Harvard-Smithsonian Center for Astrophysics|
The CfA Almanac Vol. XIII No. 2, July 2000
The radical instrument that transformed the design and construction of almost every new large optical telescope is now twice as large itself.
The original 4.5-meter-diameter Multiple Mirror Telescope (MMT) was the third-largest optical telescope in the world when dedicated on Mt. Hopkins, Arizona, in 1979. Now converted to the 6.5-meter heart of the MMT Observatory, it became the largest single-mirror optical telescope in North America when rededicated on Saturday, 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 Smithsonian Institution and the University of Arizona 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 UofA Steward Observatory Mirror Lab.
The $20-million-plus conversion project has increased the telescope's light-collecting area by 2.5 times and the diameter of its field-of-view by about 15 times. This fifteen-fold increase expands the area of sky that astronomers can view simultaneously by more than 200 times, according to Foltz. The combination of the converted MMT's much greater light-collecting power, expanded field of view, and exquisite image quality will keep this telescope in league with most other large telescopes now 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.
So why change the MMT?
Casting mirrors larger than 5 meters in diameter was impractical when the MMT was conceived. 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 UofA, casting mirrors up to 8.4 meters across has become possible and affordable. Indeed, 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 2.5 centimeters high.
The unprecedented conversion of a working world-class telescope marks the beginning of what SAO's J.T. Williams, project engineer on the MMT conversion, says is sure to be a trend--the "recycling" of existing prime astronomical observatory sites. Although Mount Hopkins is threatened by ever-increasing light pollution, according to Williams it still remains 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 and large optical- fiber-fed spectrographs to observe hundreds of galaxies at a time. In short, astronomers from the Smithsonian Institution and the University of Arizona will continue to have access to first-class observing instruments.
The late afternoon rededication festivities at the FLWO Basecamp and Visitors Center in Amado were capped off with a remote ribbon-cutting ceremony beamed by live video link from mountain top to parking lot below that officially opened the MMT to the cosmos. Nearly 200 guests witnessed this important event, including Smithsonian Under Secretary for Science Dennis O'Connor, University of Arizona President Peter Likins, SAO Director Irwin Shapiro, and Steward Observatory Director Peter Strittmatter.
FLWO motor pool mechanic and budding artist Cesar Lopez announced the winners of the contest to name his sculpture, currently displayed on the Visitors Center patio. [One of Lopez' constructions of "found materials" will be on display at the Smithsonian Institution July 25-September 4, as part of "Artists at Work: A Celebration of Staff Creation," a juried exhibit of art by 87 Institution staff members, ranging from paintings to photography and sculpture to quilts.] The dual winners were Rebecca Fleishman ("Celestial Shield") and Janice Igou ("Celestial Traveler"); and runners-up Gerald Weiss ("Comet Catcher") and Beth and Donald Sanborn ("The Visitor").
The day concluded with star-gazing after dark from the parking area next to the Visitors Center. Nearly two dozen telescopes were set up by members of the Tucson Amateur Astronomy Association (TAAA) and the Sonora Astronomical Society (SAS) from Green Valley.