PARANAL, Chile:
The lasers are installed at the European Southern Observatory’s (ESO) Very Large Telescope (VLT) on the summit of Cerro Paranal. With the completion of the latest laser installation in late 2025, the VLT’s laser system has expanded to seven beams.
Their purpose is remarkably simple: to create artificial stars in the sky so that astronomers can see real stars and distant galaxies more clearly.
Creating a star where none exists
The artificial stars are produced roughly 90 kilometres above Earth, where a thin layer of sodium atoms exists in the upper atmosphere.
When powerful laser beams are directed towards this layer, their light excites the sodium atoms. The atoms then emit photons, producing a small, glowing point of light that appears to astronomers as an artificial star.
That artificial point becomes a reference source for the telescope.
By observing how the artificial star appears to shift or distort, astronomers can determine how Earth’s constantly changing atmosphere is affecting incoming starlight. The telescope can then compensate for those distortions.
The technique is known as adaptive optics, one of the key technologies used to improve the performance of modern ground-based observatories.
Why Earth's atmosphere blurs the universe
Earth’s atmosphere is essential for life, but for astronomers it can be a major source of interference.
Light travelling from distant stars and galaxies must pass through layers of moving gases before reaching a telescope. Temperature differences, winds and changes in atmospheric density can bend and distort the light along the way.
As a result, even a powerful telescope can produce images that are less sharp than its optical system would otherwise allow.
Space telescopes avoid much of this problem because they operate above the atmosphere. But building and operating extremely large telescopes on Earth can be considerably more practical for many astronomical observations.
Adaptive optics provides a way to combine the advantages of large ground-based telescopes with sophisticated real-time correction of atmospheric distortion.
A mirror that constantly changes shape
At the heart of the system is a deformable mirror.
Light entering the telescope is reflected from its main mirror and then directed towards a much smaller mirror. Unlike a conventional mirror, this component can change its shape extremely rapidly.
The system continuously measures atmospheric distortion and adjusts the mirror accordingly. By making tiny changes to the mirror’s surface, it can compensate for much of the turbulence affecting the incoming light.
But the system needs a reference point to determine exactly how the atmosphere is distorting the image.
That is where the artificial stars created by lasers become essential.
“If you don't have a bright enough star in the sky, you can create one,” ESO project scientist and astronomer Amelia Bayo explained, describing the sodium layer as a natural region where astronomers can effectively generate a reference star when required.
Seven lasers, greater precision
The VLT’s current laser system represents a significant technological advance over earlier generations.
Previous laser systems relied on toxic liquid dyes and required lengthy engineering work to maintain stable operation. Modern systems are designed to be considerably easier to operate, allowing astronomers to activate them at the push of a button and keep them running reliably throughout an observing session.
Each of the VLT’s lasers has an output of around 22 watts and produces a beam approximately 30 centimetres in diameter.
That is vastly more powerful than an ordinary laser pointer. The individual beams are designed specifically to interact with sodium atoms high in the atmosphere and generate the artificial reference stars needed for adaptive-optics systems.
Four giant telescopes working together
The VLT is not a single telescope but a system of four giant 8.2-metre Unit Telescopes.
They are known as Antu, Kueyen, Melipal and Yepun.
Following earlier upgrades, Yepun became capable of operating four laser beams simultaneously, allowing astronomers to measure atmospheric turbulence over a broader region of the sky.
In 2026, the four main VLT telescopes also gained the ability to work together in coordinated observations, further expanding the observatory’s capabilities.
When combined, the four enormous mirrors can operate in interferometric mode, effectively functioning as a much larger virtual telescope for certain observations. This allows astronomers to investigate exceptionally fine details in astronomical objects that would be difficult to resolve with a single telescope.
A powerful tool for modern astronomy
The VLT has already contributed to some of the most important astronomical observations of the modern era.
Its observations have played a role in the study and imaging of planets beyond our Solar System and in tracking individual stars orbiting the supermassive black hole at the centre of the Milky Way.
Such observations require extremely precise measurements. Even small amounts of atmospheric distortion can obscure the details astronomers are trying to capture.
The laser-guided adaptive-optics system therefore serves as an important technological bridge be
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