VERY LARGE TELESCOPE (VLT)

THE ASTRONOMY PICTURE OF THE DAY FOR 2009 August 16

A Laser Strike at the Galactic Center

Credit: Yuri Beletsky (ESO)

Explanation: Why are these people shooting a powerful laser into the center of our Galaxy? Fortunately, this is not meant to be the first step in a Galactic war. Rather, astronomers at the Very Large Telescope (VLT) site in Chile are trying to measure the distortions of Earth's ever changing atmosphere. Constant imaging of high-altitude atoms excited by the laser -- which appear like an artificial star -- allow astronomers to instantly measure atmospheric blurring. This information is fed back to a VLT telescope mirror which is then slightly deformed to minimize this blurring. In this case, a VLT was observing our Galaxy's center, and so Earth's atmospheric blurring in that direction was needed. As for inter-galaxy warfare, when viewed from our Galaxy's center, no casualties are expected. In fact, the light from this powerful laser would combine with light from our Sun to together appear only as bright as a faint and distant star.

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ESO Very Large Telescope (VLT) - Paranal

THE FOURTH AND FINAL AUXILARY TELESCOPE (AT4) HAD ITS FIRST LIGHT ON 2006 DECEMBER 15

The Very Large Telescope (VLT) is a system of four separate optical telescopes (the Antu telescope, the Kueyen telescope, the Melipal telescope, and the Yepun telescope) organized in an array formation, built and operated by the European Southern Observatory (ESO) at the Paranal Observatory on Cerro Paranal, a 2,635 m high mountain in the Atacama desert in northern Chile. Each telescope has an 8.2 m aperture. The array is complemented by four movable Auxiliary Telescopes (ATs) of 1.8 m aperture. Working together in so-called interferometric mode, the telescopes can achieve an angular resolution of around 1 milliarcsecond, equivalent to the gap between the headlights of a car as observed from the same distance as between the Earth to the Moon. ^[1]

General information

The VLT consists of an arrangement of four large (8.2 meter diameter) telescopes, and optical elements which can combine them into an astronomical interferometer (VLTI) which is used to resolve small objects. The interferometer also includes a set of four 1.8 meter diameter movable telescopes dedicated to interferometric observations. The 8.2 meter telescopes have been named after some astronomical objects in the local Mapuche language: Antu (The Sun), Kueyen (The Moon), Melipal (The Southern Cross), and Yepun (Venus).

The VLT 8.2 meter telescopes was originally designed to be operated in three modes^[2]:

• • as a set of four independent telescopes (this is the primary mode of operation). With one such telescope, images of celestial objects as faint as magnitude 30 can be obtained in a one-hour exposure. This corresponds to seeing objects that are four billion times fainter than what can be seen with the unaided eye.

  • as a single large coherent interferometric instrument (the VLT Interferometer or VLTI), for extra resolution. This mode is occasionally used, only for observations of relatively bright sources with small angular extent.

  • as a single large incoherent instrument, for extra light-gathering capacity. The instrumentation required to bring the light to a combined incoherent focus was not built. Recently, new instrumentation proposals have been put forward for making this observing mode available^[3]. Multiple telescopes are sometimes independently pointed at the same object, either to increase the total light-gathering power, or to provide simultaneous observations with complementary instruments.

The VLTs are equipped with a large set of instruments permitting observations to be performed from the near-UV to the mid-IR (ie a large fraction of the light wavelengths accessible from the surface of the Earth), with the full range of techniques including high-resolution spectroscopy, multi-object spectroscopy, imaging, and high-resolution imaging. In particular, the VLT has several Adaptive optics systems, which at infrared wavelengths correct for the effects of the atmospheric turbulence, providing images almost as sharp as if the telescope were in space. In the near-IR, the Adaptive Optics images of the VLT are up to three times sharper than those of the Hubble Space Telescope, and the spectroscopic resolution is many times better than Hubble. The VLTs are noted for their high level of observing efficiency and automation.

The principal role of the main VLT telescopes is to operate as four independent telescopes. The interferometry (combining light from multiple telescopes) is used about 20 percent of the time for very high-resolution on bright objects e.g. Betelgeuse.

Additionally, the four 8.2 m telescopes are accompanied by four smaller Auxiliary Telescopes of 1.8 m each (two operational in 2005, the other two in 2006), which can be placed on different positions around the four big telescopes in order to provide better interferometric observations.

The VLT is operated by the European Southern Observatory.

In 2004, VLT telescopes produced some of the first infrared images of extrasolar planets GQ Lupi b and 2M1207b. Among the more recent discoveries is the discovery of the farthest gamma-ray burst and the evidence for a black hole at the centre of our Galaxy, the Milky Way. The VLT has also discovered the candidate farthest galaxy ever seen by humans, Abell 1835 IR1916.

Instruments

Instruments on the VLT:^[4]

• • FORS 1 (FOcal Reducer and low dispersion Spectrograph) is a visible light camera and Multi Object Spectrograph with a 6.8 arcminute field of view.

  • FORS 2. Like FORS 1, but with further multi-object spectroscopy.

  • ISAAC (Infrared Spectrometer And Array Camera) is a near infrared imager and spectrograph

  • UVES (Ultraviolet and Visual Echelle Spectrograph) is an ultraviolet and visible light spectrograph.

  • FLAMES (Fibre Large Area Multi-Element Spectrograph) is a multi-object fibre feed unit for UVES and GIRAFFE, the latter allowing the capability for simultaneously studying hundreds of individual stars in nearby galaxies at moderate spectral resolution in the visible.

  • NACO (NAOS-CONICA, NAOS meaning Nasmyth Adaptive Optics System and CONICA meaning COude Near Infrared CAmera) is an adaptive optics facility which produces infrared images as sharp as if taken in space and includes spectroscopic, polarimetric and coronagraphic capabilities.

  • VISIR (VLT spectrometer and imager for the mid-infrared) provides diffraction-limited imaging and spectroscopy at a range of resolutions in the 10 and 20 micrometre mid-infrared (MIR) atmospheric windows.

  • SINFONI is a medium resolution, near-infrared (1-2.5 micrometres) integral field spectrograph fed by an adaptive optics module.

  • CRIRES (CRyogenic InfraRed Echelle Spectrograph) is adaptive optics assisted and provides a resolving power of up to 100,000 in the infrared spectral range from 1 to 5 micrometres.

  • HAWK-I (High Acuity Wide field K-band Imager) is a near-infrared imager with a relatively large field of view.

  • VIMOS (VIsible Multi-Object Spectrograph) delivers visible images and spectra of up to 1,000 galaxies at a time in a 14 x 14 arcmin field of view.

  • Guest focus available for visitor instruments, such as ULTRACAM or DAZZLE.

Several second-generation VLT instruments are now under development:

• • X-Shooter, a wide-band [UV to near infrared] spectrometer designed to explore the properties of rare, unusual or unidentified sources

  • KMOS, a cryogenic infrared multi-object spectrometer intended primarily for the study of distant galaxies

  • MUSE a huge "3-dimensional" spectroscopic explorer which will provide complete visible spectra of all objects contained in "pencil beams" through the Universe

  • SPHERE, a high-contrast adaptive optics system) dedicated to the discovery and study of exoplanets.

Interferometry and the VLTI

In its interferometric operating mode, the light from the telescopes is reflected off mirrors and directed through tunnels to a central beam combining laboratory. The VLTI is intended to achieve an effective angular resolution of 0.002 arcsecond at a wavelength of 2 µm. This is comparable to the resolution achieved using other arrays such as the Navy Prototype Optical Interferometer and the CHARA array. Using the big telescopes the faintest object the VLTI can observe is magnitude 7 in the near infrared for broadband observations,^[5] similar to many other near infrared / optical interferometers without fringe tracking². At more challenging mid-infrared wavelengths, the VLTI can reach magnitude 4.5, significantly fainter than the Infrared Spatial Interferometer. When fringe tracking is introduced, the limiting magnitude of the VLTI is expected to improve by a factor of almost 1000, reaching a magnitude of about 14. This is similar to what is expected for other fringe tracking interferometers. In spectroscopic mode, the VLTI can currently reach a magnitude of 1.5. The VLTI can work in a fully integrated way, so that interferometric observations are actually quite simple to prepare and execute. The VLTI has become worldwide the first general user optical/infrared interferometric facility offered with this kind of service to the astronomical community.^[6]

Because of the many mirrors involved in the VLTI system, about 99 percent of the light is LOST before reaching the detector. Additionally, the interferometric technique is such that it is very efficient only of objects that are small enough that all their light is concentrated. For instance, an object with a relatively low surface brightness such as the moon cannot be observed, because its light is too diluted. Only targets which are at temperatures of more than 1,000°C have a surface brightness high enough to be observed in the mid-infrared, and objects must be at several thousands of degrees Celsius for near-infrared observations using the VLTI. This includes most of the stars in the solar neighborhood and many extragalactic objects such as bright active galactic nuclei, but this sensitivity limit rules out interferometric observations of most solar-system objects. Although the use of large telescope diameters and adaptive optics correction can improve the sensitivity a small amount, this cannot extend the reach of optical interferometry beyond nearby stars and the brightest active galactic nuclei.

Because the Unit Telescopes are used most of the time independently, they are used in the interferometric mode mostly during bright time (that is, close to Full Moon). At other times, interferometry is done using 1.8 meter Auxiliary Telescopes (ATs), which are dedicated to full-time interferometric measurements. The first observations using a pair of ATs were conducted in February 2005, and all the four ATs have now been commissioned. For interferometric observations on the brightest objects, there is little benefit in using 8 meter telescopes rather than 1.8 meter telescopes.

The first two instruments at the VLTI were VINCI (a test instrument used to set-up the system) and MIDI, which only allowed two telescopes to be used at any one time. With the installation of the three-telescope AMBER closure-phase instrument in 2005, the first imaging observations from the VLTI are expected soon. In 2008 the Phase Referenced Imaging and Microarcsecond Astrometry (PRIMA) instrument further enhanced the imaging capabilities of the VLTI by allowing phase-referenced imaging, although PRIMA is not expected to be available for use by the astronomic community until at least April 2009.^[7]

After falling drastically behind schedule and failing to meet some specifications, in December 2004 the VLT Interferometer became the target of a second ESO "recovery plan". This involves additional effort concentrated on more rapid improvements to fringe tracking and the performance of the main delay lines. Note that this only applies to the interferometer and not other instruments on Paranal. In 2005, the VLTI was routinely producing observations, although with a brighter limiting magnitude and poorer observing efficiency than expected.

As of March 2008^[update], the VLTI had already led to the publication of 89 peer-reviewed publications.^[8]

In popular culture

One of the large mirrors of the telescopes was a focus of an episode of American reality series World's Toughest Fixes, where a crew of engineers transported the large mirror to be cleaned and re-coated.

See also

• List of observatories

• List of optical telescopes

• List of largest optical reflecting telescopes

• List of astronomical interferometers at visible and infrared wavelengths

• Extremely Large Telescope

• Giant Magellan Telescope

References

1. ^ ESO Press release

2. ^ Science with the VLT in the ELT Era

3. ^ Espresso instrument

4. ^ Paranal Observatory Instrumentation

5. ^ AMBER - Astronomical Multi-BEam combineR

6. ^ Observing with the ESO VLT Interferometer

7. ^ van Belle, et al., The VLTI PRIMA Facility

8. ^ ESO Telescope Bibliography

External links

• Visits to the Paranal Site

• ESO VLT official site for the 8 m telescopes.

• ESO VLTI official site for the interferometer (combining the telescopes)

• Delay Lines for the Very Large Telescopes @Dutch Space

• The VLTI Delay Lines @TNO

• The VLTI Delay Line Article and images by Fred Kamphues/Mill House Engineering

• The PRIMA Star Separator

• ASTROVIRTEL Accessing Astronomical Archives as Virtual Telescopes including archives from the VLT

• Telescope to challenge moon doubters -- note description of VLTI capabilities is inaccurate

• Travelogue VLT Visit

• VLT images