Future observatory in the United States
The Thirty Meter Telescope (TMT) is a planned extremely large telescope (ELT) proposed to be built on Mauna Kea, on the island of Hawai'i. The TMT would become the largest visible-light telescope on Mauna Kea.
Scientists have been considering ELTs since the mid 1980s. In 2000, astronomers considered the possibility of a telescope with a light-gathering mirror larger than in diameter, using either small segments that create one large mirror, or a grouping of larger mirrors working as one unit. The US National Academy of Sciences recommended a telescope be the focus of U.S. interests, seeking to see it built within the decade. 
Scientists at the University of California, Santa Cruz and Caltech began development of a design that would eventually become the TMT, consisting of a 492-segment primary mirror with nine times the power of the Keck Observatory. Due to its light-gathering power and the optimal observing conditions which exist atop Mauna Kea, the TMT would enable astronomers to conduct research which is infeasible with current instruments. The TMT is designed for near-ultraviolet to mid-infrared (0.31 to 28 μm wavelengths) observations, featuring adaptive optics to assist in correcting image blur. The TMT will be at the highest altitude of all the proposed ELTs. The telescope has government-level support from several nations.
The proposed location on Mauna Kea has been controversial among the Native Hawaiian community. Demonstrations attracted press coverage after October 2014, when construction was temporarily halted due to a blockade of the roadway. When construction of the telescope was set to resume, construction was blocked by further protests each time. In 2015, Governor David Ige announced several changes to the management of Mauna Kea, including a requirement that the TMT's site will be the last new site on Mauna Kea to be developed for a telescope. The Board of Land and Natural Resources approved the TMT project, but the Supreme Court of Hawaii invalidated the building permits in December 2015, ruling that the board had not followed due process. In October 2018, the Court approved the resumption of construction; however, no further construction has occurred due to continued opposition. In July 2023 a new state appointed oversight board, which includes Native Hawaiian community representatives and cultural practitioners, began a five-year transition to assume management over Mauna Kea and its telescope sites, which may be a path forward. In April 2024, TMT's project manager apologized for the organization having "contributed to division in the community", and stated that TMT's approach to construction in Hawai'i is "very different now from TMT in 2019."
An alternate site for the Thirty Meter Telescope has been proposed for La Palma, Canary Islands, Spain, but is considered less scientifically favorable by astronomers. As of  2024[ [update]], there were no specific timelines or schedules regarding new start or completion dates.
Background.
In 2000, astronomers began considering the potential of telescopes larger than in diameter. The technology to build a mirror larger than does not exist; instead scientists considered two methods: either segmented smaller mirrors as used in the Keck Observatory, or a group of 8-meter (26') mirrors mounted to form a single unit. The US National Academy of Sciences made a suggestion that a telescope should be the focus of US astronomy interests and recommended that it be built within the decade. 
The University of California, along with Caltech, began development of a 30-meter telescope that same year. The California Extremely Large Telescope (CELT) began development, along with the Giant Segmented Mirror Telescope (GSMT), and the Very Large Optical Telescope (VLOT). These studies would eventually define the Thirty Meter Telescope. The TMT would have nine times the collecting area of the older Keck telescope using slightly smaller mirror segments in a vastly larger group. Another telescope of a large diameter in the works is the Extremely Large Telescope (ELT) being built in northern Chile.
The telescope is designed for observations from near-ultraviolet to mid-infrared (0.31 to 28 μm wavelengths). In addition, its adaptive optics system will help correct for image blur caused by the atmosphere of the Earth, helping it to reach the potential of such a large mirror. Among existing and planned extremely large telescopes, the TMT will have the highest elevation and will be the second-largest telescope once the ELT is built. Both use segments of small hexagonal mirrors—a design vastly different from the large mirrors of the Large Binocular Telescope (LBT) or the Giant Magellan Telescope (GMT). Each night, the TMT would collect 90 terabytes of data. The TMT has government-level support from the following countries: Canada, Japan and India. The United States is also contributing some funding, but less than the formal partnership.
Proposed locations.
In cooperation with AURA, the TMT project completed a multi-year evaluation of six sites:
The TMT Observatory Corporation board of directors narrowed the list to two sites, one in each hemisphere, for further consideration: Cerro Armazones in Chile's Atacama Desert and Mauna Kea on Hawaii Island. On July 21, 2009, the TMT board announced Mauna Kea as the preferred site. The final TMT site selection decision was based on a combination of scientific, financial, and political criteria. Chile is also where the European Southern Observatory is building the ELT. If both next-generation telescopes were in the same hemisphere, there would be many astronomical objects that neither could observe. The telescope was given approval by the state Board of Land and Natural Resources in April 2013. 
There has been opposition to the building of the telescope, based on potential disruption to the fragile alpine environment of Mauna Kea due to construction, traffic, and noise, which is a concern for the habitat of several species, and because Mauna Kea is a sacred site for the Native Hawaiian culture. The Hawaii Board of Land and Natural Resources conditionally approved the Mauna Kea site for the TMT in February 2011. The approval has been challenged; however, the Board officially approved the site following a hearing on February 12, 2013.
Partnerships and funding.
The Gordon and Betty Moore Foundation has committed US$200 million for construction. Caltech and the University of California have committed an additional US$50 million each. Japan, which has its own large telescope at Mauna Kea, the Subaru, is also a partner.
In 2008, the National Astronomical Observatory of Japan (NAOJ) joined TMT as a collaborator institution. The following year, the telescope cost was estimated to be $970 million to $1.4 billion. That same year, the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) joined TMT as an observer. The observer status is the first step in becoming a full partner in the construction of the TMT and participating in the engineering development and scientific use of the observatory. By 2024, China was not a partner in TMT. 
In 2010, a consortium of Indian Astronomy Research Institutes (IIA, IUCAA and ARIES) joined TMT as an observer, subject to approval of funding from the Indian government. Two years later, India and China became partners with representatives on the TMT board. Both countries agreed to share the telescope construction costs, expected to top $1 billion. India became a full member of the TMT consortium in 2014. In 2019 the India-based company Larsen &amp; Toubro (L&amp;T) were awarded the contract to build the segment support assembly (SSA), which "are complex optomechanical sub-assemblies on which each hexagonal mirror of the 30-metre primary mirror, the heart of the telescope, is mounted".
The IndiaTMT Optics Fabricating Facility (ITOFF) will be constructed at the Indian Institute of Astrophysics campus in the city of Hosakote, near Bengaluru. India will supply 80 of the 492 mirror segments for the TMT. A.N. Ramaprakash, the associate programme director of India-TMT, stated; "All sensors, actuators and SSAs for the whole telescope are being developed and manufactured in India, which will be put together in building the heart of TMT", also adding; "Since it is for the first time that India is involved in such a technically demanding astronomy project, it is also an opportunity to put to test the abilities of Indian scientists and industries, alike."
The continued financial commitment from the Canadian government had been in doubt due to economic pressures. In April 2015, Prime Minister Stephen Harper announced that Canada would commit $243.5 million over a period of 10 years. The telescope's unique enclosure was designed by Dynamic Structures Ltd. in British Columbia. In a 2019 online petition, a group of Canadian academics called on succeeding Canadian Prime Minister Justin Trudeau together with Industry Minister Navdeep Bains and Science Minister Kirsty Duncan to divest Canadian funding from the project. As of  2020[ [update]], the Canadian astronomy community has named TMT its top facility priority for the decade ahead.
Design.
The TMT would be housed in a general-purpose observatory capable of investigating a broad range of astrophysical problems. The total diameter of the dome will be with the total dome height at (comparable in height to an eighteen-storey building). The total area of the structure is projected to be within a complex.
Telescope.
The centerpiece of the TMT Observatory is to be a Ritchey-Chrétien telescope with a diameter primary mirror. This mirror is to be segmented and consist of 492 smaller (), individual hexagonal mirrors. The shape of each segment, as well as its position relative to neighboring segments, will be controlled actively.
A secondary mirror is to produce an unobstructed field-of-view of 20 arcminutes in diameter with a focal ratio of 15. A flat tertiary mirror is to direct the light path to science instruments mounted on large Nasmyth platforms. The telescope is to have an alt-azimuth mount. Target acquisition and system configuration capabilities need to be achieved within 5 minutes, or ten minutes if relocating to a newer device. To achieve these time limitations the TMT will use a software architecture linked by a service based communications system. The moving mass of the telescope, optics, and instruments will be about . The design of the facility descends from the Keck Observatory.
Adaptive optics.
Integral to the observatory is a Multi-Conjugate Adaptive Optics (MCAO) system. This MCAO system will measure atmospheric turbulence by observing a combination of natural (real) stars and artificial laser guide stars. Based on these measurements, a pair of deformable mirrors will be adjusted many times per second to correct optical wave-front distortions caused by the intervening turbulence.
This system will produce diffraction-limited images over a 30-arc-second diameter field-of-view, which means that the core of the point spread function will have a size of 0.015 arc-second at a wavelength of 2.2 micrometers, almost ten times better than the Hubble Space Telescope.
Scientific instrumentation.
Early-light capabilities.
Three instruments are planned to be available for scientific observations:
Approval process and protests.
In 2008, the TMT corporation selected two semi-finalists for further study, Mauna Kea and Cerro Amazones. In July 2009, Mauna Kea was selected. Once TMT selected Mauna Kea, the project began a regulatory and community process for approval. Mauna Kea is ranked as one of the best sites on Earth for telescope viewing and is home to 13 other telescopes built at the summit of the mountain, within the Mauna Kea Observatories grounds. Telescopes generate money for the big island, with millions of dollars in jobs and subsidies gained by the state. The TMT would be one of the most expensive telescopes ever created.
However, the proposed construction of the TMT on Mauna Kea sparked protests and demonstrations across the state of Hawaii. Mauna Kea is the most sacred mountain in Hawaiian culture as well as conservation land held in trust by the state of Hawaii.
2010-2014: Initial approval, permit and contested case hearing.
In 2010 Governor Linda Lingle of the State of Hawaii signed off on an environmental study after 14 community meetings. The BLNR held hearings on December 2 and December 3, 2010, on the application for a permit.
On February 25, 2011, the board granted the permits after multiple public hearings. This approval had conditions, in particular, that a hearing about contesting the approval be heard. A contested case hearing was held in August 2011, which led to a judgment by the hearing officer for approval in November 2012. The telescope was given approval by the state Board of Land and Natural Resources in April 2013. This process was challenged in court with a lower court ruling in May 2014. The Intermediate Court of Appeals of the State of Hawaii declined to hear an appeal regarding the permit until the Hawaii Department of Land and Natural