The Giant Magellan Telescope (GMT) in northern Chile will collect its first light from the cosmos by 2029. As part of a new class of next generation instruments known as “Extremely Large Telescopes” (ELTs), GMT will combine the power of advanced primary mirrors, flexible secondary mirrors, adaptive optics (AOs) and spectrometers to see further and with more detailed than any previous optical telescope.
At the heart of the telescope are seven monolithic mirror segments, each 8.4 m (27.6 ft) in diameter, giving it the resolution of a 24.5 m (80.4 ft) primary mirror. According to recent statements from the GMT Organization (GMTO), the University of Arizona Richard F. Caris Mirror Lab has started casting the sixth and seventh segments for the telescope’s primary mirror (completion will take the next four years).
The seven mirror segments that make up the GMT are among the largest rigid monolith mirrors in the world. In their final configuration, the six off-axis segments surround a central axial segment, giving it a primary mirror that can collect light from a surface area of 368 square meters (~ 1200 square feet). The resolution is ten times higher than that of the Hubble Space Telescope (HST).
Artist’s impression of the segmented GMT mirror. Photo credit: GMTO
As James Fanson, the GMT project manager, said in the GMTO press release:
“The most important part of a telescope is its light collecting mirror. The bigger the mirror, the deeper we can see into the universe and the more details we can observe. The Giant Magellan Telescope’s unique primary mirror design consists of seven of the largest mirrors in the world.
“The casting of the sixth mirror is an important step towards completion. As soon as the Giant Magellan Telescope is operational, images are generated that are ten times sharper than the Hubble Space Telescope. The discoveries that these mirrors will make will change our understanding of the universe. “
The casting process is conducted at the Richard F. Caris Mirror Lab, which is overseen by the University of Arizona in Tuscon, AZ. It starts with nearly 17.5 tons of high-purity borosilicate glass (also known as E6 glass), which is then melted by the world’s only spinning furnace. This “centrifugal casting” process heats the glass until it liquefies and also gives the segments their special parabolic shape.
At its peak temperature (an event known as a “high fire”), the furnace spins at a speed of 5 rpm, heating the glass to 1,165 °C (2.129 °F) for about five hours. The sixth and seventh mirror segments will reach a “high fire” by March 6, 2021. They then go through the one-month “annealing process” during which the spinning furnace slows down in order to remove internal stresses on the glass.
A GMT mirror segment is cast in the Richard F. Caris Mirror Lab. Photo credit: GMTO
This allows the mirrors to harden as they cool, which will take another 1.5 months before they reach room temperature. After cooling, the mirrors are polished for two years until their surfaces achieve an optical surface accuracy of just a few nanometers (less than a thousand the width of a human hair). Buell Jannuzi, director of the Steward Observatory and head of the astronomy department at UofA, said:
“I am extremely proud of the way the mirror laboratory has adapted to the pandemic so that our talented and dedicated members of the Richard F. Caris Mirror Laboratory can continue to safely manufacture the mirrors for the giant Magellanic telescope.”
The first two mirror segments are currently completed and stored in the mirror laboratory, while segments three to six are now at different points in their production. The fifth mirror was cast in November 2017 while the fourth mirror has finished polishing the back, while the third is more than halfway done (and has an accuracy of 70 nanometers).
An eighth replacement mirror is also planned, which will be replaced if another mirror segment needs to be serviced. The mirrors are a crucial part of the optical design that allows the GMT to have the widest field of view of any ELT telescope in the 30 meter class – like the 30 meter telescope (TMT) currently in use at Mauna Kea, Hawaii.
The secret to this is the GMT’s unique optical design, which can take advantage of any photon of light collected by the mirrors (ensuring an unprecedented level of optical efficiency). As Rebecca Bernstein, GMT’s chief scientist, explained:
“This unprecedented combination of light gathering power, efficiency and image resolution enables us to make new discoveries in all areas of astronomy, especially in areas that require the highest spatial and spectral resolutions, such as the search for other earths.
“We will have unique capabilities to study planets with high resolution. This is key to understanding whether a planet is rocky in composition like our earth, whether it contains liquid water, and whether its atmosphere contains the right combination of molecules to signal the presence of life. “
By the late 2020s, the finished mirrors will be transported from Tuscon, AZ, to the Las Campanas Observatory in the Atacama Desert in northern Chile. This arid region lies 2,500 m above sea level and is considered to be one of the best places to practice astronomy on the planet. At this altitude the sky is very clear and the distance to the urban centers ensures that there is practically no light pollution.
In addition, the region’s stable airflow enables exceptionally sharp images, and its location (in the southern hemisphere) allows the observatories to look towards the center of the Milky Way. The proximity to other observatories in the area (which carry out observations at different wavelengths) enables easy collaboration.
This includes the European Extremely Large Telescope (EELT), which is currently being built by the European Southern Observatory (ESO) at the neighboring Cerro Armazones Observatory in Chile. It will also help with observations made by ESO’s other large observatories in the region – such as the Very Large Telescope (VLT) and the Atacama Large Millimeter / Submillimeter Array (ALMA).
In addition to providing ten times the visual performance of Hubble, GMT is four times the performance of the highly anticipated James Webb Space Telescope, slated to launch on October 31, 2021. Once operational, the GMT will reveal a number of cosmic mysteries, including the early history of the universe, the role of dark matter and energy in cosmic evolution, and the possible habitability of nearby exoplanets.
Further reading: GMTO, CfA
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