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  • Steward Observatory Spins Out Major Mirrors

Photonics Spectra
Jul 2002
Spin-cast parabolic mirrors are revolutionizing telescopes and may one day allow astronomers to view extrastellar planets orbiting distant stars.

Brent D. Johnson, Senior News Editor

Tucson, at first, seems a remote spot for the world’s foremost optical sciences laboratory. Arising from the parched red crust of the Arizona landscape, manicured lawns and English gardens betray an extensive field of subterranean aquifers similar to those that planetologists hope to discover on Mars. The comparison is not inappropriate. In the outlying regions of the city, the tracts of homes and businesses quickly give way to an unearthly terrain of jagged rock, cracked and worn by heat and whipped by winds into fine brick dust.

The absence of atmospheric moisture and the relative isolation of the region from polluting light sources provide an uninterrupted supply of deep blue skies that make for pristine viewing conditions in the mountains to the north and east of the city. High above the desert at the Lowell Observatory in Flagstaff, Clyde Tombaugh discovered the planet Pluto and V.M. Slipher meticulously laid the groundwork for Edwin Hubble’s big bang theory.

Astronomers have been attracted from around the globe to conduct research at the 10-m Heinrich Hertz Submillimeter Telescope and the 1.8-m Vatican Advanced Technology Telescope, both atop Mount Graham, which rises above the Sonoran Desert to an elevation of 10,413 ft. At this vantage, the rarefied atmosphere offers little resistance to the weak pulses traveling from the ends of the universe.

Underground lab

I recently joined Jim Trexler, docent of the Smithsonian Institution’s Fred Lawrence Whipple Observatory on Mount Hopkins, for a guided tour of the University of Arizona’s Steward Observatory Mirror Laboratory. This was his first time back to the lab since it was closed to the public last year, so we were both excited about seeing the progress that had been made on the mirrors.

Figure 1.
The two 8.4-m mirrors made at the Steward Observatory Mirror Laboratory will operate jointly to provide the sensitivity of an 11.8-m mirror, at a cost less than that of a conventional 8-m telescope.

The lab is in a vault underneath the bleachers of the Arizona Wildcats’ football stadium. It is encased in concrete and anchored to the bedrock. The stadium offered an existing structure from which to mount heavy lifting apparatus to handle the mirrors. Vibration isolation for optical testing is provided by a steel and concrete tower assembly that weighs 440 tons and that rests on an air-actuated isolation system.

Since the early 1980s, the lab has revolutionized astronomy by creating large, lightweight mirrors using a spin-casting technique that produces a highly efficient parabolic reflector. Such mirrors could not have been built using traditional grinding and polishing techniques, and their high-speed f/1.1 focal ratio enables much shorter telescopes with greater light-collecting capability, thus reducing the cost of erecting new observatories.

Spin-cast parabolic mirrors

The first mirror made by the spin technique, built for the Vatican Observatory, was 1.8 m in diameter. Since then, three 3.5-, three 6.5- and two 8.4-m spin-cast primaries have been produced. Except for the 8.4-m mirrors, all are in operation. A fourth 6.5-m casting is in progress, and a third 8.4-m casting will follow.

The first 6.5-m mirror is operating at Multiple Mirror Telescope Observatory on Mount Hopkins, a joint facility of the university and the Smithsonian. The mirror’s success has been such that the Carnegie Institute, which is a competitor of the Smithsonian for federal funds, has ordered two of the 6.5-m mirrors for use in its Magellan Observatory in Las Campanas, Chile. The 6.5-m primary mirrors were all figured to an accuracy of between 15 and 20 nm rms.

The 1.8-m mirror that Steward lab made for the Vatican Observatory, which Trexler said is quite opulent by most astronomers’ standards, is accurate to within λ/50. The laboratory’s pièces de résistance, however, will be the twin 8.4-m mirrors constructed for the Large Binocular Telescope.

Figure 2.
The casting oven spins as it reaches 1160 °C and molten glass fills the honeycomb infrastructure of the Large Binocular Telescope’s first mirror.

The binocular telescope, which is now under construction at Steward, will be the fruition of 20 years of development in materials and spin-casting technology. The two large mirrors will work in tandem and permit astronomers to view the early formation of galaxies. The telescope brings together light from two 8.4-m telescopes, creating the world’s largest interferometer. The light from one telescope will cancel the light from the other to within λ/2, allowing astronomers to remove stars from the field of view and leaving only the faint spectra from extrastellar planets. When the mirrors from the telescope are combined with adaptive optics, the binocular system will be a true diffraction-limited telescope with a resolution 10 times that of the Hubble Space Telescope.

The adaptive optics that were planned for the Large Binocular Telescope were developed by the US Air Force to facilitate high-energy laser beam propagation through the atmosphere for weapons to destroy satellites. Later, the technology was applied to astronomical and other imagery applications, including laser eye surgery.

The first application of Steward-engineered adaptive optics will be on the f/15 secondary mirror that was fabricated for the Multiple Mirror Telescope. This consists of a 64-cm-diameter Zerodur (glass-ceramic) shell and a 1.8-mm-thick convex hyperbola. Behind it, 336 actuators control the figure of the secondary mirror itself, replacing the usual adaptive element that is added downstream from the telescope. This reduces the thermal noise in IR imaging.

Artificial star

On the Large Binocular Telescope, the secondaries will be concave ellipses 2 mm thick and 911 mm in diameter, and there will be almost 700 actuators.

The way the adaptive optics system works is that a laser is fired up the axis of the telescope to the ionosphere, producing an artificial star. The system measures the distortion produced by atmospheric effects, and directive optics change the shape of the mirror 500 to 1000 times per second to compensate for the interference.

Because it would be impractical to try to move an 8.4-m mirror, the binocular telescope uses a smaller secondary mirror with 672 solenoids that act like high-fidelity speakers to push and pull the mirror surface by angstroms. Early on, the designers used quartz crystals in these mirrors, but the piezoelectric effect produced by the compression of the quartz generated unwanted heat. Now they use glass.

For spectroscopy, you don’t need adaptive optics, Trexler said. But if you’re trying to view planets in distant galaxies, they’re necessary.

Constructing the telescope

The laboratory uses a unique honeycomb lattice for its mirrors that requires far less mass than previous designs and that cools more rapidly so that there is no variation between the mirror temperature and the nighttime air. The latticed mirrors weigh only one-fifth of what they would if they were solid. The integrity of the honeycomb design makes the mirrors big, stiff and light. “Bees taught us this,” Trexler said.

Once the 1662 hexagonal cores are assembled on a giant turntable, chunks of borosilicate glass are placed onto the ceramic superstructure. A large furnace is lifted into place over the mirror, and computer-controlled heating elements are fired to 1160 °C until the glass becomes the consistency of honey and sinks into the honeycomb. All the while, the enormous mirror and furnace are spinning at 7 rpm to produce the parabolic shape.

CCD cameras inside the furnace monitor surface uniformity by regulating temperature and the configuration of the mirror. As the glass liquefies and flows into the structure, metal tension bands surrounding the mirror crimp inward to regulate the expansion of the heated material and to prevent the walls of the mold from collapsing under the hydrostatic pressure. Gradually, after several hours, the parabola emerges.

After about three months of cooling, the mirror is integrated into a polishing cell, which supports it on 160 actuators that preserve its shape, compensating for adjustments in temperature and gravity. The mirror surface is polished to critical accuracy by a floating raft-stressed lap that continually assumes the shape of the parabola as it is moved over the surface. This process may take as long as a year to complete. The first of the mirrors being prepared for the binocular telescope has already been cast and is being polished to an accuracy of 20 nm.

When both mirrors have been finished, 36 suction cups and two vacuum pumps will lift them into padded boxes for transportation to Mount Graham. After they have been mounted in the telescope, the final aluminum coating will be applied to the surface.

One drawback of the mirrors is that protective coatings might interfere with collecting power; thus, the mirrors are nothing more than exposed raw metal. The metal layer is electrically deposited by exploding evaporating filaments of aluminum over the mirror under a vacuum, which results in a mirror surface that has approximately 85 percent efficiency. The deposition process takes place under a steel bell jar in most cases and occurs in situ. This aluminum coating must be chemically stripped from the mirror and reapplied approximately every two years. In part because of the coating problem, Trexler believes that, in the future, more and more mirrors will be constructed in place and that glass factories will be built on location to better serve the observatories.

The goal

When the Large Binocular Telescope is pointed toward the heavens for the first time, probably in 2003, it will view a universe that is 10 times closer than previous instruments could resolve. Some of the objects that astronomers are most hoping to see will be extrastellar planets orbiting distant stars. These planets, perhaps similar to our own, may teach us about the evolution of our solar system and the unique place that our world holds in the cosmic scheme.

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