The Very Large Telescope (VLT), a premier astronomical facility operated by the European Southern Observatory (ESO), has been a cornerstone of modern astronomy since its establishment in 1998. Perched atop Cerro Paranal in the Atacama Desert of northern Chile, the VLT is renowned for its cutting-edge technology and contributions to the exploration of the cosmos.
The facility features four individual optical telescopes, each equipped with a primary mirror spanning 8.2 meters in diameter. These telescopes are named Antu, Kueyen, Melipal, and Yepun, borrowing terms for astronomical objects from the indigenous Mapuche language. While the telescopes typically operate independently, they can be combined using interferometry to achieve exceptionally high angular resolution, enabling detailed observations of celestial phenomena.
In addition to the main telescopes, the VLT system includes four movable Auxiliary Telescopes (ATs), each with 1.8-meter apertures. These ATs further enhance the VLT’s capabilities, particularly in interferometric mode, by enabling precise measurements that push the boundaries of astronomical research. The VLT’s advanced design and strategic location in one of the driest regions on Earth ensure clear skies and minimal atmospheric interference, making it one of the most powerful observatories in the world.
The Very Large Telescope (VLT) is a highly versatile astronomical instrument, capable of observing both visible and infrared wavelengths. This dual capability allows astronomers to study a wide range of celestial phenomena, from the detailed structure of distant galaxies to the faintest objects in the cosmos.
Each of the VLT’s four individual telescopes can detect objects approximately four billion times fainter than what the human eye can perceive. This extraordinary sensitivity enables the study of some of the universe’s most elusive and dimmest features.
In addition to its sensitivity, the VLT excels in angular resolution, which determines its ability to distinguish between closely spaced objects. In single telescope mode, the VLT achieves an angular resolution of approximately 0.05 arcseconds, allowing for highly detailed observations. When all four telescopes are combined using interferometry, the array reaches an angular resolution as fine as 0.002 arcseconds—a level of detail equivalent to resolving a car on the Moon.
The Very Large Telescope (VLT) stands out as one of the most scientifically productive astronomical facilities in the world. In terms of the number of scientific papers generated from observations in visible wavelengths, it ranks second only to the Hubble Space Telescope. This underscores the VLT’s crucial role in advancing our understanding of the universe.
Over the years, the VLT has been at the forefront of groundbreaking discoveries. Among its most notable achievements are:
- The First Direct Image of an Exoplanet: The VLT was instrumental in capturing the first-ever direct image of a planet outside our solar system, a milestone in exoplanet research.
- Tracking Stars Around the Milky Way’s Supermassive Black Hole: Using its exceptional angular resolution, the VLT provided unprecedented insights into the motion of stars orbiting Sagittarius A*, the supermassive black hole at the center of our galaxy. These observations were critical in confirming the black hole’s existence and mass.
- Observations of Gamma-Ray Burst Afterglows: The VLT has contributed to the study of some of the universe’s most energetic events, including capturing the afterglow of the furthest-known gamma-ray burst, shedding light on the early universe and extreme cosmic phenomena.
These pioneering contributions highlight the VLT’s unparalleled capabilities and its vital role in addressing some of astronomy’s most profound questions.
General information
The Very Large Telescope (VLT) is a sophisticated astronomical system comprising four 8.2-meter Unit Telescopes (UTs), which can operate independently or combine their power through an astronomical interferometer (VLTI). This interferometric setup allows astronomers to resolve extremely small celestial objects with remarkable precision, pushing the boundaries of observational astronomy.
In addition to the UTs, the VLT’s interferometric capabilities are supported by four 1.8-meter Auxiliary Telescopes (ATs). These movable telescopes are specifically dedicated to interferometric observations, ensuring that the VLTI remains operational even when the UTs are engaged in other research projects.
The VLT’s development was a phased process:
- The first UT became operational in May 1998 and was made available to the astronomical community on April 1, 1999.
- The remaining UTs were progressively commissioned in 1999 and 2000, marking the beginning of the multi-telescope VLT capability.
- The ATs were installed between 2004 and 2007, enhancing the functionality of the VLTI by providing consistent support for interferometry.
This combination of large and auxiliary telescopes, along with the integration of cutting-edge interferometric techniques, makes the VLT one of the most versatile and powerful tools in modern astronomy. It enables groundbreaking research across a wide range of astronomical phenomena.
The Very Large Telescope (VLT) was designed with remarkable versatility, enabling its 8.2-meter Unit Telescopes (UTs) to operate in three distinct modes:
- Independent Telescope Mode:
The primary mode of operation, where each of the four UTs functions independently, allows simultaneous observations of different celestial objects. This setup maximizes the telescope array’s productivity and flexibility. - Interferometric Mode (VLTI):
In this mode, the UTs are combined into a single, coherent interferometric instrument known as the VLT Interferometer (VLTI). This configuration provides extremely high angular resolution, making it ideal for observing bright sources with small angular sizes, such as the detailed structures of stars or the motion of objects around black holes. - Incoherent Mode:
This mode was envisioned to combine the light-gathering capacity of all four UTs into a single large instrument, increasing the total light captured. However, the necessary instrumentation to achieve a combined incoherent focus was not initially implemented. In 2009, proposals for new instrumentation were introduced to explore the feasibility of enabling this observing mode.
Additionally, the telescopes are sometimes pointed at the same object independently. This approach either enhances the total light-gathering power or provides simultaneous observations using complementary instruments, allowing researchers to collect a broader range of data on a single target.
These operating modes highlight the VLT’s adaptability and its ability to address a wide array of astronomical challenges, from high-resolution imaging to simultaneous multi-instrument studies of the universe.
Unit telescopes
The Unit Telescopes (UTs) of the Very Large Telescope (VLT) are equipped with a comprehensive array of instruments, enabling observations across a wide range of wavelengths, from the near-ultraviolet to the mid-infrared. This extensive coverage includes a significant fraction of the light wavelengths that can be observed from Earth’s surface. The VLT’s instrumentation supports a diverse set of observational techniques, such as:
- High-resolution spectroscopy: Capturing detailed spectra of celestial objects to study their composition, motion, and physical properties.
- Multi-object spectroscopy: Simultaneously observing multiple objects in the same field of view to improve efficiency in large-scale surveys.
- Imaging: Producing detailed images of astronomical phenomena across various wavelengths.
- High-resolution imaging: Achieving exceptionally sharp images, particularly with the help of advanced adaptive optics systems.
The adaptive optics systems of the VLT are a standout feature. They compensate for atmospheric turbulence, delivering images that are nearly as sharp as those taken by telescopes in space. In the near-infrared, the VLT’s adaptive optics provide images up to three times sharper than those of the Hubble Space Telescope, a remarkable achievement. Furthermore, the VLT’s spectroscopic resolution surpasses that of Hubble, enabling highly detailed analyses of astronomical objects.
The VLT is also recognized for its high observing efficiency and automation, which streamline its operations and maximize its output. These advanced capabilities, combined with its adaptability, make the VLT one of the most powerful ground-based observatories in the world, offering unparalleled insights into the universe.
The UTs’ main mirrors have a diameter of 8.2 meters, but in reality, the telescopes’ secondary mirrors define their pupil, so lowering the useable diameter to 8.0 meters at the Nasmyth focus and 8.1 meters at the Cassegrain focus.
Compact, thermally regulated structures that spin in unison with the 8.2 m-diameter telescopes hold them. Any negative impacts on the observing circumstances, such as air turbulence in the telescope tube, which may ordinarily arise from changes in temperature and wind flow, are minimized by this design.
The primary function of the Very Large Telescope (VLT) is to operate its four Unit Telescopes (UTs) as independent instruments. These telescopes carry out a majority of the VLT’s observations, focusing on a wide range of astronomical targets. However, about 20% of the observation time is dedicated to interferometry, where light from multiple telescopes is combined to achieve very high-resolution imaging of bright objects.
One notable example of this capability is the observation of Betelgeuse, where the interferometric mode allows astronomers to discern fine details with exceptional clarity. Using this method, the VLT Interferometer (VLTI) can achieve resolution up to 25 times finer than what is possible with the individual telescopes alone.
The interferometric process requires extraordinary precision. The light beams from each telescope are combined using a sophisticated system of mirrors housed in underground tunnels. These systems ensure that the light paths are equalized with a tolerance of less than 1 micrometer (1 μm) across distances exceeding 100 meters. This meticulous alignment is critical to maintain coherence and achieve the desired resolution.
Thanks to this precision, the VLTI can reconstruct images with angular resolutions measured in milliarcseconds, enabling astronomers to study fine details of stars, planetary systems, and other celestial phenomena that would otherwise be impossible to resolve. This capability highlights the VLT’s cutting-edge contribution to advancing our understanding of the universe.
Mapuche names for the Unit Telescopes
ESO had long planned to give the four VLT Unit Telescopes “real” names in place of the initial technical designations of UT1 through UT4. Four significant Mapuche names for celestial objects were selected during the Paranal opening in March 1999. The majority of this indigenous group resides south of Santiago, Chile.
In this regard, pupils from the Chilean II Region, whose capital is Antofagasta, were invited to participate in an essay competition where they were asked to write on the meanings behind these names. It attracted a lot of entries about the host nation’s cultural heritage.
17-year-old Jorssy Albanez Castilla from Chuquicamata, near Calama, submitted the winning essay. She received the prize, an amateur telescope, during the inauguration of the Paranal site.
Since then, Antu (Sun), Kueyen (Moon), Melipal (Southern Cross), and Yepun (Evening Star) have been the names of Unit Telescopes 1–4. Because Yepun was incorrectly rendered as “Sirius” in a Spanish-Mapuche dictionary from the 1940s, there was initially considerable misunderstanding over whether it truly stood for the evening star Venus.
Auxiliary telescopes
The four 8.2-meter Unit Telescopes may be joined to form the VLTI, however they mostly observe separately, with just a few nights per year being utilized for interferometric observations. To enable the VLTI to function nightly, four smaller 1.8-meter ATs are available and devoted to interferometry.
Each AT has a spherical enclosure at the top that opens and closes. It is composed of two sets of three segments. Its function is to shield the fragile 1.8-meter telescope from the harsh desert environment. The boxy transporter part supports the enclosure and includes power supply, air conditioners, liquid cooling systems, electronics cabinets, and more. The transporter and enclosure are mechanically separated from the telescope during astronomical observations to prevent vibrations from compromising the data being recorded.
The ATs may be transported to 30 distinct viewing places because to the transporter section’s track-based operation. The VLTI may be adapted to the requirements of the observing project by repositioning the ATs since it functions more like a single telescope than the collection of telescopes together. The VLTI and the Very Large Array both include reconfigurable features.
Scientific results
As a result of the VLT, more than one peer-reviewed scientific paper is published per day on average. For example, more than 600 peer-reviewed scientific publications based on VLT data were published in 2017. Observing the afterglow of the farthest known gamma-ray burst, tracing individual stars moving around the supermassive black hole at the center of the Milky Way, and directly picturing Beta Pictoris b—the first extrasolar planet thus imaged—are just a few of the scientific discoveries made by the telescope.
The first successful test of Albert Einstein’s General Relativity on the velocity of a star traveling through the gravitational redshift, or severe gravitational field, close to the supermassive black hole, was carried out in 2018 with the assistance of the VLT.
Actually, the VLT’s SINFONI and NACO adaptive optics instruments have been employed for the observation for more than 26 years, and the 2018 method additionally made use of the beam-combiner device GRAVITY. These data were utilized to initially uncover these effects by the Max Planck Institute for Extraterrestrial Physics (MPE) Galactic Centre team.
The first identification of carbon monoxide molecules in a galaxy about 11 billion light-years distant, which had been elusive for 25 years, is one of the other discoveries that bear the fingerprints of VLT. This has made it possible for astronomers to determine the cosmic temperature at such a distant period with the highest degree of precision.
The intense flares from the supermassive black hole at the Milky Way’s center were the subject of another significant investigation. Material is stretched out as it circles in the strong gravity around the core black hole, according to the VLT and APEX collaboration.
The age of exceptionally ancient stars in the NGC 6397 cluster has also been measured by astronomers using the VLT. Two stars from the earliest period of star formation in the Universe were determined to be 13.4 ± 0.8 billion years old based on stellar evolution models.
Additionally, they used the VLT for the first time to analyze the atmosphere surrounding a super-Earth exoplanet. As the planet, known as GJ 1214b, moved in front of its parent star and some of the starlight entered the planet’s atmosphere, it was observed.
The VLT was used in seven of the top ten discoveries made at ESO’s facilities.
Technical details
Telescopes
Each Unit Telescope (UT) of the Very Large Telescope (VLT) is a Ritchey-Chrétien Cassegrain telescope, a design optimized for minimizing optical distortions. The primary mirror, made of Zerodur, measures 8.2 meters in diameter and weighs 22 tonnes. It has a focal length of 14.4 meters. The secondary mirror, crafted from lightweight beryllium, has a diameter of 1.1 meters and contributes to the telescope’s precision and stability.
The telescope includes a flat tertiary mirror that directs the light to one of two instruments positioned at the f/15 Nasmyth foci on either side, offering a system focal length of 120 meters. Alternatively, the tertiary mirror can tilt aside, allowing the light to pass through the central hole in the primary mirror to reach a third instrument at the Cassegrain focus. This innovative design allows astronomers to switch between any of the three instruments in under 5 minutes, optimizing the telescope’s efficiency to match varying observing conditions.
For interferometric observations, additional mirrors can channel light through underground tunnels to the central VLTI beam combiners, enabling high-resolution studies.
The telescope’s maximum field-of-view at the Nasmyth foci is approximately 27 arcminutes in diameter, slightly smaller than the apparent size of the full moon. However, most of the instruments are designed to observe narrower fields, focusing on specific regions of interest with extraordinary detail.
This advanced optical design and flexibility make the UTs versatile and powerful tools for exploring the universe across a wide range of wavelengths and observation modes.
Using active optics with 150 supports on the rear of the primary mirror, each telescope has an alt-azimuth mount that weighs around 350 tons in total. Computers are used to regulate the curvature of the thin (177mm thick) mirror.
Instruments
The most ambitious program ever designed for a single observatory is the VLT instrumentation program. It spans a wide spectral range, from deep ultraviolet (300 nm) to mid-infrared (24 μm) wavelengths, and comprises large-field imagers, adaptive optics adjusted cameras and spectrographs, as well as high-resolution and multi-object spectrographs.
| UT# | Telescope Name | Cassegrain-Focus | Nasmyth-Focus A | Nasmyth-Focus B |
|---|---|---|---|---|
| 1 | Antu | FORS2 | KMOS | |
| 2 | Kueyen | VISIR | FLAMES | UVES |
| 3 | Melipal | XSHOOTER | SPHERE | CRIRES |
| 4 | Yepun | ERIS | HAWK-I | MUSE |
In addition to these, GRAVITY and MATISSE are currently installed in the VLTI lab, along with ESPRESSO fed via fibre-optics (not interferometric).
newer HAWK-I or KMOS.
