Giant mirror transplanted in trend-setting telescope

Lori Stiles
May 16, 2000

A radical instrument that transformed design and construction of large telescopes is now twice as large. The original 4.5-meter-diameter Multiple Mirror Telescope was the third-largest optical telescope in the world when dedicated on Mt. Hopkins, Arizona, in 1979. Now, converted to the 6.5-meter telescope of the MMT Observatory, it was the largest single-mirror telescope in North America when it was rededicated May 20.

"The original telescope changed people's expectations of how a telescope should be designed and how a modern telescope should perform," says MMTO Director Craig B. Foltz.

The University of Arizona and the Smithsonian Institution have replaced the MMT's array of six, 1.8-meter mirrors with a single stiff, lightweight 6.5-meter borosilicate "honeycomb" mirror, spin cast and polished to extraordinary precision at the UA Steward Observatory Mirror Lab.

The $20-million conversion project has increased the telescope's light-collecting area by 2.5 times and its field-of-view by about 15 times. A 15-fold increase in field-of-view expands the area of sky that astronomers can study by more than 200 times, Foltz says. The combination of the upgraded MMT's much greater light-collecting power, expanded field of view, and exquisite image quality will keep this telescope in league with other large telescopes coming on line, he adds.

The original MMT used six identical 1.8-meter telescopes in a single altitude-azimuth (naval gun-type) mount. The light gathered by each of the six telescopes was combined at a common focus. This gave the MMT the light-gathering power equivalent to a telescope having a single 4.5-meter primary mirror.

The pointing and tracking of the MMT and the co-alignment of the individual telescopes were all under computer control. The absolute necessity of computers for operation and the use from the start of electronic light detectors made the MMT the first all-electronic telescope. Throughout its career, it remained unrivaled for telescope pointing performance and nearly unrivaled in image quality.

Why change the MMT?
"The MMT has been, throughout its history, at the forefront of modern astronomical research and has contributed significantly to our understanding of phenomena such as young stars, galaxies, quasars, gravitational lenses, and black holes," said then MMT Observatory director Frederic H. Chaffee at the beginning of the conversion process. However, Chaffee, now director of the Keck Observatories, said "The MMT wouldn_t be able to compete with the even larger telescopes expected around the year 2000."

Casting mirrors larger than 5 meters in diameter was impractical when the MMT was designed. In fact, the MMT was designed specifically to circumvent the problem of large, heavy, very expensive mirrors. Now, however, thanks to the spin-casting method pioneered by Roger Angel of the University of Arizona, casting mirrors up to 8.4 meters across has become possible and affordable.. Mirror Lab scientists have polished the 6.5-meter mirror so perfectly that if the glass were the size of the United States, a typical bump on its surface would be only one inch high.

The unprecedented conversion of a working, world-class telescope marks the beginning of what J.T. Williams, project engineer on the MMT conversion, says is sure to be the beginning of a trend - the "recycling" of existing prime astronomical observatory sites. Although the Mount Hopkins site is threatened by ever-increasing light pollution, Williams said, it remains among one of the world's best sites.

In keeping with its groundbreaking predecessor, the new telescope will make use of nascent technologies such as adaptive optics to counteract the "twinkling" of starlight; new large optical-fiber-fed spectrographs able to measure hundreds of galaxies at a time; and thus continue the access of astronomers from the University of Arizona and the Smithsonian Institution to first-class observing instruments.


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