Steward Observatory Mirror Lab to cast 2nd 8.4-meter LBT mirror

Roger Angel, Mirror Lab director, 520-621-6541, [email protected]
John Hill, LBT project director, 520-621-3940, [email protected]
Peter Strittmatter, director, Steward Observatory,
520-621-6524, [email protected]
Peter Wehinger, director's office, Steward Observatory,
520-621-7662, [email protected]
Steve Miller, Mirror Lab manager, 520-621-1022, [email protected]

(EDITORS: Note video availability at the end of this news release.)

TUCSON, Ariz. -- The University of Arizona Steward Observatory Mirror Lab is about to cast a second 8.4-meter mirror for the Large Binocular Telescope, a full binocular telescope like no other telescope in the world.

The $84 million Large Binocular Telescope (LBT) will use two 8.4-meter (27.6-foot) mirrors, positioned side by side on a single mount, spanning 22.8 meters, or almost 75 feet. Now under construction for operation in 2004, it will be the world's most powerful single telescope for optical and infrared astronomy.

Created by UA astronomer Roger Angel, scientific director of the UA Steward Observatory Mirror Lab, the LBT's binocular reflectors will have a total light-gathering power of a single 11.8-meter (39-foot) telescope.

Astronomers will use the LBT at Mount Graham International Observatory in eastern Arizona to get views at least twice as sharp as views obtained from other new generation optical/infrared telescopes in the 8-to-10 meter class.

Like other giant optical/infrared telescopes, the LBT will use adaptive optics to correct for atmospheric turbulence that blurs light from celestial objects. Adaptive optics systems instantly analyze how light is blurred by the turbulent air above a telescope and adjust or warp optic surfaces to correct for the distortion.

But unlike other giant optical/infrared telescopes, the LBT will combine the light corrected by each of its two secondary mirrors at a common focus. The binocular telescope thereby will view the universe as a 22.8-meter telescope, with more than twice the sharpness of the other giant telescopes.

Angel and Bob Fugate of the U.S. Air Force Research Laboratory, Kirtland Air Force Base, N.M., reported on the binocular telescope's image sharpness in a review of astronomy's advances in adaptive optics in the April 21 issue of Science. They compared simulated LBT views of Jupiter's moon, Io, and an actual image of Io taken by the Keck telescope in Hawaii.

Project scientists calculate that the LBT images that will be 25 times brighter and 10 times sharper than those taken by the 2.4-meter Hubble Space Telescope. Astronomers will use the LBT to study the evolution of the universe from its birth in the Big Bang through the formation of galaxies, stars, planets and chemical elements essential to life.

In the search for alien worlds, the LBT's two mirrors again give it unique capability. The mirrors will be combined as a nulling interferometer to eliminate light around a bright star and enhance light from its nearby planet or interplanetary disk.


The Mirror Lab cast the first 8.4-meter mirror for the telescope three years ago in the Lab's famous two-story spinning furnace, housed beneath the east section of the university football stadium.

The mirror is the largest single piece of optical glass ever cast. It is also the "fastest" optical glass in the giant 8-meter class ever cast: It has an extremely fast focal ratio (f/1.14), so that the mirror focuses light at a distance just slightly longer than its diameter. And because the mirror is a stiff honeycomb structure, it weighs in at only 18 tons, which is lightweight for giant mirrors.

Next year Steward Observatory Mirror Lab opticians will polish the first mirror to an accuracy 3,000 times thinner than a human hair for best possible image sharpness. The Lab developed new stressed-lap polishing technology for finishing its giant, deep-dished mirrors. The polishing technology uses computer-controlled actuators to continuously adjust the shape of the lap to the mirror.


Last week Mirror Lab team members inspected more than 41,000 pounds of glass for the second mirror. They are so practiced at the task - and the new shipment of E6 glass from Ohara in Japan is so good - that the Lab team worked ahead of schedule, said Randy Lutz, casting supervisor. The team will start loading big chunks of glass into the mirror mold inside the furnace tomorrow (May 2).

"This is the best glass we've ever seen - really high quality stuff," said Phil Muir, senior staff technician at the Lab. Muir was on the 8-member crew who inspected and sorted a record 7,000 pounds of glass in a single morning last week.

The mold holds 1,662 capped hexagonal-shaped cores made of refractory ceramic fiber material similar to the heat-shield tiles on the space shuttle.

During casting, furnace temperature will rise to 1,150 degrees Celsius (2,102 degrees Fahrenheit) and the entire furnace assembly will rotate at a rate of 6.8 rpm. The 41,000 pounds of glass will melt to a consistency of thick honey, flowing around the cores to form the glass honeycomb structure. Spinning forces the molten glass into the deep parabola required for fast focusing.

The Mirror Lab will start furnace heating Saturday, May 13, said Steve Miller, Mirror Lab manager and optical engineer. The furnace will start spinning at 7 a.m. Friday, May 19, and reach high temperature on Saturday, May 20.


May 20th marks another highlight for Steward Observatory mirror makers. The UA Steward Observatory and the Smithsonian Astrophysical Observatory will rededicate their MMT Observatory on Mount Hopkins, Ariz., on May 20. The Mirror Lab used its innovative technologies
to cast and polish the 6.5-meter primary mirror in the converted MMT.

Steward Observatory astronomers used the original MMT (Multiple-Mirror Telescope), which was configured with six 1.8 meter (72-inch) mirrors, in experiments that proved the feasibility of using the sodium layer surrounding Earth to create artificial guide stars for adaptive optics for giant telescopes. These artificial guide stars serve as essential beacons in correcting wavefront distortions.


The LBT is a collaboration between Italian astronomers, represented by the Osservatorio Astrofisico de Arcetri in Florence, the University of Arizona, Arizona State University, Northern Arizona University, the LBT Beteiligungsgesellschaft (LBTB) in Heidelberg, Germany, Ohio State
University, Research Corp. of Tucson and the University of Notre Dame.

The LBTB partners are the Max-Planck-Institute fur Astronomie in Heidelberg, Landessternwarte in Heidelberg, Astrophysikalisches Institut in Potsdam, Max-Planck-Instiut fur Extraterrestrial Physik
in Garching, near Munich, and Max-Planck -Institut fur Radioastronomie in Bonn.

The rotating LBT enclosure, all facility power and support systems and interior rooms will be completed during the coming summer, said James H. Slagle, associate director of the LBT project.
The mechanical parts of the telescope are being fabricated in Milan, Italy, and will be shipped to Arizona in early 2001. The first LBT mirror will be installed in 2003. The second mirror will be installed in 2004.

*** ***

VIDEO - A short documentary program of casting of the first 8.4-m mirror is available to news operations. Resource tapes on the Large Binocular Telescope at Mount Graham, and on a related story, steps in the conversion of the MMT telescope on Mount Hopkins, Ariz., will be available from UA News Services.

Steward Observatory
Steward Observatory Mirror Lab
Large Binocular Telescope
Center for Astronomical Adaptive Optics
Mount Graham International Observatory