By Dave Paiz
Arizona Daily Wildcat
March 3, 1998

Mirror to get a good cleaning

Scientists at the UA's Steward Observatory Mirror Laboratory held their breath last week when their pride and joy - a 40-ton ceramic-filled mirror blank - was carefully maneuvered upright for cleaning.

"This is the largest mirror cast - and by far the largest structured mirror cast - anywhere," said Steward Observatory Director Peter Strittmatter.

After a meticulous casting process that took nearly a year to complete, the new mirror blank was finally unveiled in the fall.

It will be the first of two identical 8.4-meter primary mirrors for the Large Binocular Telescope as part of the Mount Graham International Observatory being built near Safford.

Telescope mirrors have historically been made from solid pieces of glass, which had to be ground down and polished to create the necessary dished surface.

Scientists working in mountaintop observatories have realized that solid mirrors retain a great deal of heat and radiate heatwaves that hamper the mirror's ability to see clearly.

The LBT mirror is a semi-hollow circular matrix of "honeycombed" hexagonal cells, all connected to each other by half-inch thick glass "ribs." The "honeycomb" is in turn sandwiched between two plates of glass - all cast in one seamless piece.

In addition to giving the mirror its structural stability, the ribbed cells circulate air throughout the mirror and allow it to cool much faster than a solid mirror.

"Over the past 20 years we have gotten better at making (the mirrors)," said Mirror Lab Director Roger Angel, a UA regents astronomy professor who pioneered the structural design of the new mirrors.

To create this type of mirror, lab technicians first have to assemble a complex arrangement of hexagonal hearth tiles and heat-resistant ceramic cores inside their immense two-story spinning furnace.

For the LBT mirror, over 40,000 pounds of glass were slowly brought to melting temperature and spun within the furnace. The centrifugal forces generated within "threw" the glass outward - forming a natural dish shape around the ceramic casting material.

The resulting dish is 18 inches thick at the center and 30 inches thick around the outer edge. The mirror's face has an average thickness of 1.75 inches, which will eventually be ground, polished to a near-perfect one-millionth of an inch and finally given a thin aluminum coating before being put into the LBT.

Last week's move was accomplished by attaching a mechanical web of 36 strategically placed suction cups to the mirror's face with silicone rubber, then lifting the whole assembly horizontally with a powerful overhead crane. The combined weight of the mirror, the ceramic core material and the lifting assembly was nearly 45 tons.

Technicians used a remote control to manipulate the crane's every move with the utmost precision.

Angel said that at one point, the crane's brake failed and the massive load began to slip. To avert a potential disaster, supports were placed beneath the mirror to stabilize it while the brake was repaired.

After the problem was fixed, it took about 45 minutes to transfer the mirror to a specially designed holding ring.

Mirror Lab technicians are now erecting a cleaning station with a motorized scaffold around the ring. This week they will begin using high pressure water jets to blast the ceramic material out of the mirror's hollow cells.

Angel said that the LBT's colossal mirrors will give scientists their farthest-reaching, most detailed views of the solar system ever achieved in the history of ground-based astronomy.

The Mirror Lab will start work on the second 8.4-meter mirror in 1999. The Large Binocular Telescope should begin operating sometime in 2003.

"By mixing the light from these two mirrors, we'll get images 10 times sharper than the Hubble - it will be just breathtaking," Angel said.

The Large Binocular Telescope project is a joint effort among the UA, the Arcetri Astrophysical Observatory in Florence, Italy, the LBT Beteiligungsgesellschaft from Germany, Ohio State University and the LBT Research Corp. of Tucson.