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Approximately 125 astronomers,
administrators and guests attended a July 23, 2005 meeting in Tucson,
Arizona celebrating the casting of the first of seven 8.4 meter mirrors
for the Giant Magellan Telescope (GMT) . The group responsible for
the project are: Carnegie Institution, Harvard University, Smithsonian
Astrophysical Observatory, University of Arizona, University of Michigan,
Massachusetts Institute of Technology, University of Texas and Texas A&M.
When completed and placed in Chile the instrument will be the largest
telescope in the world with resolution "ten times that of the Hubble Space
Telescope".
The University of Arizona's Steward Observatory Mirror Lab is building the
mirrors in a huge spinning furnace (in order to achieve a meniscus shape
to the molten glass). Twenty tons of E6 borosilicate glass from
Ohara Glassworks in Japan is used in the casting of the first 8.4-meter
(27-foot) diameter mirror for the GMT. 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 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 held 17,000 pounds of
hearth tiles, 16,000 pounds in fiber tub walls, and 15,000 pounds of cores
and pins. Heating the furnace started on July 18. It takes six days
for the glass to reach peak temperature of 2,150 degrees Fahrenheit (1178
Celsius). The furnace can be supplied with up to 1.1 Megawatts of
electricity during casting -- enough to power an average 750 to 1,100
Tucson households, depending on the time of year. Once heated the
furnace will rotate during the "spincasting" in order to give the mirror
its concave surface. 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 because the off-axis GMT
mirror is to be a shallower, longer focal-length mirror than symmetric
types.
Richard Meserve,
president of the Carnegie Institution, said:
"This is a new epoch for astronomy.
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."
References:
http://www.gmto.org/
http://www.gmto.org/tech_overview
http://web.mit.edu/newsoffice/2004/telescope.html
http://www.officeroutlook.com/news/Science/125.htm
http://www.utexas.edu/opa/news/2005/07/mcdonald21.html
GMT Casting Meeting Photos
mouse click on any of the thumb nail
images below for an enlargement
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