SkyandTelescope.com March 2014 25

low-mass exoplanets and might even capture the fi rst

direct image of a “wet and warm” rocky planet that orbits

in its star’s habitable zone.

Though making and combining several smaller

mirrors is simpler than building one giant mirror, the

concept carries its own set of challenges. Because GMT’s

seven mirrors combine to form a single parabolic pri-

mary, the outer segments cannot be parabolic themselves.

Instead, they must be slightly saddle-shaped to match the

curve found at the outer edge of the central mirror.

The fi rst completed mirror was one of these outer seg-

ments and the fi rst of its kind. A large mirror with such

an asymmetric shape had never been built before, says

Buddy Martin, a scientist at the Mirror Lab. Its successful

completion was proof it could be done.

Attaining that proof was far from simple, demonstrat-

ing astronomical engineering of the highest order.

Spin Casting Mirrors

Manufacturing a single 8.4-meter mirror is not easy —

in fact, the Mirror Lab is the only place in the world that

does it. Yet as high-tech as the business is, it’s surpris-

ing how much is still done by hand, and in much the

same way as fi rst devised decades ago by the Mirror

Lab’s founder and scientifi c director, Roger Angel. Angel

crafted a technique called spin casting: spinning the mol-

ten glass shapes its surface into a paraboloid, reducing the

time and energy needed to grind in the curve.

“The reason we are still making the mirrors the same

way after 20 years is that it’s not possible to make them

better,” explains Angel. “I believe that if aliens out there

are using big telescopes to look at us, they would have had

to build them the same way.”

The mirror starts with the glass, a high-grade ver-

sion of what’s used in common cookware such as Pyrex.

Called E6 low-expansion borosilicate, the glass has a low

thermal coeffi cient, so it holds its painstakingly crafted

shape through temperature changes. It also melts at

relatively low temperatures, becoming viscous (like cold

honey) at 1165°°C (2129°F). “There is other glass that is

better thermally, but you can’t melt it,” says Angel.

The glass is manufactured in Japan, forged in clay pots

one ton at a time. The pots are cracked off , then the glass

is broken into chunks slightly bigger than softballs. The

outer chunks that touched the pot are discarded to ensure

that the glass used for the mirror has never been in

contact with other chemicals and will melt together seam-

lessly. All told, casting each 8.4-meter mirror requires

some 16,000 glass chunks, together weighing 19.2 tons.

Technicians carefully place each piece of glass onto a

ceramic mold, which is also largely assembled by hand.

Once the glass pieces are in place, a furnace lowers to

enclose the mold before heating it to 1165°C. The glass

melts over 1,681 hexagonal columns to take on an internal

honeycomb structure that maintains stiff ness while

eliminating 80% of the weight. As the chunks begin to

melt and seep into the honeycomb mold, the furnace and

mold together spin fi ve times per minute to shape the

mirror face into a preliminary paraboloid.

THE HONEYCOMB MOLD Technicians bolted 1,681 hexagonal

columns into place by hand. Molten glass fl owed around (not into)

the columns to create honeycomb-shaped voids that allowed air cir-

culation, maintained the mirror’s stiff ness, and lightened its weight.

Te l e s c o p e

RAY BERTRAM / STEWARD OBSERVATORY MIRROR LAB / UNIVERSITY OF ARIZONA

Giant

GMT Mirror.indd 25 12/24/13 11:46 AM