FAQ VLT/Paranal

  1. How sharp is the vision of a VLT Unit Telescope with adaptive optics?
  2. How sharp is the vision of the VLTI?
  3. How smooth are the VLT mirrors?
  4. How large, and how heavy, are the VLT primary mirrors?
  5. How heavy is a VLT Unit Telescope?
  6. Since Paranal is such a remote and dry site, how are the essentials provided?
  7. Why do we need ground-based telescopes when we have telescopes in space, free from the effects of the Earth's atmosphere?
  8. Is access to the VLT Unit Telescopes allowed at night?
  9. What is the percentage of observations performed in visitor mode versus service mode, and what are the main advantages and disadvantages offered by each mode?
  10. How many telescopes can be combined at the VLTI?
  11. A spectacular red laser beam is pictured in many photos of Paranal Observatory. How often is this laser guide star used?
  12. How many astronomers are typically on site at Paranal?
  13. How big are the buildings which house the VLT´s telescopes?
  14. Do earthquakes represent a threat for the VLT?
  15. What are the VLT´s primary mirrors made of?
  16. How do you keep the VLT mirrors clean?
  17. How much aluminium is needed to cover the surface of the VLT´s mirrors?
  18. Could the VLT take a picture of the Moon-landing sites?
  19. Why is the 3.58-metre New Technology Telescope (NTT) at La Silla called the New Technology Telescope?
  20. What kind of planets can the spectrograph HARPS see?
  21. How does the VLT Interferometer (VLTI) work?
  22. What is the maximum resolution of a telescope array, or interferometer?
  23. How many people work for the VLT on Paranal?
  24. What is its annual budget for the VLT (excluding personnel)?
  25. How much did it cost to build the VLT?
  26. How is energy produced at ESO’s Observatories?
  27. Why is solar energy not used at ESO’s observatories?

Q: How sharp is the vision of a VLT Unit Telescope with adaptive optics?

A: By removing the blurring effects of the Earth’s atmosphere, the VLT´s adaptive optics system achieves an angular resolution of about 50 milliarcseconds, which means that it can distinguish details smaller than the size of a DVD on the International Space Station, as seen from the ground.

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Q: How sharp is the vision of the VLTI?

A: When two or more telescopes are combined in interferometric mode, the spatial resolution is determined by the maximum distance between them. The VLTI, operating with two 8.2-metre Unit Telescopes, reaches a spatial resolution equivalent to a single 130-metre giant telescope, which is about 2 milliarcseconds. This is equivalent to distinguishing two points separated by the size of a sesame seed on the International Space Station as seen from the ground.

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Q: How smooth are the VLT mirrors?

A: An optical surface needs to be very accurately polished so that its imperfections are smaller than the wavelength of the collected light. Otherwise, the image will be degraded. In the case of the VLT, if we scale the diameter of the 8.2-metre primary mirrors to the size of the Earth, the largest imperfection on them would still be no larger than a pebble.

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Q: How large, and how heavy, are the VLT primary mirrors?

A: The VLT mirrors are 8.2 metres in diameter, but only 17.5 cm thick – very thin relative to their size. If you scaled a mirror down to the size of a CD, its thickness would be equivalent to just two discs placed on top of each other. Despite being very thin, the large diameter means the glass weighs 23 tonnes.

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Q: How heavy is a VLT Unit Telescope?

A: The movable structure of each VLT Unit Telescopes is the azimuth platform and it weighs 430 tonnes, which is about the same as a fully loaded jumbo jet. However, it is so perfectly balanced and resting on hydrostatic oil-film bearings, that the giant telescopes can be moved by hand.

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Q: Since Paranal is such a remote and dry site, how are the essentials provided?

A: Paranal is a very arid and isolated place, where no trace of water can be found, and the closest town is about 100 km away. For this reason, everything needed must be brought in specially. The 60 000 litres of water that are used each day are delivered by truck from Antofagasta, the capital of the region, located some 120 km north.

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Q: Why do we need ground-based telescopes when we have telescopes in space, free from the effects of the Earth's atmosphere?

A: On the ground, we can build larger telescopes than those we can economically put in orbit. Techniques such as adaptive optics help us remove the blurring effects of the Earth's atmosphere. The adaptive optics system detects the distortion introduced by the atmosphere in real time and uses this information to act on a deformable mirror hundreds of times per second in order to compensate for the atmosphere. The final result is a corrected image almost as sharp as those we get from telescopes in space.

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Q: Is access to the VLT Unit Telescopes allowed at night?

A: During observations, not even the astronomers and operator are present in the telescope building. For safety reasons and in order to avoid light and heat contamination, access to the telescopes is restricted to authorised personnel, and only when absolutely needed. Once the telescope is ready to start observations, the engineer hands it over to the astronomer and it is operated remotely from a control room in a separate building.

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Q: What is the percentage of observations performed in visitor mode versus service mode, and what are the main advantages and disadvantages offered by each mode?

A: At present, most observations are done in service mode, at 60-70% of the total time, versus 30-40% for the visitor mode. Although the VLT is exclusively operated by highly specialised ESO staff, the visitor mode offers astronomers the opportunity for direct interaction, which is particularly useful whenever any real-time decision is needed; nevertheless, any time lost for meteorological reasons cannot be reassigned. On the other hand, the service mode guarantees full flexibility to reschedule observations to match them with the most suitable atmospheric conditions.

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Q: How many telescopes can be combined at the VLTI?

A: The instruments currently available at VLTI can combine a maximum of three Unit Telescopes or three Auxiliary Telescopes at the same time. The Auxiliary Telescopes have 100% of their observing time with the VLTI, while the Unit Telescopes are usually busy with an enormous variety of observations, so they typically do not spend more than 20% of their time on VLTI. The Unit Telescopes are typically used in interferometric mode for fainter sources, when a larger collecting area is needed.

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Q: A spectacular red laser beam is pictured in many photos of Paranal Observatory. How often is this laser guide star used?

A: The laser guide star is part of the VLT´s adaptive optics system and is launched from Yepun (Unit Telescope 4). It is used for adaptive optics when no good reference stars are found in the field of view. Yepun typically observes with the laser guide star for about 25% of the time.

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Q: How many astronomers are typically on site at Paranal?

A: There are typically about ten astronomers at the site on the same shift. This represents less than 10% of the total "population" of the observatory, which, besides the technical staff, also includes logistics, administration, and general service personnel. Even among the technical staff, astronomers are not the most numerous; four times as many engineers and technicians are normally on site. This highly specialised team is in charge of optimising the performance of the machines and preventing time loss due to technical problems.

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Q: How big are the buildings which house the VLT´s telescopes?

A: The enclosures of the VLT Unit Telescopes are 25 metres high, about the same as an eight-floor building. Their very compact design makes them small in relation to the size of the telescope. The pathfinder project for many VLT technologies was the 3.5-metre NTT, which has been in operation at La Silla Observatory since 1989. The compact size of the NTT building can easily be compared with the much larger one of the ESO 3.6-metre Telescope, inaugurated at the same observatory in 1976. If the VLT Unit Telescopes were built to the design of the ESO 3.6-metre building, a dome 68 metres in diameter and a building almost 100 metres high would be needed! This will be roughly the size of the E-ELT enclosure.

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Q: Do earthquakes represent a threat for the VLT?

A: The VLT is designed with anti-seismic technology, specially chosen to protect the most delicate components of the telescopes. If there is a strong earthquake (more than magnitude 7) a special system is automatically activated, anchoring the primary mirror to the cell. This prevents it from falling, breaking, or causing damage to the structure. During its lifetime, Paranal Observatory has experienced frequent minor seismic events and a few significant earthquakes, and the anti-seismic technology has passed each test with flying colours.

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Q: What are the VLT´s primary mirrors made of?

A: The VLT primary mirrors are made of a special glass-ceramic with almost no thermal expansion, called Zerodur, which has an opaque-beige appearance. The reflective surface is a layer of aluminium only 80 nanometres thick. If we were to scale the mirror to the diameter of the Earth, this aluminium layer would still be only 12 cm thick.

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Q: How do you keep the VLT mirrors clean?

A: As the telescopes are open to the sky during the whole night, the mirrors accumulate dust on their surface, losing reflectivity. They therefore need to be cleaned from time to time. The cleaning is a very delicate operation that takes place every 18 months and involves removing the aluminium by chemical washing and then recoating the mirror.

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Q: How much aluminium is needed to cover the surface of the VLT´s mirrors?

A: The amount of aluminium needed to coat a single VLT primary mirror is about 12 grams, which is less than the amount in a soft drink can. Although the layer covers the whole of the mirror's 8.2-metre diameter, it is spread only 80 nanometres thick.

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Q: Could the VLT take a picture of the Moon-landing sites?

A: Yes, but the images would not be detailed enough to show the equipment left behind by the astronauts. Using its adaptive optics system, the VLT has already taken one of the sharpest ever images of the lunar surface as seen from Earth: http://www.eso.org/public/news/eso0222/. However, the smallest details visible in this image are still about one hundred metres on the surface of the Moon, while the parts of the lunar modules which are left on the Moon are less than 10 metres in size. A telescope 200 metres in diameter would be needed to show them. Although the VLT, when used as an interferometer (VLTI), reaches the same equivalent resolution, it cannot be used to observe the Moon. You may be wondering whether the Hubble Space Telescope would have better luck. In fact, while a space telescope is not affected by the atmosphere of the Earth, it is not substantially closer to the Moon. Also, the Hubble is smaller than the VLT, so it isn’t able to obtain images that show the surface of the Moon with higher resolution. The sharpest images of the lunar landers have been taken by the Lunar Reconnaissance Orbiter: Apollo Landing Sites Revisited.

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Q: Why is the 3.58-metre New Technology Telescope (NTT) at La Silla called the New Technology Telescope?

A: Inaugurated in 1989, the 3.58-metre New Technology Telescope (NTT) broke new ground for telescope engineering and design: the altazimuth mounting and the very compact enclosure result in a much smaller and more functional building than could have been achieved with the traditional housing, for a telescope of the same size. The design of the building also incorporates an innovative thermal control and ventilation system, to prevent the degradation of the image due to local turbulence. The NTT was the first telescope in the world to have a computer-controlled main mirror (active optics), a technology developed at ESO and now applied to most of the world’s current large telescopes. The purpose of the active optics system is to maintain the perfect shape of the main mirror, compensating for the action of gravity on it.

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Q: What kind of planets can the spectrograph HARPS see?

A: HARPS does not directly "see" planets. It detects them indirectly, measuring the velocity at which a star wobbles because of the gravitational pull of one or more orbiting planets. By measuring and analysing these variations in velocity, astronomers can calculate the mass and the orbits of the planets around a star. The less massive the planet, the tinier the effect it produces on the star, and the more precise the instrument needed to detect it is. HARPS is able to detect movements at velocities of just a metre per second — the speed of a person walking — in a star hundreds of light-years away. This has allowed planets only a few times more massive than the Earth to be discovered. HARPS is currently the most powerful exoplanet hunter in the world.

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Q: How does the VLT Interferometer (VLTI) work?

A: The power of the VLT Interferometer does not come from adding the light beams from the individual telescopes to gather more light. Instead, the light waves are made to interfere with each other to produce patterns of light and dark fringes, a little like the way ripples on water can combine to produce either bigger waves or cancel to produce calm water. This technique is called interferometry. The fringes give us information about the structure of the observed object, but they are not an image of it. If an image is needed, it must be reconstructed by mathematically combining the information from many sets of fringes. However, many important scientific questions can be answered without making an image of an object. For the interferometry to work, the light waves must be combined very precisely using a complex system of mirrors in underground tunnels. The paths along which the light travels must be kept accurate to a fraction of the wavelength of the light, to control the phase of the waves. The accuracy required in positioning the mirrors is less than 1/1000 mm over a hundred metres. Thanks to this technological feat, the VLTI can reconstruct images with an angular resolution of 2 milliarcseconds, and allow astronomers to see details up to 16 times finer than with the individual VLT Unit Telescopes.

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Q: What is the maximum resolution of a telescope array, or interferometer?

A: For a telescope array, or interferometer, the resolution (the finest details it can distinguish) depends on the separation of the individual elements of the array. The resolution improves as the separation of the elements increases, which is why widely spread interferometer arrays are important for high resolution observations. This is different from a single telescope, where the resolution depends instead on the diameter of the individual telescope (see previous FAQ).

The angular size θ (measured in radians) of the smallest details that the array can distinguish is given by θ≈λ/B, where λ is the wavelength of the light and B the maximum “baseline”, the separation between a pair of elements in the array. This expression can be compared with the equivalent one for a single telescope of θ≈λ/D, where D is the diameter of the telescope. In other words, the interferometer can be thought of as having the resolution of a single telescope as large as the whole array.

For example, the Atacama Large Millimeter/submillimeter Array (ALMA) will have maximum baselines between antennas of about 16 kilometres. When observing at wavelengths of 1 millimetre, ALMA's effective resolution will be around 13 milliarcseconds, or less than four millionths of a degree.

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Q: How many people work for the VLT on Paranal?

A: 242 full time equivalents (FTE), and out of these 174 FTEs work on Paranal (2011). These figures cover VLT operations related activities - Paranal operations, upgrades, end-to-end science support, dataflow and control software (2011 figures).

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Q: What is its annual budget for the VLT (excluding personnel)?

A: The annual budget is 16.9 million Euro without personnel costs (2011 figure).

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Q: How much did it cost to build the VLT?

A: In excess of 330 M€ were spent in ESO Member States for the construction of the VLT, which started operations in 1999 (UT1). This excludes personnel costs within ESO itself.

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Q: How is energy produced at ESO’s Observatories?

A: Energy production is one of the most difficult parts when setting up a remote operation away from the electrical grid. At Paranal, and at ALMA, the electrical power is produced on site by using multi-fuel turbines, which are more efficient than traditional diesel generators and can use "cleaner" fuels such as LPG (liquefied petroleum gas) and natural gas. The generators work in "island mode”, i.e. not connected to a grid. However, due to new opportunities in the energy markets, including Chile, the choice for the E-ELT might include options for the use of renewable energy. In addition, the ALMA power distribution system has been designed to be ready for connection to a renewable energy plant.

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Q: Why is solar energy not used at ESO’s observatories?

A: In the planning phases of the observatories, both solar and wind power generation were considered. However, it turned out that the costs of this would have been too great. With solar power, for example, the system to store the energy collected during the day for use at night would have been extremely expensive. Finally, as these power sources are not guaranteed to be constantly available, a traditional generator system on site would have been necessary for backup as well. However, as the available technology (and its associated cost) develops, there may be interesting options for the usage of renewable energy in the future.

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