Extremely Large Telescope

A 39-metre mirror being assembled on a Chilean mountaintop from 798 hexagons — larger than every other optical telescope on Earth combined.

Under constructionCerro Armazones39 m

Project cost €1.5 billion (development cost)

The Extremely Large Telescope's dome under construction on Cerro Armazones at sunset, surrounded by cranes. ESO/G. Vecchia

01 — Interpretation

What these numbers mean

Aperture39 m

One hundred and ninety times the diameter of an 8-inch Dobsonian, and roughly 37,000 times its light-gathering area. Put differently: the ELT's mirror has more collecting area than every other 8-metre-class optical telescope in the world added together.

Segments798

Each is about 1.4 m across — a size an amateur mirror-maker would recognise — and the whole surface must be held to tens of nanometres across 39 metres. That is the same trick Keck proved at 36 segments, scaled up by a factor of twenty.

02 — Specifications

The instrument

Aperture: 39000
Optical design: Five-mirror anastigmat; 798 hexagonal primary segments
Mount: Altazimuth

03 — Mission

The observatory

Status: Under construction
Operator: European Southern Observatory
Site: Cerro Armazones, Atacama Desert, Chile
Altitude: 3046
Instruments: 798-segment primary; integrated adaptive optics (M4 deformable mirror)

04 — Scale

Against your telescope

Aperture is aperture. The same measurement that describes this instrument describes the telescope in your garage — which is the only honest way to feel the difference.

Extremely Large Telescope39000 mm

Sky-Watcher Flextube 350P SynScan GoTo Collapsible Dobsonian355 mm · 110× smaller

Sky-Watcher Flextube 300P SynScan GoTo Collapsible Dobsonian305 mm · 128× smaller

Celestron StarSense Explorer 12" Dobsonian305 mm · 128× smaller

05 — Context

What changes at thirty-nine metres

Aperture buys two things: light, and resolution. The ELT gets an unreasonable amount of both, and that changes the kind of question it can be pointed at rather than merely doing existing work faster.

The headline case is planets around other stars. Imaging a planet directly means separating it from a star that is millions of times brighter and, from here, almost exactly in the same direction. Resolution scales with aperture, so 39 metres separates what 10 metres cannot. Rocky planets in the habitable zones of nearby stars move from impossible to plausible.

The second case is faintness. Light that has travelled since the first galaxies formed arrives in quantities a smaller mirror cannot usefully collect — the photons are there, but too few per hour to measure. Collecting area is the entire answer, and the ELT has more of it than the rest of the world's large optical telescopes put together.

It is worth noticing that these are jobs a space telescope cannot simply take over. Webb's mirror is 6.5 metres because that is what fit in a rocket. Nothing on the ground has to fold.

06 — Context

Not finished, and what that means

The ELT does not yet work. It is under construction on Cerro Armazones, and this page will say so until it does.

ESO's published plan is about twelve years of construction with a technical first light targeted for 2029. That is a schedule rather than a fact, and schedules for instruments of this size have moved before, so it is recorded here as ESO's stated intention and nothing more. When the telescope achieves first light, that date will appear in the specifications above; until then the field is deliberately empty.

What exists today is substantial: the dome structure, the main telescope structure, and segment production running at a steady rate. The primary mirror's 798 hexagons are being cast, polished and delivered over years, because there is no other way to make a mirror of this size.

The stated cost is €1.45 billion. It is quoted here in euros, as ESO publishes it — converting it to dollars would mean choosing an exchange rate and a date, and inventing a precision that the figure does not have.

Sources

Where these numbers come from

Elsewhere

More great telescopes

Vera C. Rubin Observatory8360 mm

An 8.4-metre survey telescope that photographs the entire southern sky every few nights, for ten years, to find what changed.

W. M. Keck Observatory10000 mm

Two ten-metre telescopes whose mirrors are not mirrors at all, but thirty-six hexagons each, held in formation to a fraction of a wavelength.

Very Large Telescope8200 mm

Four 8.2-metre telescopes on a Chilean ridge that can observe separately, or combine their light into one instrument no single mirror could be.

Gran Telescopio Canarias10400 mm

The largest single optical aperture working anywhere on Earth — 10.4 metres of mirror on a volcanic ridge above the Atlantic.

Hale Telescope5100 mm

The 200-inch that held the record for forty-five years, still working on a California mountain, and still the telescope every larger one was measured against.

Arecibo Observatory305000 mm

A 305-metre dish built into a Puerto Rican sinkhole that could not point at anything — and, for fifty-seven years, heard more than anything else on Earth.

Atacama Large Millimeter/submillimeter Array

Sixty-six antennas on a plateau five kilometres above sea level, which together resolve detail as a single dish sixteen kilometres wide would.

All ground telescopes →