The Sky at Night – The Very Large Telescope

The Sky at Night - The Very Large Telescope

The Sky at Night – The Very Large Telescope – Nestled in the heart of Chile’s arid Atacama Desert, the Very Large Telescope (VLT) stands as a monumental testament to humanity’s quest for cosmic knowledge. This premier observatory has catalyzed paradigm-shifting astronomical discoveries, earning its place in the annals of science through contributions that have garnered Nobel recognition and profoundly reshaped cosmic comprehension. Situated within the Paranal Observatory in one of Earth’s most desiccated environments, the VLT benefits from unparalleled atmospheric conditions for stellar observation. The site’s minimal moisture significantly mitigates atmospheric distortion, granting an almost unadulterated view of the celestial expanse.



Comprising four individual telescopes, each anchored by an 8.2-metre mirror, the VLT possesses the unique capability to operate in isolation or synchronize for enhanced observational potency. This configuration harnesses an expansive light capture area, producing ground-based images with a clarity that rivals space-derived visuals. In this exploration, we gain insights from the dedicated professionals sustaining this exceptional facility’s operations. Maintenance, Support, and Engineering Head, Maxime Boccas, provides an exclusive overview of the biennial mirror maintenance protocol—a delicate process involving the vaporization of aluminium particles to ensure optimal reflectivity.



The VLT’s sophisticated array of instruments supports astronomers like Dr. Joe Anderson in dissecting the universe’s light, aiding in the elucidation of cosmic phenomena. As the leading visible-light astronomical observatory globally, the VLT’s diverse instrument suite facilitates extensive spectral readings, pivotal to groundbreaking discoveries concerning exoplanets, black holes, and gamma-ray bursts. Overseeing the logistical orchestration of this remote desert facility is Vanessa Peidro, responsible for the seamless operation of infrastructure essential to supporting the onsite staff, which numbers between 150-160 individuals at any given moment.



Our exploration continues with physicist Francoise Delplancke-Stroebele and her associate Frederic Gonte, spearheading the VLT’s forthcoming Gravity+ upgrade. They elucidate the principles of interferometry—a revolutionary technique enabling the VLT’s telescopes to operate in concert, pivotal in the recent identification of an elusive ‘vampire star.’ While the VLT continues to be a nexus for cutting-edge astronomical research, the horizon heralds an evolution into an even more formidable entity. The in-development Extremely Large Telescope (ELT) promises a transformative leap in observational capabilities. Upon completion, this behemoth, mirroring a cathedral’s grandeur atop a secluded mountain, will represent one of humanity’s most ambitious engineering endeavors, poised to redefine our understanding of the cosmos.


The Sky at Night – The Very Large Telescope



A Groundbreaking Observatory in the Driest Desert on Earth

In the heart of Chile’s Atacama Desert lies one of the most advanced observatories in the world – the Very Large Telescope (VLT). This astronomical powerhouse has been at the forefront of ground-based optical astronomy for 25 years. With its huge 8.2 meter mirrors and cutting-edge instruments, the VLT has been instrumental in some of the greatest astronomical discoveries of all time. These breakthroughs have led to Nobel Prizes and transformed our understanding of the universe.

The VLT is located at the Paranal Observatory, situated on the Paranal mountain at an elevation of 2,635 meters. This high-altitude site was chosen specifically because the Atacama Desert is one of the driest places on Earth, comparable only to the dryness of the Poles. The lack of moisture in the air results in steadier atmospheric conditions. This allows the VLT to achieve sharper images, free of distortions caused by water vapor. The “Very Large” in Very Large Telescope is certainly no exaggeration. This astronomical facility is comprised of an array of four main telescopes with 8.2 meter wide mirrors. Additional smaller telescopes can also be incorporated to further enhance observations. These gigantic mirrors are designed to collect as much light as possible from faint and distant cosmic objects. In fact, each 8.2 meter mirror surface area is equivalent to that of a tennis court!

The Very Large Telescope - 1
The Very Large Telescope – 1

Despite their mammoth size, these mirrors are engineered with utmost precision. They are a mere 17.5 cm thick, as any additional mass would cause the mirrors to collapse under their own weight. The telescopes utilize advanced adaptive optics to counteract atmospheric turbulence. This results in images almost as sharp as those taken from space. When operating together in synchronized array, the light gathering power is equivalent to an single aperture 16 meters across.


Charting the Skies for a Quarter Century

The Very Large Telescope array achieved “first light” in 1998, after over a decade of planning and construction. While not the first 8-10 meter class optical telescope, the VLT was pioneering for its ambitious four telescope design. This novel approach allowed the facility to undertake observations like never before.

Within its first year of operation, the fledgling VLT made headlines for obtaining the first image of a planetary system beyond our own Solar System. This initial discovery was made using just a single Unit Telescope, before the full array was even complete. It provided a tantalizing preview of the array’s immense research potential. Over the subsequent 25 years, the VLT has amassed an illustrious series of astronomical firsts. It enabled the first image of an exoplanet. It provided unprecedented views of distant galaxies in the early universe. The VLT has even delved into the mysteries surrounding the supermassive black hole at the Milky Way’s center.

The Paranal Observatory has since expanded beyond the original four Unit Telescopes. The additional four moveable 1.8 meter Auxiliary Telescopes paved the way for powerful optical interferometry between the telescopes. This method combines light from multiple telescopes to achieve the resolution of a much larger single telescope.

Meanwhile, a constant cycle of instrument upgrades ensures the VLT remains on the cutting-edge. State-of-the-art instruments outfit each Unit Telescope to capture different wavelengths of light. From infrared to visible to ultraviolet, the VLT provides full spectral coverage to reveal the secrets of cosmic phenomena.

The Sky at Night - VLT
The Sky at Night – VLT

Unparalleled Discoveries that Rewrote Astronomy

The VLT’s immense light gathering power, high resolution imaging, and versatile instrument suite have enabled discoveries that reshaped our understanding of the universe. In 2011, the VLT captured the first ever image of a supermassive black hole’s powerful jets. This rare glimpse into these elusive processes proved that black hole jets emit light by accelerating charged particles. These observations provided clues into the mechanics at play near these enigmatic objects.

The following year, the VLT Interferometer made the first direct measurement of an exoplanet’s orbit. By tracking the motion of the planet Beta Pictoris b, this technique demonstrated the viability of mapping exoplanet orbits optically. This was a pivotal step towards studying alien solar systems in greater detail.

One of the VLT’s most profound contributions was in the discovery of accelerated cosmic expansion. Distant supernovae observed with the VLT showed that the universe’s expansion is accelerating over time, contrary to expectations. This breakthrough research earned the 2011 Nobel Prize in Physics and opened up new physics frontiers.

In the hunt for extrasolar planets, the VLT has spotted some of the smallest and most Earth-like exoplanets found to date. Its precise radial velocity measurements enabled the discovery of a planet only twice the size of Earth orbiting the nearest star, Proxima Centauri. Finding potentially habitable worlds like these brings us closer to definitively answering whether we are alone in this galaxy.


Peering Deeper into the Cosmos than Ever Before

The VLT’s capabilities go far beyond mere imaging. Its array of cutting-edge scientific instruments provide astronomers with multiple windows into cosmic mysteries.

The FOcal Reducer and low dispersion Spectrograph (FORS) is a versatile instrument capable of photometric imaging and spectroscopy over visible wavelengths. It is particularly adept at spectroscopy of faint objects. This was the pioneering instrument used to obtain the VLT’s very first light image. For ultra-sharp infrared imaging, the Nasmyth Adaptive Optics System (NAOS) works in concert with a near-infrared camera called CONICA. Together, they provide infrared imaging sharper than could be achieved from space. NAOS-CONICA was one of the first adaptive optics systems to utilize a laser guide star for atmospheric corrections.

High resolution spectroscopic data across optical and infrared wavelengths is obtained using the VLT instrument CRIRES. This instrument was pivotal in precisely measuring carbon monoxide in distant galaxies to reveal galactic growth over cosmic history. CRIRES also enabled insights into exoplanet atmospheres through detailed spectroscopy.

For optical interferometry using the additional Auxiliary Telescopes, the AMBER instrument combines three telescopes together. It obtains extremely high spatial resolution observations in the near-infrared spectrum. AMBER chaotic orbits of the stars at the Galactic Center. This provided clues to understanding the presence of the central black hole.


Inside the VLT: Telescope Maintenance and Cutting-Edge Upgrades

At the heart of the VLT are its set of four 8.2 meter Unit Telescopes. These gigantic mirrors work tirelessly year-round to gather faint light from astronomical objects. But even world-class telescopes need occasional maintenance.

About every two years, the Unit Telescopes’ mirrors must be cleaned and recoated to restore peak performance. This laborious process requires shutting down telescope operations completely for almost two weeks. Each colossal 23 ton mirror is removed from the telescope structure and transported to Paranal’s mirror maintenance facility.

First, the existing aluminium coating is chemically stripped away to expose the bare glass surface. A rotating robotic arm equipped with jets then performs an initial cleansing of dust with soapy water. After drying, the newly stripped mirror is installed within a vacuum chamber for the recoating process.

Inside the vacuum chamber, aluminium is deposited onto the glass in a process known as sputtering. By bombarding a pure aluminium source with energetic ions, aluminium particles are ejected and uniformly coat the glass. The coating builds up just 200 nanometers thick, about 1/1000 the thickness of a human hair. Yet this microscopically thin layer is enough to create the mirror’s reflective surface.

Engineers must perform this elaborate cleaning and recoating ritual every couple years to revive each mirror’s optical performance. But that is just one aspect of the VLT’s continuous cycle of maintenance and upgrades.

Keeping the VLT at its technological peak involves upgrading its instruments, hardware, and software on a regular basis. One major initiative currently underway is the conversion to adaptive optics on all four Unit Telescopes. This includes adding state-of-the-art lasers and deformable mirrors that correct for atmospheric distortion in real-time. Sharper images allow astronomers to better study exoplanet properties and galactic characteristics.

The instrument GRAVITY is also undergoing an upgrade to “GRAVITY+.” This will increase its sensitivity using the newly enhanced adaptive optics systems on the telescopes. GRAVITY combines light from multiple Unit Telescopes to obtain extremely high resolution observations. Its upgrade will usher in a new era of interferometric capability.


Life at the VLT: The People behind the Science

The Paranal Observatory operates 24 hours per day, 365 days per year to maximize observing time on the cosmos. But what is it like for the astronomers, engineers, and support staff that keep this research mecca humming?

To maintain round-the-clock operations, staff live and work at the Paranal Residencia located a short distance from the telescopes. This on-site accommodation and office complex even features its own swimming pool and tennis court to prevent staff from getting “cabin fever” while off-duty.

A typical day shift involves astronomers calibrating instruments, preparing observing routines for the night, and analyzing data from previous observations. Night duties include controlling the telescopes, monitoring instrument performance, and conducting observations.

“I’ve been here ten years, and seeing that sunset every night is still an amazing experience,” says astronomer Joe Anderson. “The engineers get the telescopes ready, then hand them over to us for a night of observing galaxies, stars, black holes – all the wonders in the universe.”

The working day begins around noon with a daily handover meeting between the outgoing night crew and incoming day crew. Staff work alternating shifts to provide round-the-clock operational coverage. Night shifts feature small crews of astronomers, operators, and technical specialists. Larger teams handle daytime instrument maintenance, calibrations, and data processing.

Paranal staff enjoy an unusual work-life arrangement. Astronomers typically spend a third of their working year either at Paranal or at La Silla, ESO’s other Chilean observatory. The remaining two thirds are dedicated to research and academics at home institutions. This provides a balance between hands-on telescope work and dedicated research time.

The VLT’s geographical isolation presents staffing challenges. Recruiting scientists willing to routinely travel to and temporarily live at an astronomical observatory in the Atacama Desert is no simple task. But those who accept the challenge are rewarded with access to the southern hemisphere’s most powerful telescopes.


Cutting Through Cosmic Mysteries with Interferometry

The full power of the Very Large Telescope is unleashed when its four Unit Telescopes work in concert as an interferometer. This observing mode synthesizes the telescopes into a virtual single telescope up to 200 meters in diameter.

Interferometry works by combining light waves from multiple telescopes observing the same astronomical object. Light is directed through underground tunnels from each telescope to a central beam combining instrument. The light waves are then brought into precise alignment with extreme accuracy.

As physicist Françoise Delplancke-Stroebele explains, “Delay Lines” within the tunnels add adjustable delays to the light paths. This ensures light from all the telescopes arrives at the instrument in perfect unison. Correctly delaying and combining the light waves creates measurable interference patterns. Analyzing these patterns reveals details about the observed object’s characteristics and structure.

The VLT’s advanced GRAVITY instrument takes this concept to an extreme. GRAVITY coherently combines light from all four VLT Unit Telescopes plus four relocatable Auxiliary Telescopes. This provides imaging resolution equivalent to a single telescope over 200 meters across, with incredible sensitivity.

GRAVITY has already unlocked some of the Paranal Observatory’s most significant discoveries. One example is resolving the 40-year mystery of why the Milky Way’s supermassive black hole is so quiet. Interferometry showed that powerful magnetic fields cause gas clouds around the black hole to orbit orderly rather than fall inwards.

Another mystery solved with GRAVITY surrounded the closest known black hole to Earth. Discovered in 2020, GRAVITY observations proved the strange motion of the black hole’s companion star is caused by a binary star pair. The black hole itself is an inactive invisible partner, disproving theories that the black hole is actively feeding on the star’s gas.

But the upgrades don’t stop with GRAVITY. Engineers are now hard at work on GRAVITY+, implementing adaptive optics and enhanced wavefront control. This will boost interferometric sensitivity even further, providing astronomers with unprecedented galactic imaging power.


Powering Paranal: The Operational Engine Behind the VLT

Away from the glamour of the telescopes, a legion of support staff work tirelessly to keep Paranal humming. Teams of mechanics, electricians, system engineers, and administrators maintain the high-tech observatory’s extensive infrastructure.

The Executive Director of Paranal looks after both the telescopes’ operations and the needs of those who tend to them. Food, accommodation, transportation – it all falls under the responsibility of Elena Zuffanelli. “We have built a little city here. There is complex logistics between all the components,” says Zuffanelli. Many support roles mirror those you would find in a small town. Diesel mechanics service a fleet of buses, trucks,Generators provide a steady supply of electricity. Air conditioning technicians work to maintain indoor temperatures and humidity levels.

When you operate a mini city in the driest desert on the planet, water becomes a most precious resource. Paranal’s supply is trucked in from the Pacific Ocean, 70 km away. The observatory’s water usage is tightly optimized to reduce environmental impact. Even sewage is treated on-site and recycled into irrigation water.

Meanwhile, at the Residencia accommodation complex, logistics manager Vanessa Peidro looks after living essentials for those working at Paranal. Her team prepares over 4,000 meals per month to feed the observatory’s 150+ residents. They also coordinate recreation activities vital for physical and mental well-being.

Maintaining Paranal requires an array of support talents – plumbing, carpentry, electronics, and heavy equipment operation. Support staff may not operate the telescopes themselves, but their skills are equally crucial to Paranal’s success.


The Future: Construction of the Extremely Large Telescope

While the VLT remains a world-leading observatory, even greater astronomical potential lies on the horizon at Paranal. Less than an hour down the road, ESO is constructing what will become the largest optical/near-infrared telescope in the world – the Extremely Large Telescope.

With a mammoth 39 meter primary mirror, the ELT will collect 15 times more light than the VLT’s 8 meter mirrors can achieve individually. Its immense light grasp will allow astronomers to study fainter objects in the universe in greater detail than ever feasible before.

ELT Project Manager Roberto Tamai describes the ELT’s construction on Cerro Armazones as one of the most challenging technical endeavors in the world. “You don’t construct telescopes of this size very often. It’s as unique as landing someone on the moon”. The logistics of the €1.4 billion project are staggering. The mammoth telescope structure sits atop a concrete pier weighing over 8,500 tons and sunk deep into the mountain. Habitable space within totals over 5,000 square meters, about the size of a soccer field.

When complete in 2027, the ELT will usher in a new era of astronomical discovery. Its unparalleled power promises insights into exoplanets, the first stars born after the Big Bang, the nature of dark matter, and much more. Twenty-five years after the VLT’s first light, Paranal continues pushing observable frontiers.


Conclusion: The VLT’s Enduring Legacy in Astronomy

Over its 25+ years of operations, the Very Large Telescope has amassed one of the most illustrious track records in observational astronomy. Its array of advanced instruments cast light on cosmic mysteries in our own galaxy and at the furthest edges of space and time.

Discovering extrasolar planets, revealing the accelerating expansion of the universe, and imaging black hole jets are just a sample of the VLT’s prolific contributions. It remains at the forefront of astronomy by continually enhancing its capabilities through instrument upgrades like GRAVITY+.

Meanwhile, construction of the Extremely Large Telescope points to an even more exciting future. Its 39 meter aperture will collect unprecedented amounts of light to reveal finer details of faint, distant objects than any previous telescope.

The VLT’s success relies not just on advanced technology, but on the collaboration of top scientists, engineers, and support staff. Their collective expertise built and operates one of humanity’s greatest tools for studying the cosmos. The VLT will continue pushing boundaries of knowledge for years to come.


Frequently Asked Questions


What is the Very Large Telescope?

The Very Large Telescope (VLT) is an astronomical observatory operated by the European Southern Observatory (ESO) in Chile’s Atacama Desert. It consists of four main 8.2 meter telescopes and four smaller 1.8 meter auxiliary telescopes.

Where is the Very Large Telescope located?

The VLT is located on the 2,635 meter summit of Cerro Paranal in Chile’s Atacama Desert. This dry, high-altitude site provides excellent conditions for observing the night sky.

What are some key discoveries made by the VLT?

The VLT has contributed to diverse breakthroughs like directly imaging exoplanets, proving the universe’s accelerating expansion, and capturing the first image of powerful jets from a black hole’s environment. Its observations have provided insights into the nature of dark matter, galaxy formation, and more.

How does interferometry work on the VLT?

Interferometry combines light from multiple VLT telescopes to achieve images at higher resolution. Light waves are precisely aligned using tunnels and delay lines to create interference patterns. Analyzing these patterns reveals finer details about cosmic objects.

What capabilities will the Extremely Large Telescope have?

With a 39 meter primary mirror, the ELT will collect far more light than existing telescopes. It will enable more detailed spectroscopy of exoplanets and fainter objects at the edge of the observable universe. Its sensitivity will provide new insights into many cosmic mysteries.


Scroll to Top