Halfway Home: ESO’s Extremely Large Telescope Progresses in Construction Milestone

by Santiago Fernandez
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ESO's Extremely Large Telescope construction

A drone shot from June 2023 captures the impressive growth of ESO’s Extremely Large Telescope (ELT) at the Cerro Armazones site in the Atacama Desert, Chile. The rapidly evolving structure of the telescope dome – soon to take on the characteristic round shape – is being assembled by a dedicated team of engineers and construction workers. The stunning backdrop is accentuated by the Milky Way’s core, our resident galaxy, along with the Large and Small Magellanic clouds, dwarf galaxies in our galactic orbit. Image credit: ESO.

The construction of the European Southern Observatory’s (ESO’s) Extremely Large Telescope (ELT) has now surpassed the halfway point. Upon completion, this behemoth will stand as the largest telescope for visible and infrared light globally, made possible by its innovative five-mirror optical design. The project’s swift progress is impressive, considering the pandemic-induced challenges, with expectations that the latter half will be accomplished more rapidly. The telescope is scheduled to commence scientific operations in 2028.

Poised atop Cerro Armazones in the Atacama Desert, Chile, ESO’s ELT is a groundbreaking ground-based telescope. Its 39-meter main mirror will crown it as the world’s largest telescope for visible and infrared light, thus deemed the world’s largest eye on the sky. Construction progress is substantial, with the project now passing the 50% completion mark.

A drone image from late June 2023 provides an in-depth view of the ELT’s construction site. The rapid assembly of the telescope dome structure is evident, with the steel structure on the verge of forming the traditional round shape characteristic of telescope domes. The image provides a sense of scale through human figures at the base, indicating the dome’s sheer size. The shadow of Cerro Armazones can be seen projected onto the desert landscape behind the telescope. Image credit: ESO.

Mirrors and other components for the telescope are underway across Europe, showcasing impressive progress. The pioneering five-mirror design includes a massive main mirror (M1) comprising 798 hexagonal segments, with more than 70% of blanks and supports for these segments already manufactured. Other mirrors, M2 and M3, are cast and being polished. The progress on M4, an adaptive, flexible mirror adjusting its shape a thousand times per second to correct air turbulence-induced distortions, is particularly noteworthy. All six laser sources, crucial to the ELT’s adaptive optics system, have been produced and are under testing at ESO.

ESO’s ELT construction at Cerro Armazones, captured in June 2023, reveals the accelerating pace of dome structure assembly, poised to form the typical round shape. The base of the telescope structure is centrally visible in the image. Image credit: ESO.

All other systems required to finalize the ELT, including the control system and assembly and commissioning equipment, are also making significant strides in development or production. Four of the first scientific instruments the ELT will be equipped with are in their final design phase, with manufacturing about to commence. Much of the support infrastructure for the ELT is already in place at or near Cerro Armazones, including a technical building for storage and coating of different ELT mirrors and a renewable energy supplying photovoltaic plant that began operations last year.

The construction of ESO’s ELT was initiated nine years ago with a groundbreaking ceremony, and the top of Cerro Armazones was flattened in 2014 to accommodate the giant telescope.

Despite the complexities of the project and COVID-19 induced challenges, the final half of the ELT is expected to be completed faster than the first. With production processes now streamlined and in full swing, the remaining 50% of the ELT is expected to take a mere five years. However, the construction of such an advanced and colossal telescope like the ELT remains risk-laden until its completion and successful operation.

“The ELT is the largest of the next generation of ground-based optical and near-infrared telescopes and the most advanced in its construction,” comments ESO Director General Xavier Barcons. “Reaching 50% completion is a significant achievement considering the inherent challenges in large, complex projects. This milestone is a testament to the unwavering commitment of everyone at ESO, the support of ESO Member States, and the contributions from our partners in industry and instrument consortia. It is a proud moment to see the ELT reach this point.”

The ELT, set to start scientific observations in 2028, will address key astronomical questions, such as the existence of extraterrestrial life, the universality of physical laws, and the formation of the first stars and galaxies. This telescope will greatly expand our understanding of the Universe, prompting us to reassess our position in the cosmos.


The ELT’s completion percentage is calculated based on the ‘earned value’ metric, a project management tool used to evaluate a project’s progress by considering schedule and cost. As of now, the ELT is 50% through the project plan.

Frequently Asked Questions (FAQs) about ESO’s Extremely Large Telescope construction

What is the European Southern Observatory’s Extremely Large Telescope?

The European Southern Observatory’s Extremely Large Telescope (ESO’s ELT) is a ground-based telescope currently under construction. Upon completion, it will be the world’s largest telescope for visible and infrared light. It’s located at Cerro Armazones, in Chile’s Atacama Desert.

What is the current status of the ELT’s construction?

As of June 2023, the construction of the ELT has reached the halfway mark. Despite challenges posed by the COVID-19 pandemic, progress has been swift, and the completion of the second half of the project is expected to be significantly quicker.

What is the expected operational date of the ELT?

The ELT is scheduled to start scientific operations in 2028.

What makes the ELT unique in its design?

The ELT features a pioneering five-mirror optical design and will have a 39-meter main mirror, the largest of its kind. It will also incorporate an adaptive, flexible mirror that will adjust its shape a thousand times a second to correct for distortions caused by air turbulence.

What scientific questions will the ELT address?

The ELT will tackle key astronomical questions such as: Are we alone in the Universe? Are the laws of physics Universal? How did the first stars and galaxies form? Its observations will greatly expand our understanding of the Universe and our place in it.

Who is responsible for the construction of the ELT?

The construction of the ELT is a project by the European Southern Observatory (ESO), with support from member states and various industry and instrument consortia partners. The telescope mirrors and other components are being built by companies across Europe.

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