Tech Briefs Magazine - February 2023 - 31

Imaging
Sixth Mirror Casting Brings Giant Magellan Telescope
Closer to Completion
Once the telescope becomes fully operational, it will have a light collecting area of 3,961
square feet.
University of Arizona, Tucson, AZ
A
gigantic furnace slowly started spinning
underneath the stands of Arizona
Stadium at the University of Arizona
in March 2021. Fire-engine red,
massive in size and resembling a sci-fi
version of a Dutch oven, the furnace is
the only one of its kind, and its sole purpose
is to produce the world's biggest
and most advanced telescope mirrors.
During the process, nearly 20 tons (17.5
metric tons) of extremely pure borosilicate
glass made by Ohara Corporation in
Japan are becoming a honeycomb mirror
measuring 27.6 feet (8.4 meters) across.
The mirror is the sixth of seven segments
that together will make up the primary
mirror of the Giant Magellan Telescope,
currently under construction in Chile's
Atacama Desert. The telescope will see farther
into the universe and capture more
detail than any optical telescope before.
Arranged in a flower-like array, seven
individual mirror segments will come together
to form a primary mirror surface
measuring 82 feet (25 meters) in diameter.
Due to the primary mirror's large size
and deep curvature, the shape of each
outer segment is unlike any mirror ever
made and required major innovations
for polishing and measuring the surface.
Spin-casting is the first step of a fouryear
process to create each mirror. The
other steps are carried out in different
parts of the Mirror Lab, so up to four mirror
segments can be manufactured in parallel.
" The spin-casting is undeniably the
most spectacular part of the manufacturing
process, " said polishing scientist Buddy
Martin. He explained that after the casting,
the surface " roughness " is about 2.5
millimeters, or a tenth of an inch, on average.
" The polishing and constant measuring
are what turn this amazing piece of
glass into a mirror, " he said. " By the time
we finish polishing, it will be accurate to
better than 25 nanometers. That's how
smooth the surface has to be in order to
make the sharpest possible images. "
It took engineers and technicians four
months to build the mold that will form
the sixth lightweight honeycomb mirror,
and it took nine hours to cover the mold
with glass. With the mold in place and
the glass loaded, they closed the furnace's
lid, which measures 33 feet (10
meters) in diameter.
Underneath the stands of the Arizona Wildcats Football Stadium, engineers of UArizona's Richard F.
Caris Mirror Lab manufacture the world's largest and most lightweight telescope mirrors. At the
center of the process is a giant spinning furnace, the only one of its kind. (Image: Damien Jemison,
Giant Magellan Telescope - GMTO Corporation)
Tech Briefs, February 2023
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The furnace began heating the glass
on March 1 and reached a peak temperature
of 2129 °F (1165 °C) at 2 p.m.
on March 6. The day prior, the furnace
began rotating at five revolutions per
minute, so as the glass melts, it forces it
outward and up the sides of the mold to
form a curved surface. At peak temperature,
the glass takes on the consistency of
honey and slowly flows into the mold to
form the honeycomb structure.
The mirror then enters a month-long
" annealing " process, during which the
molten glass cools while the furnace
spins at a slower rate in order to remove
internal stresses and toughen the glass.
During this period, the mirror must have
a uniform temperature, which is
achieved by cooling it very slowly for just
over a month. It takes another month
and a half to cool to room temperature.
Once the mold has been removed, the
mirror " blank " is then ready for grinding
and polishing to refine the surface.
With the first two giant mirrors completed
and placed in storage in Tucson,
the sixth mirror joins three others in various
stages of production at the Mirror
Lab. The third mirror's front surface
polishing process is less than a year from
completion. The fourth mirror has completed
rear surface polishing, and load
spreaders are being attached to allow the
mirror to be manipulated during operation.
The fifth mirror was cast in November
2017, and the seventh mirror is expected
to be cast in 2023. An eighth
mirror segment will also be made and
can be swapped in when another segment
requires maintenance.
Once the Giant Magellan Telescope
becomes fully operational, it will have a
light collecting area of 3,961 square feet
(368 square meters) - enough to see the
torch engraved on a dime from nearly
100 miles away. It also will offer the widest
field of view of any Extremely Large Telescope
in the 30-meter (98-foot) class. The
31

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