The Thirty Meter Telescope, which will be built atop Mauna Kea, on the island of Hawai’i, will utilize a “calotte” enclosure. The 66 m diameter enclosure dome will be able to rotate along two axes. The rotating structure will be located atop a fixed base that is connected to a one-story support building. Courtesy of TMT Observatory Corporation
Construction is expected to begin this year on the Thirty Meter Telescope, which will be located within the Mauna Kea Science Preserve on the island of Hawai’i.
April 1, 2014—The barren summit of Mauna Kea—a shield volcano that last erupted before recorded history began—might appear to be an inhospitable location: covered in broken volcanic rock, containing little to no ground water, and with a thin atmosphere that makes breathing difficult. However, its elevation of 4,205 m above sea level—a whopping 9,750 m from the ocean floor—makes it the highest point in the Pacific Basin and the highest island mountain in the world. Conditions atop the mountain are perfect for astronomical observations, and the mountaintop already hosts 13 telescopes operated by astronomers from 11 different countries. The Thirty Meter Telescope, which will be the world’s most advanced telescope upon its completion in 2022, is slated to join this group.
Standing above 40 percent of the earth’s atmosphere, the Mauna Kea Science Preserve is unique in the world. The air is remarkably dry, due to a tropical-inversion cloud layer measuring approximately 600 m thick that is located well below the summit, according to the University of Hawai’i’s Institute for Astronomy website. (The university operates the Mauna Kea Science Preserve under a lease from the State of Hawai’i). The thick cloud layer that surrounds the mountain below the summit isolates the upper atmosphere from the lower, moist maritime air and ensures that the summit skies are “pure, dry, and free from atmospheric pollutants,” according to the institute’s website. Not only does this provide air quality that enables telescopes to measure infrared and submillimeter radiation from space, it also creates a virtually cloud-free environment; the proportion of clear nights atop the mountain are among the highest in the world, according to the institute. “Mauna Kea is an excellent site for astronomy and several of the TMT partners already had facilities on Mauna Kea,” says Paul Gillett, the facilities department head for the Pasadena-based TMT Observatory Corporation, which is managing the design and construction of the telescope’s enclosure and adjacent facilities.
The Thirty Meter Telescope is so named because its primary mirror will measure 30 m in diameter. The mirror will be formed by 492 hexagon-shaped segments measuring 1.44 m across, and fitted together with a 2.5 mm gap, a decision that makes construction and transportation of the pieces possible. The mirror’s size is unprecedented, and for this reason it will offer a higher resolution and the ability see fainter images than ever before. This will enable astronomers to study the earth’s solar system, stars located throughout the Milky Way, and neighboring galaxies, as well as those galaxies forming at the edges of the observable universe.
The observatory, foreground, will be located on the Mauna Kea
Science Preserve, which currently houses 13 telescopes operated
by astronomers from 11 countries. A thick cloud layer that
surrounds the mountain below the summit isolates the upper, dry
atmosphere from the lower, moist maritime air. Courtesy of TMT
The observatory will be composed of the telescope, located atop its own isolated foundation, and a 66 m diameter outer enclosure dome that can rotate along two axes. A one-story support building will be located adjacent to the enclosure and will provide interior access to the enclosure’s base.
The telescope support structure, referred to as the “telescope pier,” will be constructed of reinforced concrete and consist of inner and outer circular walls, an integral upper floor slab, interior walls, and the independent foundation, according to the final geotechnical report, which was issued by the Honolulu office of the global engineering firm URS Corporation. Currently, the telescope pier’s foundation is expected to be a continuous spread ring footing. Because the telescope’s ability to point at, and track, objects in the sky is of paramount importance, the telescope pier is being designed so that no more than 1 mm of settlement occurs as the telescope is rotated, according to the report.
The observatory enclosure that will encase the telescope pier structure will measure approximately 56.7 m in height and 66 m in diameter, according to the report. The fixed base of the enclosure will include 32 evenly spaced columns located on a radius of 30.7 m, each of which will also be located atop the continuous spread ring footing that will support the enclosure. “The erection of the enclosure is the biggest challenge, first for [its] size, and second for the environmental conditions,” Gillett says. “Mauna Kea can be windy, so planning the lifts (and waiting for calm days) is critical. Also, the high altitude makes work more difficult.”
The enclosure will be rimmed with 92 ventilation doors that can be opened to provide air to the structure on calm nights. “The enclosure is kept closed during the day and cooled to the predicted temperature of the upcoming night,” says Gillett. “The purpose is to maintain the temperature of the telescope as close to the ambient air temperature [as possible] during observing.”
Ground anchors that can counteract the uplift forces created by wind are also being considered for the enclosure, which is designed as a “calotte enclosure,” comprising a rotating base, a cap cover, and a circular aperture. “The ‘calotte’ allows us to make a very compact enclosure,” Gillett says. “The distance between the telescope and the inside enclosure is about half a meters. So it’s a tight concept, and allows us to get the dome as small as possible.” However, the building is still the equivalent of a 16- or 18-story building, he notes.
The telescope’s enclosure will be rimmed with ventilation doors
that can be opened to provide air to the structure on calm nights.
During the day, these doors will be closed so that the telescope
can be cooled to the expected evening temperature. Courtesy of
TMT Observatory Corporation
The base and cap of the steel enclosure are a part of a continuous metal plate-covered spherical shell, which is split by an interface plane inclined at 32.5 degrees. The combined rotation of the base section and the cap section, which move independently, will enable the telescope to track objects across the sky, Gillett explains. “If you rotate those two axes simultaneously, you can point that aperture or shutter at any point in the sky you want to,” he says.
The aluminum shutter that closes the aperture opening is also a rotating structure, comprising an open framework of steel tubing and located within the inner edge of the enclosure, as shown in this video.
The telescope’s cost through to the end of construction is $1.3 billion, in 2011dollars, according to Gordon Squires, Ph.D., the communications and media relations lead for the telescope. Squires wrote in response to written questions posed by Civil Engineering online.
The telescope is being funded by a $250-million grant from the Gordon & Betty Moore Foundation and by both financial and in-kind contributions from the telescope’s partners: the California Institute of Technology, the University of California system, the Association of Canadian Universities for Research in Astronomy, the National Astronomical Observatory of Japan, a consortium of Chinese institutions led by the National Astronomical Observatories of the Chinese Academy of Sciences, and institutions in India supported by the Department of Science and Technology of India.
“The funding model is interesting and unique,” Squires said. “As well as making cash contributions, all of the partners are planning in-kind contributions (for example, mirror segments) and a world-cost model had to be developed to value these contributions.” Doing so was important, he notes, because the value of each partner’s contribution to the telescope “ultimately determines the partnership share and observing time on the telescope.”
The University of Hawai’i approved the lease for the telescope at the end of February. Construction is planned to begin later this year, and the first civil work on site will be completed in 2015, according to Gillett. Construction of the enclosure will then begin in 2015, with an estimated completion in 2020.
On-site installation of the telescope will begin in 2018, with completion slated for 2020, according to Gillett. In 2020, a period of assembly, integration, and verification that will last two years will begin.