The age of extremely large telescopes is here.

We astronomers always want more: the most distant galaxy, the weakest star, the planet most similar to Earth… And for that we need more photons! In other words, bigger telescopes.

But it has taken 400 years of development to usher in a new era, that of extremely large telescopes. The ELT, a marvel at the height of the Sagrada Familia and the Eiffel Tower, and currently under construction in Chile, will be the first of these giant telescopes.

From Galileo to Herschel

Galileo Galilei did not invent the telescope, but almost. With one of his devices, a telescope with a 3.7 cm lens, he saw for the first time in 1610 the four “Galilean” satellites of Jupiter, the phases of Venus and the rings of Saturn.

Almost 200 years later, William Herschel – discoverer of Uranus, infrared radiation and much more – finished making a telescope for the king of Spain. This telescope had a primary mirror of about 60 cm, which is still preserved in the National Astronomical Observatory, next to the El Retiro park, in Madrid.

So important were the discoveries of these astronomers that, another 200 years later, two of the largest telescopes at the Roque de Los Muchachos Observatory, on La Palma, bear their name: the Telescopio Nazionale Galileo (TNG) and the William Herschel Telescope ( wht). The first is Italian and the diameter of its mirror is 3.56 m in diameter; the second is (Hispanic-)Dutch-British and its mirror measures 4.2 m.

From lenses to mirrors

In astronomy “size matters”. The larger the diameter of the lens or primary mirror, the larger the photon-collecting area and, therefore, the greater the ability to see very faint objects, such as very distant galaxies or tiny objects in the Solar System.

Since the area of ​​a circle is π (pi) multiplied by the radius squared (A = π * R2 ), Herschel’s telescope for the King of Spain collected about 300 times more photons than Galileo’s, and the TNG and WHT another 5,000 times more than Herschel’s.

In 2023, although the best observatories are in Chile and Hawaii, the largest optical-infrared telescope in the world is at Roque de Los Muchachos. The primary mirror of the Gran Telescopio Canarias (GTC) is not a large circular and monolithic lens, like those of the TNG or the WHT, but is a collection of 36 hexagonal glass-ceramic mirrors, each measuring 1.9 m between vertices. These mirrors operate together as if they were a single circular mirror 10.4 m in diameter. GTC has an aperture 3,000 times larger than Galileo’s telescope, and it collects 8 million more photons!

ESO’s Very Large Telescope (VLT) during nighttime observations. In this image, captured from the platform during twilight, you can see the four main telescopes of 8.2 m in diameter. From left to right, Antu, Kueyen, Melipal and Yepun, Mapuche names of the VLTs.
THOSE. Brunier

From large telescopes to very large telescopes

Size matters, but it’s not everything. A telescope can be like a large mobile phone but with 4G and a regular screen and camera, or a bit more compact but with 5G, super retina XDR display and 48MP Main Pro camera system. And more expensive, too.

In this respect, astronomy and telephony are identical, and the best non-space telescope instruments (with the best “displays and cameras”) are all in the four Very Large Telescope units at the Paranal Observatory in Chile. Interestingly, we also use a three-letter acronym to designate it: VLT. However, the names of the instruments are more suggestive: MUSE, ESPRESSO, X-Shooter, ERIS…

The VLT and its instrumentation belong to the European Southern Organization (ESO, for its acronym in English), and Spain is part of it. Therefore, Spanish astronomers have access to the ESO telescopes in Chile, in addition to all those in La Palma (TNG, WHT, GTC), those in Tenerife, those in Calar Alto in Almería… This is one of the reasons why which Spain is a great world power in astronomy.

file 20230831 29 ww6we2.jpg?ixlib=rb 1.1
This image, taken in early August 2023, shows the construction site of ESO’s Extremely Large Telescope (ELT). Located atop Cerro Armazones in Chile’s Atacama Desert, the telescope passed the 50% completion milestone in July 2023.
G. Hüdepohl (, CC BY

And the three giant telescopes in development

There are currently three major international projects for extremely large telescopes, some still on the table, others already underway. Their acronyms, of course, have three letters.

The most retarded giant of all, the TMT or Thirty Meter Telescope, is supposed to have 492 hexagonal mirrors of 1.4 m each. All together they will be equivalent to, as its name says, a 30 m mirror. The problem is that the Americans and their Japanese, Chinese, Indian and Canadian colleagues have been trying to find the ideal place to locate it for more than a decade. And, at this rate, they will never find it.

The GMT, Giant Magellan Telescope, which has begun construction at the Las Campanas Observatory in Chile, will soon consist of seven 8.4 m mirrors. Together they will have an equivalent diameter of 25.4 m. The Americans, with their Australian and Brazilian colleagues, plan to have their first light in 2029.

But the first extremely large telescope that will begin operating and that will also be the largest for many decades is the ELT, acronym for (note its imaginative name) Extremely Large Telescope. As you can imagine, the international community has its eyes on Chile and the construction of the prodigy (in which, by the way, Spanish companies participate).

The prodigious machine at the height of the Eiffel Tower

The dome structure that will house the ELT, with a mirror equivalent of 39.3 m, composed of almost 800 hexagonal segments of 1.4 m each, is being completed as I write these lines.

The complete dome will weigh about 6,100 tons, and the structure that supports the multimirror another 2,800 tons. To this must be added all the optical surfaces, the instruments (HARMONI, METIS, ANDES…) and, of course, the building with everything necessary to operate this prodigious machine: power generators, aluminization plants (to keep the mirrors perfectly reflective), kilometers and kilometers of pipes, cables and fiber optics… The ELT will be a monument at the height of the Eiffel Tower, the Statue of Liberty, and even the Sagrada Familia.

What can we see with the ELT? Beyond Orion, that’s for sure, and with more precision than Hubble now does. Let the show begin!

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