Glass for 2nd Magellan mirror heats up next week
by Lori Stiles

Dateline: September 4, 1998

Makers of giant telescope mirrors at The University of Arizona in
Tucson will begin casting a second mirror for the Magellan Project
in Chile next week. They plan to start heating the furnace on
Wednesday afternoon, Sept. 9.

The Magellan Project, led by the Carnegie Institution of Washington,
will add two 6.5-meter (21-foot) optical telescopes to Las Campanas
Observatory. UA, Harvard University, the University of Michigan and
the Massachusetts Institute of Technology are partners in the $72
million project. Magellan 1 will begin operations late in 1999.
On-site construction for Magellan 2 begins December 1998. "First
light" for Magellan 2 is planned in 2002.

When the UA Steward Observatory Mirror Laboratory, directed by UA
Regents' Professor of astronomy Roger Angel, successfully cast a
first 6.5-meter mirror six years ago, the achievement was recognized
as the most significant breakthrough in big telescope mirror-making
in a half century.

Lab astronomers and staff changed hardly anything for casting a
second 6.5-meter mirror in 1994 -- the Magellan 1 mirror. And they
continue to change hardly anything for casting their third 6.5-meter
mirror, the Magellan 2 mirror. Probably the most striking difference
is that mirror making at the Steward Observatory lab has become a
faster, more streamlined operation.

Astronomers and lab staff began assembling the mold for Magellan 2
only six months ago. Last Friday, the casting team loaded 22,750
pounds of glass into the mold and lowered the furnace dome. UA
astronomer John M. Hill, who directs Mirror Lab castings, has
scheduled furnace heating to start on at 2:30 p.m. on Sept. 9.

Today Hill and the casting team were tending to a few glitches with
the video cameras mounted on the rotating furnace. These cameras
give real-time views of the glass throughout the casting process.
The images can be viewed at near real-time at the Steward
Observatory Mirror Lab web site:

http://medusa.as.arizona.edu/mlab/mlab.html

The best views will be Monday, Sept. 14, in the afternoon and
evening, during rapid rotation, when the furnace spins at 7.5 rpm.
That's when the glass blocks soften and slump like warm Jell-O, then
melt and smooth into a clear glass surface, and ooze into ribs
between the cores of the mold. Peak furnace temperature reaches
2,156 degrees Fahrenheit.

Glass annealing and cooling -- which is just as exciting to watch
as it sounds unless you are the scientist/engineer monitoring for
any stresses developing in the glass -- should last until just after
Thanksgiving. The cooled furnace could be opened Nov. 30.

Meanwhile, the Mirror Lab's computer-controlled, stressed-lap
polisher will finish polishing the Magellan 1 mirror in about two
months from now. Lab staff are also currently installing support
mechanisms in the Magellan 1 mirror cell, the large piece that
cradles the primary mirror in the telescope. When the mirror and
the mirror cell are finished, staff will install the mirror into
the mirror cell for testing as an intregal unit. The mirror will
then be removed from the cell so both parts can be shipped
separately from Tucson to South America next summer. When
reassembled, Magellan 1 will see first light sometime in late 1999.

The Steward Observatory Mirror Lab produces stiff, lightweight,
short focal-length telescope mirrors as large as 8.4-meters in
diameter using unique and stunning technologies.

Casting involves spinning a 2-story furnace so that tons of glass
loaded into a great ceramic tub melt over and around more than a
thousand hexagonal ceramic cores within the tub -- the mirror mold.
The walls and removable dome of the furnace are lined with heating
elements whose temperatures are precisely controlled by computer.

The ceramic cores are cleaned from the cooled mirror blank with
water guns, creating the hollow "honeycombs" between the face plate
and the back plate of the mirror.

Lab scientists created an innovative computer-controlled "stressed
lap" for mirror polishing. They also developed the world's largest
holograms for use in mirror testing at the Mirror Lab's multi-story
test tower.

Great steel "lifting spiders" hoisted by 40-ton and 55-ton cranes
and air carts move mirror blanks around the lab.

The enclosure and auxiliary building for Magellan 1 on Manqui Peak
at Las Campanas were completed this year, according to Matt Johns
of the Carnegie Institution, manager for the Magellan Project. The
twin Magellan telescopes will be 60 meters apart and connected by
an underground tunnel so it will be possible to electronically
combine the light from the two telescopes.

More about the Magellan Project is available at the web site:

http://www.ociw.edu/~johns/magellan.html .

This article forwarded by our friend and ex-Committee member:

Chris Stewart