UA Set to Cast First Mirror for World's Largest Telescope in July

UA SET TO CAST FIRST MIRROR FOR WORLD'S LARGEST TELESCOPE IN JULY
From Lori Stiles, UA News Services, 520-621-1877
June 27, 2005


The University of Arizona's Steward Observatory Mirror Lab is
pre-firing its
huge spinning furnace and inspecting tons of glass for casting a first
8.4-meter (27-foot) diameter mirror for the Giant Magellan Telescope
(GMT).
The casting is scheduled for Saturday, July 23.

With this milestone step, the GMT becomes the first extremely large
ground-based telescope to start construction.

The completed GMT telescope primary mirror will consist of six
8.4-meter
off-axis mirrors surrounding a seventh, on-axis central mirror. (An
off-axis
mirror focuses light at an angle away from its axis, unlike a
symmetrical
mirror that focuses light along its axis.) This arrangement will give
the
GMT four-and-one-half times the collecting area of any current optical
telescope and the resolving power of a 25.6-meter (84-foot) diameter
telescope, or 10 times the resolution of the Hubble Space Telescope.

'Spin-casting' single-piece telescope mirrors that are giant, stiff
yet
lightweight is an ingenious, awesome process that was conceived and
developed by University of Arizona Regents' Professor of astronomy J.
Roger
P. Angel. Casting giant monolithic mirrors is accomplished at only one
place
in the world -- the Steward Observatory Mirror Laboratory.

The casting team, headed by Randy Lutz, installed about 50 cores a day
for
a total 1,681 cores during seven weeks in April - May. The team bolted
each
core at precisely measured angles to hearth tile and adjoining cores in
this
operation. The crew daubed all the glued junctures with blue "smurf" -
a
concoction the color of the blue smurf cartoon characters -- to prevent
glass from sticking to the mold.

At this point, the mold holds 17,000 pounds of hearth tiles, 16,000
pounds
in fiber tub walls, and 15,000 pounds of cores and pins. The casting
team
has now cleaned and inspected the completed mold, lowered the furnace
cover
into place, and begun pre-firing on June 16.

Team members actively 'pilot' the furnace by computer as temperatures
ramp
up during the first eight days of the heating process, then shut power
off
to complete the two-week pre-firing. Pre-firing centers core glue
joints,
burns out any impurities and stresses the mold. The casting team will
inspect the mold for any needed repairs after pre-firing.

Some of the most visually stunning steps in casting are glass
inspection
and loading. The team began inspecting 90 shipping crates of glass on
June
24. Glass loading is scheduled for the second week of July, said Steve
Miller, Mirror Lab manager.

The 40,000 pounds of borosilicate glass that will make the 27-foot
diameter
(8.4 meter) GMT mirror comes from Ohara Glassworks in Japan. Ohara made
the
glass from sand that comes from the gulf coast of Florida.

The Mirror Lab will start heating the furnace July 18. It takes six
days
for the glass to reach peak temperature at 2,150 degrees Fahrenheit
(1,178
Celsius). At this temperature, the glass begins to flow like honey at
room
temperature. The thick liquid glass flows between the hexagonal cores
in the
mold to create a "honeycomb" structure. The final honeycomb mirror
blank
will weigh about a fifth as much as a solid glass mirror of its size.

The bearings on the rotating furnace will turn a 100-ton load during
spincasting. The furnace can be supplied with up to 1.1 Megawatts of
electricity during casting -- enough electricity to power an average
750 to
1,100 Tucson households, depending on the time of year.

The oven's rotation rate determines the depth of the curve spun into
the
shape of the mirror, or the mirror's focal length. The GMT mirror will
spin
5 times a minute, slower than the two 8.4-meter mirrors the Lab made
for the
Large Binocular Telescope (LBT), because the off-axis GMT mirror is to
be a
shallower, longer focal-length mirror than the symmetric LBT primaries.

"This is a new epoch for astronomy," Richard Meserve, president of the
Carnegie Institution, said. "The fabrication of the off-axis mirror is
a
path-breaking event that will advance scientific discovery. Everyone in
the
eight-member GMT consortium is excited that we're in production."

The Giant Magellan Telescope consortium currently includes the
Carnegie
Observatories, Harvard University, Smithsonian Astrophysical
Observatory,
University of Arizona, University of Michigan, Massachusetts Institute
of
Technology, University of Texas at Austin, and Texas A & M University.

"The fact that we are already in production is directly related to the
successful technology developed for the twin 6.5-meter (21-foot)
Magellan
telescopes at Carnegie's Las Campanas Observatory in Chile," said Matt
Johns, assistant director of the Carnegie Observatories and GMT project
manager. "The Magellan telescopes have proved to be the best natural
imaging
telescopes on the ground."

Mirror cooling is a carefully controlled process that will take 11 to
12
weeks. After the mirror is completely cooled, the lab will wash the
ceramic
cores out of the mirror's glass honeycomb cells. Then the mirror will
be
ground and polished to an accuracy of plus-or-minus 15 to 20 nanometers
(a
nanometer is a billionth of a meter). The mirror will be coated with a
layer
of reflective aluminum only 100 nanometers thick at the observatory
site.

The GMT is slated for completion in 2016 at a site in northern Chile.
With
its powerful resolution and enormous collecting area, it will be able
to
probe the most important questions in astronomy, including the birth of
stars and planetary systems in our Milky Way, the mysteries of black
holes,
and the genesis of galaxies.

Detailed information about the GMT design and science goals is online
at
http://www.gmto.org/

---------------------------------------------------------
Science contact information
Peter Strittmatter, Director, Steward Observatory
520-621-6524

J. Roger Angel, Director, SO Mirror Lab
520-621-6541

Stephen M. Miller, Manager, SO Mirror Lab
520-621-9753

Matt Johns, GMT Project Manager
Assistant Director, Carnegie Obsv.
626-304-0288

Mirror Lab media contact -
Lori Stiles, UA News Services
520-626-4402

Photos/video: Download high-res images by clicking on ImageBase, under
the
Services column on the left of the UA News Web page at
http://uanews.org
Search words - "gmt" "gmto" "mirror lab" or "steward observatory"
New images will be posted as steps in the casting process occur.
New b-roll video of steps in the process will be available from UA News
Services after the mirror is cast July 23.

Related Web sites
Giant Magellan Telescope - http://www.gmto.org/
Steward Observatory Mirror Lab - http://mirrorlab.as.arizona.edu/

GMT consortium media contacts:
Tina McDowell, Carnegie Observatories,
Smithsonian Astrophysical Observatories:
David (Cambridge)
Christine Pulliam, (Cambridge)
Dan Brocious, (Tucson)
Lori Stiles, University of Arizona,
Rebecca Johnson, University of Texas at Austin

Steve Bradt, Harvard University,
Laura Bailey, University of Michigan,
Elizabeth Thompson, MIT,
Sherylon Carroll, Texas A&M University,

WOW That's a lot of glass. Spin casting was done to make the barrels
for the navy 16'inch guns Steel and glass have served humankind well.
I know I won't be around when the GMT is showing nightbat a clear
picture that shows a BH,but I had my day,for my spacetime had the
Hubble. Beert

nightbat wrote

G=EMC^2 Glazier wrote:

WOW That's a lot of glass. Spin casting was done to make the barrels
for the navy 16'inch guns Steel and glass have served humankind well.
I know I won't be around when the GMT is showing nightbat a clear
picture that shows a BH,but I had my day,for my spacetime had the
Hubble. Beert

nightbat

Better yet, your space time had the profound Captain nightbat
that pointed you always in the right direction. Dr. Hubble ruined Dr.
Einstein's space time, the Hubble can't ruin but only affirm the
nightbat's because scientists finally found a piece of the tip of the
mysterious chased comet dog you and astro everybody had been wondering
about for so long. It's done Officer Bert, the long hunt is over, have
you ordered your nightbat Earth Science Team Star T-shirt yet, what are
you waiting for? Only net Earth Science Team Officers presently are
eligible at this space time, later all my world wide big and little "
Black Comet " fans will be able to get one.

carry on,
the nightbat

G=EMC^2 Glazier wrote:
WOW That's a lot of glass. Spin casting was done to make the barrels
for the navy 16'inch guns Steel and glass have served humankind well.
I know I won't be around when the GMT is showing nightbat a clear
picture that shows a BH,but I had my day,for my spacetime had the
Hubble. Beert


Hi Beert,

Why would they use spin to cast a naval gun?

They use spin for casting reflector mirrors for large telescopes
because they found that a spinning liquid naturally settles into the
shape of a parabola, just the shape you need for a reflecting mirror.
The spinning liquid glass takes on the parabolic shape and then freezes
into exactly that shape. That is what has allowed them to make such
large telescopes nowadays. Otherwise it is difficult to work with such
a large piece of glass, grind it, etc., without cracking it.

Double-A

Hi Double-A Those 16 inch barrels for navy guns were spin casted in a
Waltham Ma. foundry. The reason for this method were numerous,but I can
only think of two. First it gave them greater strength then just
casting,and it was fast. Am I right in thinking multiple mirrors for a
telescope became possible because a computer can line them up? Bert