Mirror LaboratorySteward ObservatoryUniversity of ArizonaTucson, AZ 85721-0061

More information about the Steward Observatory Mirror Laboratory at the University of Arizona can be found at:

http://mirrorlab.as.arizona.edu

Mirror Lab Contact Information

The Mirror Lab is building a new generation of optical and infrared telescopes

Size1 Focal Ratio2 Date3 Telescope/Observatory

1.8 1.0 1985 Vatican Advanced Technology Telescope 1.2 1.9 1987 Smithsonian Astrophysical Observatory3.5 1.75 1988 Astrophysical Research Consortium3.5 1.75 1988 Wisconsin-Indiana-Yale-NOAO3.5 1.5 1989 US Air Force Starfire Optical Range6.5 1.25 1992 Multiple Mirror Telescope Conversion6.5 1.25 1994 Magellan I, Las Campanas Observatory8.4 1.14 1997 Large Binocular Telescope I6.5 1.25 1998 Magellan II, Las Campanas Observatory8.4 1.14 2000 Large Binocular Telescope II6.5 1.25 2002 Lockheed Martin Corporation8.4 2.14* 2005 Giant Magellan Telescope I3.8 3.4 2006 Test Optic, Mirror Lab8.4 1.2 2007 Large Synoptic Survey Telescope

1Meters 2Focal length/diameter 3Casting date *Off-axis segment

Mirrors in the Mirror Lab casting bay: the two 8.4-m mirrors for LBT are in the foreground, and a 6.5-m mirror is in the background (January 2003). Photos by Ray Bertram except as noted.

Extraordinary sensitivity and ability to resolve fine detail come only with larger mirrors. The 6.5 and 8.4-meter class telescopes are bringing breakthroughs in the search for planetary systems around other stars, the search for extraterrestrial life, and the understanding of the structure and evolution of the universe.

MIRROR LABORATORY

S T E W A R D O B S E R V A T O R Y

The Giant Magellan Telescope, with seven 8.4-m mirrors, will have an equivalent circular aperture of 21.5 m and an edge to edge diameter of 25.5m. Graphic by Todd Mason / Carnegie Observatories.

Steward Observatory Mirror Lab Castings

To contact us, email mirrorlab@as.arizona.eduor call (520) 621-6524

Steward Observatory Mirror Laboratory

Beneath the east section of the Arizona football

stadium, a technological revolution is in progress: a team of enterprising scientists, engineers, and technicians are producing giant, lightweight mirrors of unprecedented power for a new generation of optical and infrared

astronomical telescopes. In 1980 Roger Angel, Mirror Lab founder and scientific director, began to develop the spin-casting technique. Since 1993, the Mirror Lab has produced four 6.5-meter (21.32 ft.) mirrors and three 8.4-meter (27.55 ft.) mirrors.

The Mirror Lab’s casting technology represents a radical departure from earlier mirror designs. The mirrors are spun cast with a honeycomb structure that is lightweight and yet has high stiffness. This design allows cool night air to be pumped into the mirror structure, allowing the glass temperature to reach equilibrium with the outside temperature in less than an hour, thereby resulting in significantly sharper images.

What is spin casting?

The Mirror Lab team creates mirrors by melting borosilicate glass (similar to Pyrex) into a array of hexagonal columns, resulting in

a honeycomb structure. The oven rotates as the glass melts, resulting in a front surface with a concave parabolic shape. The columns are composed of a ceramic material, alumina silica, that is strong enough to support the weight and heat of the molten glass at a temperature of 1180 C (2156 F). The glass pieces, weighing 4-5 kg each, are individually inspected for flaws and bubbles, and then are carefully placed on top of the ceramic columns. More than 20 tons of glass are used to produce each 8.4-meter mirror.

How is a mirror shaped and polished?

The mirror is installed on the Large Optical Gen-

erator (LOG) and shaped using a diamond-grinding wheel. First the back surface is ground and polished to make it mechanically flat. Then the mirror is turned over, installed in a mirror polishing cell, and the process of grinding and polishing the optical surface

begins. Force actuators support the mirror during grinding and polishing with a computer-controlled stressed-lap polishing tool.

Polishing is an iterative process with repeated optical tests that prog-ress through finer and finer stages. The mirror’s figure or surface accuracy is measured using a laser to create an interferogram that is converted into a precise topographic map. Optical tests of the figure are carried out by moving the mirror from the polishing machine to a seismically-isolated test tower. The glass is slowly ground and pol-ished using cerium oxide. In the final stages, a thickness of only 100 atoms is removed. The polishing process continues until the mirror has a paraboloidal shape accurate to better than ±25 nanometers (one millionth of an inch).

How does a mirror become part of a telescope?

Before leaving the Mirror Lab, the mirror and mirror cell undergo final testing under the optical test tower. Then the mirror and

cell are separately transported to the mountain site and reassembled in the telescope mounting. Once installed, an aluminum coating is applied to the optical surface in a vacuum chamber. Force actuators support the mirror to preserve its shape against gravity, wind, and temperature. Observatory personnel then test and adjust the mirror-support system and its pointing and tracking software. Finally, astronomers may begin observing, utilizing 25 years of innovative Mirror Lab technology. At the Mirror Lab, arrays of large mirrors are being designed and built for ever larger telescopes in the pursuit of knowledge and extraordinary discoveries.

How is a spun-cast mirror made?

Oven pilots carefully monitor the temperature of the glass during a week of heating, followed by 11-12 weeks of cooling. When the

oven reaches its maximum of 1180 C, it is held at a constant temperature for a few hours while the oven spins at about seven revolutions per minute. Air bubbles in the molten glass rise to the surface and escape.

After reaching maximum temperature, the liquid glass fills the mold and

controlled cooling begins. After four or five days of rapid spinning, the rate of rotation is gradually reduced. Over the next 11 to 12 weeks, the glass goes through a carefully controlled cooling process. When the glass reaches room temperature the oven stops spinning. Paraboloidal mirror surfaces are created by spin casting. This process creates optical surfaces with fast focal ratios (f/1.1 – f/1.2) resulting in much shorter focal lengths. The short focal length means that giant telescopes can be housed in relatively small domes or enclosures. The 1-meter (3.28 ft.) refractor at Yerkes Observatory (1897) has a focal length of 15 meters (49.2 ft.) whereas the Large Binocular Telescope (LBT) has a focal length of 9.6 meters (31.49 ft.) and an aperture of 11.8 meters (38.7 ft.).

What happens after the glass cools?

After the glass reaches room temperature, the casting crew members remove the oven cover and walls and then install a

series of support disks bonded to the front surface of the mirror blank. These disks are coupled to a steel lifting fixture that distributes the weight of the glass. The mirror blank is lifted from the oven floor using a 45-ton crane. Then it is turned from horizontal to vertical position. A plastic enclosure is assembled around the mirror blank, and the casting team members don wet suits to flush out the ceramic columns trapped inside the glass with a high-pressure water stream. The mirror is then ready for rough grinding. Ron Smallwood, UAPD