating was specified. Planning and implementing
the silver coating process for Gemini began with the
design of twin 9-meter-wide coating chambers
located at the observatory facilities in Chile and
Hawaii. Each coating plant (originally built by the
Royal Greenwich Observatory in the UK)
incorporates devices called magnetrons to “sputter”
a coating on the mirror. The sputtering process is
necessary when applying multi-layered coatings on
the Gemini mirrors in order to accurately control the
thickness of the various materials deposited on the
mirror=92s surface. A similar process is commonly used
to apply coatings on architectural glass to reduce air-
conditioning costs and produce an aesthetic reflection
and color to glass on buildings. However, this is the first
time these steps have been applied to a large
astronomical telescope mirror.

The coating is built up in a stack of four individual
layers to assure that the silver adheres to the glass
base of the mirror and is protected from
environmental elements and chemical reactions. As
anyone with silverware knows, tarnish on silver
reduces the reflection of light. The degradation of an
unprotected coating on a telescope mirror would
have a profound impact on its performance. Tests
done at Gemini with dozens of small mirror samples
over the past few years show that the silvered
coating applied to the Gemini mirror should remain
highly reflective and usable for at least a year
between recoatings.

In addition to the large primary mirror, the
telescope’s 1-meter secondary mirror and a third
mirror that directs light into scientific instruments
were also coated using the same protected silver
coatings. The combination of these three mirror
coatings as well as other design considerations are
all responsible for the dramatic increase in Gemini’s
sensitivity to thermal infrared radiation.

A key measure of a telescope’s performance in the
infrared is its emissivity (how much heat it actually
emits compared to the total amount it can
theoretically emit) in the thermal or mid-infrared part
of the spectrum. These emissions result in a
background noise against which astronomical
sources must be measured. Gemini has the lowest
total thermal emissivity of any large astronomical
telescope on the ground, with values under 4% prior
to receiving its silver coating. With this new coating,
Gemini South’s emissivity will drop to about 2%. At
some wavelengths this has the same effect on
sensitivity as increasing the diameter of the Gemini
telescope from 8 to more than 11 meters! The result
is a significant increase in the quality and amount of
Gemini’s infrared data, which allows detection of
objects that would otherwise be lost in the noise
generated by heat radiating from the telescope. It is
common among other ground-based telescopes to
have emissivity values in excess of 10%.

The recoating procedure was successfully
performed on May 31, and the newly coated Gemini
South mirror has been re-installed and calibrated in
the telescope. Engineers are currently testing the
systems before returning the telescope to full
operations. The Gemini North mirror on Mauna Kea
will undergo the same coating process before the
end of this year.

Why Silver?

The reason astronomers wish to use silver as the
surface on a telescope mirror lies in its ability to
reflect some types of infrared radiation more
effectively than aluminum. However, it is not just the
amount of infrared light that is reflected but also the
amount of radiation actually emitted from the mirror
(its thermal emissivity) that makes silver so
attractive. This is a significant issue when observing
in the mid-infrared (thermal) region of the spectrum,
which is essentially the study of heat from space.
“The main advantage of silver is that it reduces the
total thermal emission of the telescope. This in turn
increases the sensitivity of the mid-infrared
instruments on the telescope and allows us to see
warm objects like stellar and planetary nurseries
significantly better,” said Scott Fisher a mid-infrared
astronomer at Gemini.

The advantage comes at a price however. To use
silver, the coating must be applied in several layers,
each with a very precise and uniform thickness. To
do this, devices called magnetrons are used to apply
the coating. They work by surrounding an extremely
pure metal plate (called the target) with a plasma
cloud of gas (argon or nitrogen) that knocks atoms
out from the target and deposits them uniformly on
the mirror (which rotates slowly under the
magnetron). Each layer is extremely thin; with the
silver layer only about 0.1 microns thick or about
1/200 the thickness of a human hair. The total
amount of silver deposited on the mirror is
approximately equal to 50 grams.

Studying Heat Originating from Space:

Some of the most intriguing objects in the universe
emit radiation in the infrared part of the spectrum.
Often described as “heat radiation,” infrared light is
redder than the red light we see with our eyes.
Sources that emit in these wavelengths are sought
after by astronomers since most of their infrared
radiation can pass through clouds of obscuring gas
dust and reveal secrets otherwise shrouded from
view. The infrared wavelength regime is split into
three main regions, near- , mid- and far-infrared.
Near-infrared is just beyond what the human eye can
see (redder than red), mid-infrared (often called
thermal infrared) represents longer wavelengths of
light usually associated with heat sources in space,
and far-infrared represents cooler regions.

Gemini’s silver coating will enable the most
significant improvements in the thermal infrared part
of the spectrum. Studies in this wavelength range
include star- and planet-formation regions, with
intense research that seeks to understand how our
own solar system formed some five billion years

Gemini South Mirror and Coating Specifications:

  • Mirror Diameter: 8.1-meters (26.6 feet)
  • Mirror Thickness: 20 cm (8 inches)
  • Mirror Mass: 24 tons
  • Surface Area of Mirror: 50 square meters
  • Silver Coating Reflectivity: ~98.75% (at mid-infrared wavelengths)


Peter Michaud

Gemini Observatory, Hilo HI

(808) 974-2510 (Desk)

(808) 937-0845 (Cell)

Antonieta Garcia

Gemini Observatory, La Serena Chile

011-56-51-205-600 (Desk)

011-56-99-182-858 (Cell)

Maxime Boccas

Gemini Observatory, La Serena Chile


Background Information on Gemini Observatory

The Gemini Observatory is an international
collaboration that has built two identical 8-meter
telescopes. The Frederick C. Gillett Gemini
Telescope is located on Mauna Kea, Hawai`i
(Gemini North) and the Gemini South telescope is
located on Cerro Pach=F3n in central Chile (Gemini
South), and hence provide full coverage of both
hemispheres of the sky. Both telescopes incorporate
new technologies that allow large, relatively thin
mirrors under active control to collect and focus both
optical and infrared radiation from space.

The Gemini Observatory provides the astronomical
communities in each partner country with state-of-
the-art astronomical facilities that allocate observing
time in proportion to each country’s contribution. In
addition to financial support, each country also
contributes significant scientific and technical
resources. The national research agencies that form
the Gemini partnership include: the US National
Science Foundation (NSF), the UK Particle Physics
and Astronomy Research Council (PPARC), the
Canadian National Research Council (NRC), the
Chilean Comision Nacional de Investigacion
Cientifica y Tecnol=F3gica (CONICYT), the Australian
Research Council (ARC), the Argentinean Consejo
Nacional de Investigaciones Cientificas y Tecnicas
(CONICET) and the Brazilian Conselho Nacional de
Desenvolvimento Cientifico e Tecnologico (CNPq).
The Observatory is managed by the Association of
Universities for Research in Astronomy, Inc. (AURA)
under a cooperative agreement with the NSF. The
NSF also serves as the executive agency for the
international partnership.

SpaceRef staff editor.