Extremely Large Telescope

The ELT will be the largest optical telescope ever built, with first light set for March 2029

ESO's Extremely Large Telescope (ELT) is a 39 m segmented telescope for optical and infrared light, from the visible to the mid-infrared, with a collecting area of 978 square meters. Its concave primary mirror, M1, is the largest optical primary ever made. The science goals are direct images of exoplanets and their atmospheres, the formation of stars and galaxies, and tests of dark matter and dark energy.

The European Southern Observatory (ESO), the intergovernmental organization that runs all its observatories in Chile, builds and will operate the ELT. ESO Council approved it in June 2012 and gave the construction green light in December 2014, at about EUR 1 billion for the first phase; with first-generation instruments the 2017 figure was EUR 1.15 billion. In December 2025 the Cost to Complete rose by almost EUR 90 million, which ESO absorbs into a very tight long-term budget. One report puts telescope and instruments together at about EUR 2.6 billion; the sources disagree on the total.

In March 2025 ESO moved the schedule: telescope first light in March 2029 and the first observations with scientific instruments in December 2030. ESO named contract delays, harsh weather, slower technology development and failures of manufacturing equipment as the causes, and the Director General confirmed the dates in December 2025. In July 2026 the project was more than 75% complete, and in mid-2026 the main structure turned on its vertical axis for the first time.

The telescope stands on Cerro Armazones in the Atacama Desert, at 3,046 m, about 20 km from ESO's Paranal Observatory. Chile and ESO signed a land agreement for 189 square kilometers around the mountain in October 2011, the land was donated in October 2013, and the summit was partly flattened in June 2014. See the Extremely Large Telescope listing, the largest optical telescopes and more observatories in Chile.

798 hexagonal segments, each held to about 2 nanometers, make the 39-meter primary mirror

The ELT folds its light through five mirrors. M1 is 798 hexagonal segments, each about 1.45 m across, about 5 cm thick and about 250 kg with its support, with 4 mm gaps between them. ESO ordered 931 segments: the extra 133 are a seventh sector of spares, so that two segments a day can come out for recoating and every segment is recoated every 18 months. Each segment is polished to surface errors no greater than about 8 nm, and all of them must stay aligned to about 2 nm to act as one surface. A further 18 spare blanks, for 949 in all, appear in one report; ESO counts 931.

SCHOTT casts the Zerodur glass-ceramic blanks in Mainz, the first in January 2018, for M1 and for M2, M3 and the M4 shells. Safran Reosc polishes, tests and integrates the M1 segments in a EUR 30 million, 5,000 square meter building in Poitiers, and said on 6 October 2026 that all 931 will be finished by the end of 2026. It polishes M2 to M5 at Saint-Pierre-du-Perray.

M2 is 4.25 m across and 3,533 kg, and M3 is 4.0 m and 3,258 kg, both Zerodur; each mirror with its cell weighs about 12 t. ESO's timeline page gives M3 as 3.75 m. SENER of Spain built the M2, M3 and M5 cells and the manipulator that handles M1 segment assemblies. M5, a flat elliptical tip-tilt mirror about 2.7 m by 2.2 m, is six brazed segments of silicon carbide from Mersen Boostec of France, polished by Safran Reosc. A separate commissioning M5 uses a SCHOTT blank polished by Glyndwr Innovations in the UK.

Supports, actuators and edge sensors: about 2,500 actuators and 9,000 sensors under M1

TNO of the Netherlands designed and prototyped the 27-point whiffletree that carries each segment, CESA of Spain built design and qualification units, and VDL ETG Projects made the series supports. Physik Instrumente (PI) of Germany supplied about 2,500 PACT position actuators, three per segment, with 2 nm accuracy over a 10 mm stroke; nine warping harness actuators per segment correct its shape. The FAMES consortium of Fogale (France) and Micro-Epsilon (Germany) made about 9,000 edge sensors, 4,500 pairs, that measure piston, gap and shear between neighbors. IDOM of Spain led the M1 Local Coherencer, the interferometer that phases the segments, with optics from UPC CD6 in Barcelona; it reached ESO in Munich for validation in May 2026. PROCON of Germany integrated the M1 local control cabinets, with heat exchangers from AAVID Thermalloy.

Coating: segments recoated on site

AGC of Belgium designed and built the coating plants for the M1 segments, and Fagerstrom Industrikonsult of Sweden the plant that washes and strips the old coating.

M4 reshapes its 2.4-meter surface up to 1,000 times a second with more than 5,000 actuators

The ELT builds adaptive optics into the telescope itself. M4 is a flat adaptive mirror 2.4 m across, made of six petals of glass-ceramic only 1.95 mm thick. More than 5,000 voice coil actuators reshape it up to 1,000 times a second, and its position is sensed 70,000 times a second. AdOptica, the Italian partnership of Microgate and A.D.S. International that also builds the deformable secondaries of the VLT, has the final design and manufacturing contract, with INAF as subcontractor. Safran Reosc thinned the 12 shells, six plus a spare set, from SCHOTT blanks that start 35 mm thick. The shells are ready for integration at AdOptica, which takes about a year and a half; the unit is then calibrated in a European test tower and shipped to Chile, with completion planned for 2028.

Laser guide stars give the wavefront sensors a reference anywhere in the sky. TOPTICA Photonics of Germany supplied the laser sources and TNO the laser projection subunits. Thales Alenia Space built the adaptive optics wavefront sensing cameras. MORFEO, formerly MAORY, the multi-conjugate adaptive optics relay led by INAF, goes to its Final Design Review in 2026. The maker of the ELT real-time adaptive optics computer hardware is not publicly named.

An Italian consortium builds the 80-meter dome and the 3,500-metric-ton structure, ESO's largest contract

The dome is about 80 m high and about 93 m across, a rotating mass of about 6,100 t on 36 trolleys, with slit doors that open 41 m wide. The main structure weighs about 3,500 t today and will pass 4,600 t once the mirrors and instruments are in. Both come from ESO's largest contract, about EUR 400 million, signed in May 2016 with the ACe Consortium of Astaldi and Cimolai, with EIE Group as nominated subcontractor. ESO's contract list now names the members as Cimolai and Webuild, which absorbed Astaldi.

The dome reached its full height with its doors installed in April 2025, the doors first moved later that year, and ESO held the topping-out ceremony in 2025. By July 2026 most of the cladding was on.

ICAFAL of Chile built the road and the summit platform between 2013 and 2017. Abengoa of Spain built the ELT Technical Facility at Paranal in 2018 and 2019. Siemens supplied the 23 kV and 0.4 kV power substations, SAESA of Chile holds the electrical supply contract to November 2049, and Hexagon of Wetzlar supplied the laser trackers used for alignment.

MICADO and METIS will be the first instruments, with Teledyne detectors inside most of them

The first-light instruments are MICADO, an imaging camera led by the Max Planck Institute for Extraterrestrial Physics (MPE); METIS, a mid-infrared imager and spectrograph led by NOVA in the Netherlands; HARMONI, an integral field spectrograph led by STFC in the UK; and MORFEO. In December 2025 MICADO and METIS were in manufacturing, assembly, integration and test, and they will be installed first. HARMONI was rescoped jointly by its consortium and ESO: it now covers 0.75 to 2.4 microns at 12 milliarcseconds, at resolving powers of about 3,000 and 7,000, under new leadership. The second generation, MOSAIC, whose agreement was signed at the end of 2025, and ANDES, is in Phase B.

Teledyne supplied the infrared detectors for HARMONI, MICADO and METIS, CCD220 detectors for MORFEO, MICADO, HARMONI and the PFS-A and PDS stations, and MUSE-type detectors. First Light Imaging of France supplied C-RED cameras for MORFEO. IDOM built the main system of the PFS-A prefocal station, and Bertin of France the powered optics of the phasing and diagnostic station (PDS). AS Scientific Products of the UK supplies the liquid nitrogen infrastructure.

ESO tests machine learning for adaptive optics control on an NVIDIA GPU testbench

Cosylab built the core integration infrastructure for the control software. iTTi and N7 Space of Poland provide central control software engineering through 2027, and Critical Software of Portugal ran independent verification and validation. FEV etamax of Germany supplies dataflow software components under a contract that runs to November 2026.

ESO's GHOST testbench, a real-time computer on NVIDIA GPUs built on the COSMIC platform, tests machine learning for predictive adaptive optics control and contrast control, work that supports the planned ELT Planetary Camera and Spectrograph. The makers of the ELT archive and data center hardware are not publicly named.

Cerro Armazones is closed to the public; Paranal next door runs free Saturday tours

The ELT site is not open for public visits. Access needs special authorization from the ELT Site Manager, a helmet, a vest, safety shoes and a signed waiver.

ESO's Paranal Observatory, home of the Very Large Telescope about 20 km away, offers free guided tours every Saturday at 10:00 and 14:00 from the Paranal Visitor Centre, booked only through ESO's online form. The tour does not include the ELT.

Sources

1. ESO: the ELT primary mirror (M1)
2. Wikipedia: Extremely Large Telescope
3. Space.com: the heart of a giant telescope
4. ESO: travel to the ELT site
5. ESO: ELT timeline
6. ESO: ELT industrial contracts
7. ESO: ELT status update from the Director General, December 2025
8. ESO: largest ELT contract signed for the dome and structure
9. UPC: CD6 completes the M1 Local Coherencer optics
10. ESO: the ELT secondary and tertiary mirrors
11. ESO: the ELT adaptive mirror (M4)
12. ESO: the ELT tip-tilt mirror (M5)
13. ESO: the ELT dome
14. Space.com: the ELT structure rotates for the first time
15. ESO: ELT construction progress, July 2026
16. ESO: new ELT schedule
17. ESO: Annual Report 2025
18. Safran: industrial success at ELT scale
19. Ideal Investisseur: Safran completes the 931 ELT segments
20. ESO: M4 contract announcement, June 2026
21. Astrobiology: HARMONI design and science capabilities
22. arXiv: the GHOST testbench for machine-learning adaptive optics
23. ESO: visiting Paranal
24. Pulse: the ELT nears first light
25. Inspenet: the ELT mirror segments