|The bell jar (right) designed to house the
aluminization system for the mirrors used on the LBT. Photo Credit:
Dan Pappalardo (OSU)|
COLUMBUS, Ohio - In an airplane hangar in Columbus, OH, some 80 tons of
steel, electronics, and cryogenic equipment are about to come together --
all to deposit one ounce of aluminum as a near-perfect, whisper-thin
coating on a giant telescope mirror.
The successful operation of the coating system will represent a
milestone in the construction of the Large Binocular
Telescope (LBT), and a critical phase in Ohio State University's
participation in the LBT project.
In 2004, when construction is to be completed on Mt. Graham in Arizona,
the LBT will be the worlds largest optical and infrared telescope. Before
that can happen, Ohio State research scientist Bruce Atwood and his
colleagues will have to perfect the process for covering the LBTs primary
mirrors with a reflective coating of aluminum.
LBT Primary Mirror: The first
8.3 meter mirror and the team who cast it at the Steward Observatory
Mirror Laboratory beneath the University of Arizona football
stadium. Photo Credit: Lori Stiles and John Florence (University of
In a 9.1-meter (30-foot) diameter steel cell, an 8.4-meter (28-foot)
sample, or dummy, mirror will simulate one of the LBT mirrors.
On November 26, the cell will seal against a similarly sized lid,
called a bell jar, to create a clean, high-vacuum environment for
Atwood explained that the coating would be an essential part of the
LBT. The telescopes concrete foundation, the support structure, the glass
mirrors -- the entire LBT building -- is being made just to support this
thin coating of reflective aluminum. Without it, the telescope cant see,
The coating project improves on existing technology. Engineers have
created vacuums in similar bell jars to test the space-worthiness of
satellites, and have been coating smaller telescope mirrors inside such
vessels for more than 50 years.
To meet the challenge of scaling up the technology for very large
mirrors, Atwood and his team have had to devise new ways to create the
vacuum and assure a very uniform film.
Our expertise is working the kinks out of the procedure so it can be
repeated on the final LBT mirrors on Mt. Graham in 2004, Atwood said.
Atwoods team includes research scientists Paul Byard, Tom O'Brien,
Ralph Belville, Dave Brewer, David Steinbrecher, Dan Pappalardo, Jerry
Mason, and Ed Teiga, all of the Imaging Sciences
Laboratory in Ohio States Department of
Telescope Structure. Built by
Ansaldo-Camozzi Energy Special Components in Milan, Italy, the
structure that will hold the
mirrors for the Large Binocular
Telescope was completed in November, 2001. The structure has been
shipped to Mt. Graham and is currently in the process of being moved
up the mountain. Photo Credit: Patrick Osmer
After Atwood supervised initial tests of the bell jar and cell in
Italy, LBT scientists took apart the equipment and shipped it to Rickenbacker International
Airport, formerly Rickenbacker Air Force Base, in Columbus, OH. Ohio
State rented a hangar at the airport to house the heavy parts, which
arrived in several crates, each the size of a small house.
Cranes will soon place the 44-ton mirror cell and 22-ton bell jar onto
wide-set train rails.
We have one of the widest, shortest, and slowest railroads in America,
On November 20, a carriage will bring the two massive parts together at
a slow 10 inches per minute, so the scientists can assure a precise
Together, the equipment will form a fat disk that resembles an
oversized diving bell tipped on its side.
Once the bell jar and
dummy mirror cell are sealed, mechanical vacuum pumps will siphon away
most of the air inside; all but a tiny fraction of one percent will
remain. Then the Ohio State scientists will employ a special cryogenic
system they designed in which extremely cold charcoal will absorb almost
all the remaining air molecules.
After the vacuum is established, the scientists will super-heat an
array of 25 crucibles, each containing a quarter of an ounce of aluminum.
The aluminum will melt, then boil inside the chamber. Just as steam
escapes from a boiling pot and condenses as water on the lid, some of the
evaporated aluminum will float through the chamber and condense on the
surface of the mirror. Molecules of a scattering gas will spread the
molecules of gaseous aluminum so they deposit evenly.
If all goes well, the aluminum will cool to form a smooth, uniform
coating, with no bumps or bare spots to spoil the mirrors reflection.
LBT Enclosure. Completed
in June 2002, the building that encloses the LBT sits atop Mt.
Graham, 10,298 feet above sea level. The enclosure is 50 meters high
and the rotating part weighs 1250 tons.Photo Credit: John Hill
Then the Ohio State scientists will dismantle the equipment once again,
and ship it by truck and barge to Mt. Graham, where coating of the final
telescope mirrors will take place.
The LBTs mirrors will belong to a new generation of telescopes designed
to produce sharper images through adaptive and active optics.
Computer-controlled pistons will adjust the shape of smaller secondary
mirrors, to help astronomers see through turbulent gas in Earths
atmosphere that would blur the view of distant stars and galaxies.
With its giant mirrors, the $100 million LBT is to have 25 times the
light-collecting area and 10 times the image resolution of the Hubble Space Telescope. Through
financial and in-kind contributions, Ohio State holds a one-eighth share
of the cost of the LBT project. One such in-kind contribution comes form
perfecting the aluminization process, for which Ohio State will earn $1
million worth of observing time on the LBT -- the equivalent of about 20
Other partners in the LBT project include the University of Arizona,
with participation form Arizona State University and Northern Arizona
University; a group of Italian observatories led by the Arcetri
Astrophysical Observatory in Florence; and a group of German institutes
and observatories led by the Max Planck Institute for Astronomy in
Heidelberg -- each of which hold a one-fourth share of the project. The
Research Corporation, a private foundation in Tucson, Arizona, holds the
remaining one-eighth share, and has given portions of its observing time
to Ohio State, the University of Notre Dame, the University of Virginia,
and the University of Minnesota.
With its grant from the Research Corporation, Ohio State will actually
enjoy a total of one-sixth of the available observing time on the