ESO switches on the largest ever optical telescope in Chile

13 Feb 2018

The Very Large Telescope during one of its rotations to observe the cosmos. Image: ESO/G.Hüdepohl (

As one of the newest members of ESO, Irish astronomers will gain access to the newly upgraded Very Large Telescope.

Our ability to search the cosmos for new scientific understanding just increased substantially with news that the European Southern Observatory’s Very Large Telescope (VLT) has achieved a decades-old goal for the organisation.

In a statement, ESO said that, for the first time, its four Unit Telescopes (UTs) were combined to create a giant telescope that achieved first light with an instrument called ESPRESSO.

In doing so, the VLT, fulfils a dream conceived by ESO back in the 1980s. Located in the Atacama desert in Chile, it is now the largest optical telescope, with a 16-metre aperture.

Two of the main scientific goals of ESPRESSO are the discovery and characterisation of Earth-like planets and the search for possible variability of the fundamental constants of physics.

That is why the combination of all four UTs will prove essential in order to observe the most distant and faint quasars as it relies on the ultra-high stability of the instrument and an extremely stable reference light source.

To make the observations, ESPRESSO receives light from each of the UTs at a distance of 69 metres away thanks to a complex system of mirrors, prisms and lenses.


Combining light from the Unit Telescopes in this way makes the VLT the largest optical telescope in existence, in terms of collecting area. Image: ESO/L Calçada

A ‘game-changer’

Until now, the only way to use all four telescopes was for the study of extremely fine detail in comparatively bright objects.

This was because interferometry, which combines the beams ‘coherently’, couldn’t exploit the huge light-gathering potential of the combined telescopes to study faint objects.

Described as a “game-changer” by ESO, the major observation boost will be a boon for the collection of high-resolution spectrographs.

“ESPRESSO working with all four UTs gives us an enticing foretaste of what the next generation of telescopes, such as ESO’s Extremely Large Telescope, will offer in a few years,” said ESO director general Xavier Barcons.

This latest development will give Irish astronomers access to unprecedented capabilities following the country’s decision to join the multinational organisation in October 2017.

Colm Gorey was a senior journalist with Silicon Republic