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SPIE Professional October 2010

Astronomy Renaissance

A new era of extremely large telescopes is about to begin.

By Kathy Sheehan

Although more than a dozen 6- to 10-meter telescopes built over the last 20 years are producing "fantastic science," advances in glass making, telescope design, instrument engineering, and adaptive optics has sparked a kind of renaissance in the astronomy world. A new generation of huge ground-based telescopes, more than twice the size of the largest in current use, is being designed and expected to go online around 2018.

Members of the astronomy community are hoping the Giant Magellan Telescope (GMT) at 25 meters, the Thirty Meter Telescope (TMT), and the 42-meter European Extremely Large Telescope (E-ELT) will be able to solve the deepest mysteries of the universe such as whether there are other planets that can support human life, how the first stars were formed, and what is the nature and origin of dark energy.

Robert GilmozziRobert Gilmozzi, head of the European Southern Observatory's Telescope Division and principal investigator of the E-ELT, says that big telescopes such as the Keck, Very Large Telescope, Gemini, and Subaru "are producing fantastic science."

At the same time, however, they are producing new questions that may only be answered well after these new extremely large telescopes (ELTs) see first light.

"The playground for these new telescopes is the whole history of the universe," he noted in a plenary talk at SPIE Astronomical Telescopes and Instrumentation earlier this year. With 10 and 20 times the collecting power of current generation telescopes and the ability to combine data from other observatories—both ground- and space-based, the ELTs under development are expected to have the ability to catch the first glimpses of the creation of the universe.

For instance, the European ELT will be able to measure the dark energy that is accelerating the expansion of the universe, Gilmozzi says. "You will actually see the universe accelerate in real time, … catching it, if you will, in the act."

Precision optics and mirrors

If the next-generation telescopes are to successfully address fundamental questions in cosmology and astrophysics and explore the universe at the time of the Big Bang, there will be a high demand on the quality and precision of the optics, instruments, technologies, and materials.

All three of the ELTs will be built with segmented mirrors, a design technology pioneered by TMT Project Scientist and SPIE Fellow Jerry Nelson of the University of California at Santa Cruz. Using this design, the segmented mirrors form a bowlshaped reflecting surface that is lighter and more powerful than a monolithic mirror.

The GMT will have large segments, about 8.4 meters in diameter, and the TMT and E-ELT will have close-packed, more economically sized segments.

The large, primary mirrors for the GMT are being built at the University of Arizona's Steward Observatory Mirror Lab. SCHOTT, which has been supplying large-format mirror substrates made of its Zerodur material since 1968, is one of the organizations under consideration to provide secondary mirrors for the GMT and multiple segmented pieces for the E-ELT and TMT.

A SCHOTT team has been in intensive discussions with the organizations building the new telescopes, especially with regard to the specifications for the thermal expansion behavior and the homogeneity of the mirror material.

"The requirements for the 42-meter E-ELT and the TMT in California directly match Zerodur," says Peter Hartmann, director of marketing and customer relations at SCHOTT and recently elected to the SPIE Board of Directors.

SCHOTT is also involved with a number of other astronomical projects using specialized mirrors. One is the Advanced Technology Solar Telescope, which will use a SCHOTT-built 4-meter off-axis, aspheric mirror. When completed and in operation in Hawaii, estimated around 2018, the ATST will be the world's largest solar telescope.

"We are really proud that we have been awarded the order for the ATST primary mirror," says Thomas Westerhoff, manager of all SCHOTT activities for the Zerodur product line, who has co-authored numerous papers for SPIE astronomy meetings.

Giant Magellan Telescope

The Giant Magellan Telescope is the smallest of the three proposed superlarge telescopes and will rely on seven big mirrors, rather than hundreds of small ones, as its primary mirror.

Giant Magellan Telescope
Artist's rendering of the seven mirrors on the GMT. Courtesy of the GMTO Corp.

To be located in Las Campanas, Chile, its design calls for six off-axis paraboloid mirrors, each 8.4 meters (27.5 feet) across, arranged in a hexagonal circle around a central seventh mirror. These mirrors will form a single 25.3-meterdiameter primary mirror assembly. Polishing of the first, 20-ton mirror is now underway at the Steward Observatory lab.

The GMT secondary mirror will be composed of seven thin adaptive shells with each segment mapping to a single primary mirror segment. The adaptive secondary mirror will provide diffraction-limited performance over modest fields of view and ground-layer adaptive optics over a field of 10-20 arcminutes in diameter.

An international consortium of leading educational and research institutions from the United States, South Korea, and Australia will build and operate the GMT at an estimated cost of U.S. $700 million. The non-profit Giant Magellan Telescope Corp. is based in California.

Thirty Meter Telescope

The TMT will be the world's most advanced ground-based optical, nearinfrared, and mid-infrared observatory, according to a consortium of the Association of Canadian Universities for Research in Astronomy, the California Institute of Technology, and the University of California, which will build it on Mauna Kea in Hawaii.

Thirty Meter Telescope
An artist's rendering of the backside of the primary mirror assembly for the TMT, with its 492 aspheric mirrors. Courtesy of TMT Observatory Corp.

A 30-meter-diameter primary mirror (above) will be composed of 492 hexagonal, aspheric segments, each approximately 1.44 meters wide, with a thickness in the range of 50 mm. The design calls for 4-meter-class secondary and tertiary mirrors. The TMT should detect the first luminous objects in the universe, catching the "act" of the first formation of the stars, according to Gilmozzi.

It will also study the physics of objects that the future James Webb Space Telescope is expected to find. The projected cost of the TMT is about US $1 billion.

European Telescope will be extremely large

The European Southern Observatory plans to build the 42-meter E-ELT in Chile's Atacama Desert, about 20 kilometers from the Very Large Telescope, at a cost of about €960 million.

European Extremely Large Telescope
An architectural concept drawing of the planned E-ELT. Courtesy of ESO/Swinburne Astronomy Productions.

The E-ELT is designed as a Nasmyth telescope with adaptive optics built into the telescope. This drove the optical layout to five mirror segments: a three-mirror anastigmat and two flat-folding mirrors providing the adaptive optics.

The adaptive optics system has 2.5-meter mirror supported by 5000 or more actuators to distort its own shape 1000 times per second and a 2.7-meter mirror for the final image corrections.

The 42-meter-diameter primary mirror assembly will be composed of nearly 1000 hexagonal segments, allowing the E-ELT to gather 15 times more light than the largest optical telescopes operating today. Each segment will be about 1.43 meters wide from corner to corner and 50 mm thick. The secondary mirror will be as large as 6 meters in diameter. The 4.2-meter tertiary mirror will relay the light to the adaptive optics system.

SCHOTT sees exciting times in astronomy

"There seems to be a renaissance in astronomy right now with larger instruments and better science," says Arnie Bazensky, manager of field sales, Western U.S. Operations, for SCHOTT North America.

The new generation of extremely large telescopes being developed "will be able to look very far back into time to see light that emanates very close to the Big Bang," he says, and we'll be able to learn about the formation of stars and galaxies and the creation of our universe.

"The question that most astronomers ask is: 'Are we alone here?' These tools are being developed to provide that."

The team developing the Thirty Meter Telescope (TMT) gathered industry representatives to discuss the project a few years ago, Bazensky says. Photos and framed correspondence from leading scientists of the early 20th century, including Albert Einstein, filled the meeting room.

"It was very inspirational to see the continuity of the thought" among the group of scientists developing the TMT, Bazensky says, "especially now that we can provide such great tools and such great materials. It's very exciting."

U.S. priorities set for next 10 years

The National Academies Astro2010 Survey Committee recommends that the United States become a partner in the GMT or TMT projects by committing funds for construction and operations. Its August 2010 report, "New Worlds, New Horizons in Astronomy and Astrophysics," prioritizes the most important activities for astronomy and astrophysics over the next 10 years.

A large optical and nearinfrared telescope like the GMT or TMT will provide a spectroscopic complement to the James Webb Space Telescope (JWST), the Atacama Large Millimeter Array (ALMA), and the Large Synoptic Survey Telescope (LSST), it said. The LSST, an 8.4-meter telescope to be sited in Chile, was ranked as the highest priority among the planned ground-based astronomy projects.

For space-based programs, the committee recommended priority support for the Wide Field InfraRed Survey Telescope (WFIRST), the Laser Interferometer Space Antenna (LISA), and the International X-ray Observatory (IXO).

More: Astronomy and astrophysics decadal survey recommendations released

Astrophysicists win Kavli Prize

photo of SPIE Fellow Jerry NelsonSPIE Fellow and TMT Project Scientist Jerry Nelson, founding director of the Center for Adaptive Optics at University of California, Santa Cruz, was among eight scientists recognized earlier this year with the Kavli Prize.

He shared the award for astrophysics with Roger Angel of the University of Arizona and British scientist Raymond Wilson of the European Southern Observatory and formerly of Imperial College London.

Working separately on adaptive optics systems, Nelson and Angel improved the structure of telescopes, making them more powerful and allowing them to provide higher-resolution images.

Wilson, also a pioneer in adaptive optics, has helped astronomers gaze further into space by using computers to correct for the distorting effects of gravity, wind and temperature on telescopes.

See an SPIE Newsroom video interview with Nelson at spie.org/nelson-video.  

Have a question or comment about this article? Write to us at spieprofessional@spie.org.

DOI: 10.1117/2.4201010.04

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