The Extremely Large Telescope (ELT) is an astronomical observatory under construction. When completed, it will be the world's largest optical and mid-infrared extremely large telescope. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile, 23 km from the existing facilities at Paranal Observatory.
The design consists of a reflecting telescope with a segmented primary mirror and a diameter secondary mirror. The telescope is equipped with adaptive optics, six laser guide star units, and various large-scale scientific instruments. The observatory's design will gather 100 million times more light than the human eye, equivalent to about 10 times more light than the largest optical telescopes in existence as of 2025, with the ability to correct for atmospheric distortion. It has around 250 times the light-gathering area of the Hubble Space Telescope and, according to the ELT's specifications, will provide images 15 times sharper than those from Hubble.
The project was originally called the European Extremely Large Telescope (E-ELT), but the name was shortened in 2017. The ELT is intended to advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes, the nature of the Universe's dark sector, and to detect water and organic molecules in protoplanetary disks around other stars. As planned in 2011, the facility was expected to take 11 years to construct, from 2014 to 2025.
On 11 June 2012, the ESO Council approved the ELT programme's plans to begin civil works at the telescope site, with the construction of the telescope itself pending final agreement with governments of some member states. Construction work on the ELT site started in June 2014. By December 2014, ESO had secured over 90% of the total funding and authorized construction of the telescope to start, estimated to cost around one billion euros for the first construction phase. The first stone of the telescope was ceremonially laid on 26 May 2017, initiating the construction of the dome's main structure and telescope. The telescope passed the halfway point in its development and construction in July 2023, with the expected completion and first light set for March 2029.
History
On 26 April 2010, the European Southern Observatory (ESO) Council selected Cerro Armazones, Chile, as the baseline site for the planned ELT. Other sites that were under discussion included Cerro Macon, Salta, in Argentina; Roque de los Muchachos Observatory, on the Canary Islands; and sites in North Africa, Morocco, and Antarctica.
Early designs included a segmented primary mirror with a diameter of and an area of about , with a secondary mirror with a diameter of . However, in 2011 a proposal was put forward to reduce overall size by 13% to 978 m2, with a diameter primary mirror and a diameter secondary mirror. This reduced projected costs from 1.275 billion to 1.055 billion euros and should allow the telescope to be finished sooner. The smaller secondary is a particularly important change; places it within the capabilities of multiple manufacturers, and the lighter mirror unit avoids the need for high-strength materials in the secondary mirror support spider.
ESO's Director General commented in a 2011 press release that "With the new E-ELT design we can still satisfy the bold science goals and also ensure that the construction can be completed in only 10–11 years." The ESO Council endorsed the revised baseline design in June 2011 and expected a construction proposal for approval in December 2011. Funding was subsequently included in the 2012 budget for initial work to begin in early 2012. The project received preliminary approval in June 2012. ESO approved the start of construction in December 2014, with over 90% funding of the nominal budget secured.
The design phase of the 5-mirror anastigmat was fully funded within the ESO budget. With the 2011 changes in the baseline design (such as a reduction in the size of the primary mirror from 42 m to 39 m), in 2017 the construction cost was estimated to be €1.15 billion (including first generation instruments). In 2014, the start of operations was planned for 2024. Actual construction officially began on 26 May 2017, and a technical first light is planned for March 2029. First scientific observations are planned for December 2030.
Planning
ESO focused on the current design after a feasibility study concluded the proposed diameter, Overwhelmingly Large Telescope, would cost €1.5 billion (£1 billion), and be too complex. Both current fabrication technology and road transportation constraints limit single mirrors to being roughly per piece. The next-largest telescopes currently in use are the Keck Telescopes, the Gran Telescopio Canarias and the Southern African Large Telescope, which each use small hexagonal mirrors fitted together to make a composite mirror slightly over across. The ELT uses a similar design, as well as techniques to work around atmospheric distortion of incoming light, known as adaptive optics.
A 40-metre-class mirror will allow the study of the atmospheres of extrasolar planets. The ELT is the highest priority in the European planning activities for research infrastructures, such as the Astronet Science Vision and Infrastructure Roadmap and the ESFRI Roadmap. The telescope underwent a Phase B study in 2014 that included "contracts with industry to design and manufacture prototypes of key elements like the primary mirror segments, the adaptive fourth mirror or the mechanical structure (...) [and] concept studies for eight instruments".
Design
The ELT will use a novel design with a total of five mirrors. The first three mirrors are curved (non-spherical) and form a three-mirror anastigmat design for excellent image quality over the 10-arcminute field of view (one-third of the width of the full Moon). The fourth and fifth mirrors are flat, and respectively provide adaptive optics correction for atmospheric distortions (mirror 4) and tip-tilt correction for image stabilization (mirror 5). The fourth and fifth mirrors also send the light sideways to one of two Nasmyth focal stations at either side of the telescope structure, allowing multiple large instruments to be mounted simultaneously.
Primary mirror The primary mirror will be composed of 798 hexagonal segments, each approximately across and with a thickness of . Two segments will be re-coated and replaced each working day, to keep the mirror always clean and highly reflective.
Edge sensors constantly measure the positions of the primary mirror segments relative to their immediate neighbours. 2394 position actuators (3 for each segment) use this information to adjust the system, keeping the overall surface shape unchanged against deformations caused by external factors such as wind, gravity, temperature changes and vibrations.
In January 2017, ESO awarded the contract for the fabrication of the 4608 edge sensors to the FAMES consortium, which is composed of French company Fogale and German company Micro-Epsilon. These sensors can measure relative positions to an accuracy of a few nanometres, the most accurate ever used in a telescope.
In May 2017, ESO awarded two additional contracts for the segments of the primary mirror. One was awarded to the German company Schott AG who manufactures the blanks of the 798 segments, as well as a maintenance set of 133 additional segments, one for each unique segment shape in the surface. Once the ELT is in operation, the maintenance set allows segments to be removed, replaced, and recoated on a rotating basis without leaving temporary gaps in the surface. The segments were cast from the same low-expansion ceramic Zerodur as the existing Very Large Telescope mirrors in Chile.
The other segment contract was awarded to the French company, Safran Reosc, a subsidiary of Safran Electronics & Defense. They receive the mirror blanks from Schott, and polish one mirror segment per day to meet the 7-year deadline. During this process, each segment is polished until it has no surface irregularity greater than 7.5 nm root mean square. Afterward, Safran Reosc mounts, tests, and completes all optical testing before delivery. This is the second-largest contract for ELT construction and the third-largest contract ESO has ever signed.
The segment support system units for the primary mirror were designed and are produced by CESA (Spain) and VDL (the Netherlands). The contracts signed with ESO also include the delivery of detailed and complete instructions and engineering drawings for their production. Additionally, they include the development of the procedures required to integrate the supports with the ELT glass segments; to handle and transport the segment assemblies; and to operate and maintain them.
As of July 2023, over 70% of the mirror segment blanks and their supporting structures had been manufactured, and by early 2024 tens of segments had been polished.
Secondary mirror Making the secondary mirror is a major challenge as it is highly convex, and aspheric. It is also very large; at in diameter and weighing , it will be the largest secondary mirror ever employed on an optical telescope and the largest convex mirror ever produced.
In January 2017, ESO awarded a contract for the mirror blank to Schott AG, who cast it later the same year from Zerodur. In May 2017, Schott AG was also awarded the contract for the much larger primary segment of the mirror.
Complex support cells are also necessary to ensure the flexible secondary and tertiary mirrors retain their correct shape and position; these support cells will be provided by SENER. Like the tertiary mirror, the secondary mirror will be mounted on 32 points, with 14 along its edges and 18 on the back. The entire assembly will be mounted on a hexapod, allowing its position to be aligned every few minutes to sub-micrometer precision. Deformations on the secondary mirror have a much smaller effect on the final image compared to errors on the tertiary, quaternary, or quinary mirrors.
The pre-formed glass-ceramic blank of the secondary mirror is being polished and tested by Safran Reosc. The mirror will be shaped and polished to a precision of 15 nanometres (15 billionths of a metre) over the optical surface.
By early 2024 this mirror was reported to be close to final accuracy.
Tertiary mirror The concave tertiary mirror, also cast from Zerodur, will be an unusual feature of the telescope. Most current large telescopes, including the VLT and the NASA/ESA Hubble Space Telescope but excluding the Vera C. Rubin Observatory, use two curved mirrors to form an image. In these cases, a small, flat tertiary mirror is sometimes introduced to divert the light to a convenient focus. However, in the ELT the tertiary mirror also has a curved surface, as the use of three mirrors delivers a better final image quality over a larger field of view than would be possible with a two-mirror design.
Much like the secondary mirror (with which it shares many design characteristics), the tertiary mirror will be slightly deformable to regularly allow deviations to be corrected. Both mirrors will be mounted on 32 points, with 18 on their backside and 14 along their edges.
As of July 2023, the tertiary mirror had been cast and was being polished.
Quaternary mirror