GMT diagram.
DAMIEN JEMISON/GMTO
Since the 1990s, two revolutionary technologies have enabled telescopes to measure the cosmos with more precision than ever before. The first is interferometry, which uses multiple telescope observations to stitch together a picture with the same resolution as one enormous telescope, which is useful for doing things like imaging the surfaces of other stars. The test tower at the Mirror Lab uses interferometry to create contour maps of the glass surfaces.
The second major breakthrough in telescope technology, which the GMT will use, is adaptive optics.
"The telescope is basically always getting knocked out of focus from the atmosphere," says Laird Close, professor of astronomy at the University of Arizona. "So if you could use actuators to adjust the focus 1,000 times per second, you can have a telescope that is always in focus."
Keeping the telescope in focus is exactly what the adaptive secondary mirroris designed to do. This 3.25 meter diameter mirror will also have seven circular segments to match the primary mirror segments, except the secondary mirror segments will only be 1.6 mm thick.
"It's so thin that if you picked it up with your hands, it would break," says Close. "We spend a lot of time trying not to break the mirrors."
The adaptive secondary mirror will be constantly warped by hundreds of actuators to correct for the atmospheric refraction that bends light before it reaches the surface of our planet. Sodium lasers beamed from the telescope up into the sky will serve as guide stars to calibrate the adaptive optics system, which uses algorithms to predict atmospheric changes and warp the secondary mirror in real time.
GMTO
Even in the high Atacama Desert, atmospheric interference can significantly reduce the precision of astronomy observations. Adaptive optics systems allow telescopes like GMT to achieve unprecedented accuracy, similar to a telescope orbiting in space.
Other large ground-based telescopes—such as the Thirty Meter Telescope(TMT) planned for construction on the volcano Mauna Kea in Hawaii and the Extremely Large Telescope (ELT) which will live to the north of GMT in Chile—will have even larger primary mirrors than the Giant Magellan Telescope. These two telescopes, however, will use hundreds of hexagonal mirror segments that are each about a meter and a half across (492 segments for the TMT and 798 for the EMT), rather than seven gargantuan monolithic mirrors like GMT.
As a result, more gaps will exist in the light collecting areas of the TMT and ELT. The light will need to bounce around more before it reaches the instruments, and adaptive optics algorithms will be required to stitch everything together to a greater extent than with GMT, which can use a backup secondary mirror that is rigid to conduct observations while its flexible adaptive optics system is being cleaned and maintenced.
There is also the next flagship NASA space telescope, the James Webb Space Telescope (JWST), with 18 hexagonal mirror segments measuring 2.4 meters across for a total primary mirror aperture of 6.5 meters. James Webb will be dwarfed by the GMT's 24.5 meter aperture, but out beyond the atmosphere it will have an unparalleled view of the stars. Additionally, James Webb will take observations primarily in infrared, while GMT is optimized for light in the visible spectrum.
GMT construction site as of February 2017.
GMTO
"I think it will be very complimentary to Webb," says McCarthy. "The visible part of the spectrum is very rich in information."
James Webb, for example, will be particularly suited to studying the very early universe, as light that has been traveling for billions of light-years is Doppler shifted significantly into the infrared part of the spectrum. Infrared light can also be detected piercing through clouds and nebulae in space, revealing the objects lurking behind. Optical observations, like those of Hubble and GMT, are well-suited to spectroscopy and detecting the compositions of bodies such as exoplanets. The sheer size of GMT will also make it capable of detecting incredibly dim and distant objects.
Before the Giant Magellan Telescope can power on, however, there is still much work to be done. The fifth mirror is currently cooling in the furnace, and three more need to be cast—one extra outer mirror to keep the telescope at full capability as mirrors are swapped out to be cleaned and recoated with aluminum. The 17-ton mirrors in their articulated, temperature-controlled, shock-absorbing cases also need to be shipped from Arizona to Chile—by way of port in either Houston or Los Angeles.
Everything is at stake. McCarthy stresses the need to avoid a situation like the first light of Hubble, when it was discovered that the mirrors were not perfectly shaped, requiring new instrumentation to be installed to account for the incorrectly focused beams of light. And, of course, there is the possibility of losing a mirror on the voyage across the ocean and up into the mountains.
"You have all your eggs," says McCarthy. "Do you put them all in one basket, or do you put them in multiple baskets? ... Maybe I'll just go with them because if the boat goes down, it won't be my problem."
GMTO
The Giant Magellan Telescope is truly a telescope of a new era. The first large-scale observatories were established by the scientific societies of the Enlightenment with the financial backing of monarchs, such as the Royal Observatory in Greenwich and the Paris Observatory. In subsequent centuries, wealthy benefactors sponsored the construction of giant telescopes, such as James Lick who is buried underneath the concrete of his eponymous observatory on the summit of Mt. Hamilton near San Jose, California. In the modern era, taxpayers foot the bill for major astronomy projects from agencies like NSF, NASA, and the European Southern Observatory (ESO).
The GMT project represents a paradigm shift. Private interests, international partners, universities, and science institutions across the world have come together to build this great monument to science in the high Atacama. Boeing is conducting fluid dynamics research for the telescope's housing structure, and NASA's Jet Propulsion Laboratory is providing mirror insight gleaned from the mistakes made on Hubble. Other than the University of Arizona's Steward Observatory, the GMT is sponsored by the Carnegie Institution for Science in Washington D.C., the São Paulo Research Foundation of Brazil, the Korea Astronomy and Space Science Institute, Harvard, the Smithsonian Institution, Arizona State University, the McDonald Observatory of the University of Texas at Austin, Texas A&M, the University of Chicago, Astronomy Australia Limited, and the Australian National University, among other private benefactors.
"We're going to be fighting tooth and nail for time," said Males, referring to the University of Arizona's need to share the telescope's observation time with so many other institutions. But the very fact that science agencies around the world are chomping at the bit to get involved with the Giant Magellan Telescope is a testament to its capability, versatility, and the profundity that astronomers expect to encounter through its 119-ton mirror, glinting on a mountaintop in the Atacama Desert.
"We do not make this to sell for a profit," says Dae Wook Kim. "We make it for all mankind."
Mirror Lab staff member Linda Warren places the last piece of glass into the mold for GMT mirror 5.