The 2020 U.S. Decadal Survey on Astronomy and Astrophysics, a type of survey in which astronomers set priorities to guide federal budgets during the next 10 years, is in the process of listing its most important projects. Asteroid detection and Mars exploration are high on the list, as is continued support of a new cadre of extremely large telescopes already under construction around the world, with effective mirror apertures measuring 20, 30, or 40 m in diameter. Astronomers hope these giant ground-based light buckets will provide capabilities exceeding anything else humans have ever built, and will witness new scientific paradigms such as life on other planets, the birth of new stars, and the formation of galaxies. The Giant Magellan Telescope (GMT), under construction on the summit of Las Campanas Peak in Chile, will help unravel the mysteries of the universe. It features seven massive 8.4-m monolith mirrors arranged around a central mirror to form a single optical surface with an effective aperture of 24.5 m in diameter. Optical telescopes on Earth require very fine angular resolution to unravel mysteries of the universe, such as dark matter and Earth-like exoplanets, which means the bigger the telescopes’ mirrors the better1. However, the more massive the mirror, the harder it is to build. The W. M. Keck Observatory in Hawaii offered an early model for solving this challenge by constructing its 10-m-wide twin telescope mirrors from a honeycomb of smaller, hexagonal mirror segments. Subsequent telescopes have applied a similar approach to aim for even greater light-gathering capabilities. The World’s Largest Optical Telescopes by 2030, Ranked by Light-Gathering Capabilities Collecting light is only part of the challenge, however. Essential for all extremely large telescopes today is adaptive optics technology, which places a very thin, deformable mirror in the telescope’s optical path. The mirror is able to change shape thousands of times per second in real time to correct for atmospheric distortions, enabling astronomers to erase the blurring caused by the turbulent air in the atmosphere. (For more on adaptive optics, see accompanying article “Bringing Habitable Planets to Light with Faster Adaptive Optics”.) Delays expected The next generation of ground-based telescopes are marvels of modern engineering, with each design standing on the shoulders of the giant observatories before it. The most ambitious project to date may be the European Southern Observatory’s Extremely Large Telescope (ELT). With a primary mirror measuring 39.3 m in diameter, the ELT will be the world’s biggest optical telescope when it begins operation in Chile in late 2025. The project remains on schedule, despite unforeseen delays such as the COVID-19 pandemic, but delays are often inherent to such complex undertakings. Current first-light estimates for the Giant Magellan Telescope (GMT) are for 2029, and for the Thirty Meter Telescope (TMT), 2030. A decade ago, these were expected to be nearly complete by now. Hot from the oven: The first of hundreds of ZERODUR ELT mirror segments was transferred from the melting tank to the annealing furnace in December 2018. Courtesy of SCHOTT AG. “When you build a schedule of a telescope this large, you have little prior experience to base it on. You can only estimate how long a particular development will take,” said James Fanson, project manager at the Giant Magellan Telescope Organization (GMTO) in Los Angeles. “Everything we’re doing is new ground, so occasionally you run into problems along the way that cause a delay.” Progress on several large telescopes was affected by interruptions in construction, vendor closures, and logistical difficulties due to the COVID-19 pandemic. Other production activities continued normally. Production of ZERODUR ELT segment blanks at SCHOTT in Mainz, Germany, continued uninterrupted because the technology is considered essential to several other important high-tech applications. The primary mirror of the European Southern Observatory’s Extremely Large Telescope (ELT) will comprise 798 hexagonal segments measuring 1.45 m each from corner to corner. The ELT and each of the next-generation giant ground-based optical telescopes will have adaptive optics systems to correct for atmospheric aberrations. Courtesy of ESO/L. Calçada/ACe Consortium. “Despite travel restrictions during the pandemic, COVID-19 was unable to impair SCHOTT’s progress on segment blank production for the world’s largest eye on the sky,” said Thomas Westerhoff, vice president of strategic business for the ZERODUR material at SCHOTT. Designers working on many other aspects of the telescopes have largely been able to continue their work from home. But other project milestones have been delayed, such as on-site construction, in-person readiness checks, and hardware development in laboratories. For example, the polishing of several ELT mirror segments scheduled to begin in April at Safran Reosc in Poitiers, France, had to be postponed because the factory closed to ensure workers’ safety. The long-term effects of COVID-19 on giant telescopes may take years to be fully gauged. The next big thing Part of the challenge for extremely large telescopes is the time required to master such huge, complex new designs. The GMT, a megapowerful multipurpose observatory under construction at the top of the Chilean Andes, has seven large primary mirrors, each measuring 8.4 m in diameter. Six of these giant monolithic mirrors will be arrayed around a central mirror to create an effective aperture of 25.4 m, giving the GMT a resolving power 10× that of Hubble. Each mirror takes years to finish, from the initial castings through the grinding and polishing of their surfaces to meet a precision threshold of 25 nm. Size comparison of the primary mirrors of the giant telescopes under construction. The Thirty Meter Telescope (TMT) will have 492 hexagonal mirrors, while the GMT will have seven round segments. The NSF Vera C. Rubin Observatory, previously named the Large Synoptic Survey Telescope, will have a ring-shaped primary mirror measuring 8.4 m in diameter. The GMT and the TMT are expected to see first light around 2030. Courtesy of ESO.org. Unlike the segmented hexagonal mirrors of two other extremely large telescopes currently being built — the ELT and the TMT — the GMT’s six off-axis primary mirrors have complex shapes that had never been manufactured before. These are paired with seven secondary mirror segments that must all be aligned as if they are a single functioning monolithic mirror. This is a much more complicated task than aligning the primary and secondary mirrors in a conventional telescope. “Nobody has ever built such a giant, doubly segmented optical telescope before,” Fanson said. “We’ve never had to align so many mirrors.” Furthermore, very high-resolution imaging with multiple mirrors means the adaptive optics must follow multiple stars to compensate for atmospheric turbulence. To make this complex alignment process more efficient, the GMT will incorporate a new precision measurement technology called Absolute Multiline Technology from Etalon GmbH in Germany. The system is a sort of laser truss or harness of more than 100 lasers operating at 1532 nm from the edges of each mirror to monitor the leg length between each set of mirrors to a precision of 0.5 µm per meter of optical path2. Knowing the precise lengths of several points between the surfaces of the mirrors enables operators to determine and maintain the position of all the telescope optics and laser guide stars. A precision metrology system from Etalon GmbH monitors the distance between the GMT’s primary and secondary sets of mirrors to keep them aligned to a precision of 0.5 µm per meter of optical path. Courtesy of GMTO. The GMT’s mirrors, which will take many years to mold, cool, grind, and polish, are being built at the University of Arizona’s Richard F. Caris Mirror Lab in Tucson, Ariz. Casting and polishing of primary mirrors M1 and M2 are complete. Primary mirror M3 is currently undergoing fine grinding of its front surface, and M4, the central mirror, is undergoing polishing of its rear surface. Mirrors M6 and M7 are expected to be cast in 2021. “It’s always remarkable when you take 20 tons of pressured glass loaded into a frame, put it in a giant ceramic oven, close everything up, start spinning it, and watch it melt through cameras,” said Robert Shelton, president of GMTO. “Months later,” added Fanson, “you take the soufflé out of the oven while you hold your breath. Fortunately, with the expertise at the University of Arizona, it comes out just right.” Shipping these massive optics to their final mountaintop destination is in itself a feat. The GMT primary mirrors must be shipped by sea from Houston through the Panama Canal, all the way to Chile. TMT progress The 30-m-diameter primary mirror of the TMT comprises 492 hexagonal segments and a structural design intended to minimize its visible and environmental impacts on the peak of the highest mountain in Hawaii, Maunakea (which reverted to the traditional single-word Hawaiian spelling in 2014). The project involves a five-nation partnership of the U.S., China, Japan, India, and Canada and will hopefully fill a unique role by observing at wavelengths from the ultraviolet to the mid-infrared at resolutions 12× sharper than Hubble. During the past decade, however, the project has been saddled with legal challenges and delays. Native Hawaiian protestors stalled construction that was scheduled to begin in July 2019. But the TMT partners are committed to helping address issues surrounding Native Hawaiian sovereignty and inequality. The calotte-style design of the TMT enables full movement of the telescope and optical systems while minimizing the size and footprint of the facility on Maunakea, Hawaii. Courtesy of TMT International Observatory. “The partner institutions have a long history of doing science on Maunakea, and there’s a human component to having been involved in the community for decades,” said Gordon Squires, vice president of external relations for the TMT International Observatory. “We’ve made a commitment to listen and work with Native Hawaiians and the state of Hawaii to help initiate solutions to their complaints. COVID-19 has made this more difficult. But how beautiful would it be if the telescope helped make progress with these long-standing cultural issues?” TMT partners have designated a secondary site for construction on La Palma, one of Spain’s Canary Islands, as a backup plan. But the location in Hawaii was selected, among other reasons, to complement the GMT in Chile with a separation in latitude that enables full-sky coverage and a higher altitude. In spite of the protests and the pandemic, TMT announced that its first carbon fiber metrology frame was built and delivered to the TMT technical laboratory near Pasadena, Calif., in April. Built by high-tech carbon fiber composite designer Rock West Composites in San Diego, the metrology frame for the TMT’s primary mirror (M1) will enable accurate placement of the mirror segments during installation. Nine additional metrology frames will help align clusters of fixed mirror frames. Despite delays, these extremely large telescope projects will hopefully become operational in time for their builders to see the fruits of their labor. “Every time we build a telescope that looks at the universe in a new and more powerful way, we discover things we had not imagined we would be able to observe,” Fanson said. “For me, one of the most exciting things about the giant telescope is that we will see things we don’t know how to anticipate.” References 1. V.C. Coffey (November 2014). Breaking ground now: next-gen giant telescopes. Photonics Spectra. 2. A. Rakich et al. (2016) A 3-D metrology system for the GMT. Proc SPIE, Vol. 9906, 990614, www.doi.org/10.1117/12.2234301. Go Big and Stay Home The counterpart to the European Extremely Large Telescope is the U.S. Extremely Large Telescope Program, encompassing the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT). On March 25 — which will go down in history as a date that occurred during an alarming period of exponential COVID-19 growth — the Science Advisory Committees for both TMT and GMT were scheduled to commence their first joint meeting in Pasadena, Calif., to plan and discuss the future science collaboration between the two large telescope projects. Instead, to ensure the safety of the scientists, the meeting was held virtually, with all the participants reporting from home. It was the first meeting of its kind, both because it was held remotely and because of the scope of the partnership between the two organizations.