eso0020 — Organisation Release

With the VLT Interferometer towards Sharper Vision

The Nova-ESO VLTI Expertise Centre Opens in Leiden (The Netherlands)

24 May 2000

European science and technology will gain further strength when the new, front-line Nova-ESO VLTI Expertise Centre (NEVEC) opens in Leiden (The Netherlands) this week. It is a joint venture of the Netherlands Research School for Astronomy (NOVA) (itself a collaboration between the Universities of Amsterdam, Groningen, Leiden, and Utrecht) and the European Southern Observatory (ESO). It is concerned with the Very Large Telescope Interferometer (VLTI).

The inaugural ceremony is preceded by a scientific workshop on ground and space-based optical interferometry.

NEVEC: A Technology Centre of Excellence

As a joint project of NOVA and ESO, NEVEC will develop in the coming years the expertise to exploit the unique interferometric possibilities of the Very Large Telescope (VLT) - now being built on Paranal mountain in Chile. Its primary goals are the

  • development of instrument modeling, data reduction and calibration techniques for the VLTI;
  • accumulation of expertise relevant for second-generation VLTI instruments; and
  • education in the use of the VLTI and related matters.

NEVEC will develop optical equipment, simulations and software to enable interferometry with VLT [1]. The new Center provides a strong impulse to Dutch participation in the VLTI. With direct involvement in this R&D work, the scientists at NOVA will be in the front row to do observations with this unique research facility, bound to produce top-level research and many exciting new discoveries.

The ESO VLTI at Paranal

The ESO VLT facility at Paranal (Chile) consists of four Unit Telescopes with 8.2-metre mirrors and several 1.8-metre auxiliary telescopes that move on rails. While each of the large telescopes can be used individually for astronomical observations, a prime feature of the VLT is the possibility to combine all of these telescopes into the Very Large Telescope Interferometer (VLTI).

In the interferometric mode, the light beams from the VLT telescopes are brought together at a common focal point in the Interferometry Laboratory that is placed at the centre of the observing platform on top of Paranal. In principle, this can be done in such a way that the resulting (reconstructed) image appears to come from a virtual telescope with a diameter that is equal to the largest distance between two of the individual telescopes, i.e., up to about 200 metres.

The theoretically achievable image sharpness of an astronomical telescope is proportional to its diameter (or, for an interferometer, the largest distance between two of its component telescopes). The interferometric observing technique will thus allow the VLTI to produce images as sharp as 0.001 arcsec (at wavelength 1 µm) - this corresponds to viewing the shape of a golfball at more than 8,000 km distance. The VLTI will do even better when this technique is later extended to shorter wavelengths in the visible part of the spectrum - it may ultimately distinguish human-size objects on the surface of the Moon (a 2-metre object at this distance, about 400,000 km, subtends an angle of about 0.001 arcsec).

However, interferometry with the VLT demands that the wavefronts of light from the individual telescopes that are up to 200 meters apart must be matched exactly, with less than 1 wavelength of difference. This demands continuous mechanical stability to a fraction of 1 µm (0.001 mm) for the heavy components over such large distances, and is a technically formidable challenge. This is achieved by electronic feed-back loops that measure and adjust the distances during the observations. In addition, continuous and automatic correction of image distortions from air turbulence in the telescopes' field of view is performed by means of adaptive optics [2].

VLTI technology at ESO, industry and institutes

The VLT Interferometer is based on front-line technologies introduced and advanced by ESO, and its many parts are now being constructed at various sites in Europe.

In 1998, Fokker Space (also in Leiden, The Netherlands) was awarded a contract for the delivery of the three Delay Lines of the VLTI. This mechanical-optical system will compensate the optical path differences of the light beams from the individual telescopes. It is necessary to ensure that the light from all telescopes arrives in the same phase at the focal point of the interferometer. Otherwise, the very sharp interferometric images cannot be obtained. More details are available in the corresponding ESO Press Release eso9811 and recent video sequences, included in ESO Video News Reel No. 9, c.f. below.

Also in 1998, the company AMOS (Liège, Belgium) was awarded an ESO contract for the delivery of the three 1.8-metre Auxiliary Telescopes (ATs) and of the full set of on-site equipment for the 30 AT observing stations, c.f. ESO Press Photos eso9832. This work is now in progress at the factory - various scenes are incorporated into ESO Video News Reel No. 9.

Several instruments for imaging and spectroscopy are currently being developed for the VLTI. The first will be the VLT Interferometer Commissioning Instrument (VINCI) that is the test and first-light instrument for the VLT Interferometer. It is being built by a consortium of French and German institutes under ESO contract.

The VLTI Near-Infrared / Red Focal Instrument (AMBER) is a collaborative project between five institutes in France, Germany and Italy, under ESO contract. It will operate with two 8.2-m UTs in the wavelength range between 1 and 2.5 µm during a first phase (2001-2003). The wavelength coverage will be extended in a second phase down to 0.6 µm (600 nm) at the time the ATs become operational. Main scientific objectives are the investigation at very high-angular resolution of disks and jets around young stellar objects and dust tori at active galaxy nuclei with spectroscopic observations.

The Phase-Referenced Imaging and Microarcsecond Astrometry (PRIMA) device is managed by ESO and will allow simultaneous interferometric observations of two objects - each with a maximum size of 2 arcsec - and provide exceedingly accurate positional measurements. This will be of importance for many different kinds of astronomical investigations, for instance the search for planetary companions by means of accurate astrometry.

The MID-Infrared interferometric instrument (MIDI) is a project collaboration between eight institutes in France, Germany and the Netherlands [1], under ESO contract. The actual design of MIDI is optimized for operation at 10 µm and a possible extension to 20 µm is being considered.


[1] The NEVEC Centre is involved in the MIDI project for the VLTI. Another joint project between ESO and NOVA is the Wide-Field Imager OMEGACAM for the VLT Survey Telescope (VST) that will be placed at Paranal.

[2] Adaptive Optics systems allow to continuously "re-focus" an astronomical telescope in order to compensate for the atmospheric turbulence and thus to obtain the sharpest possible images. The work at ESO is described on the Adaptive Optics Team Homepage.

More information

The Inauguration of the new Centre will take place on Friday, May 26, 2000, at the Gorlaeus Laboratory (Lecture Hall no. 1), Einsteinweg 55 2333 CC Leiden; the programme is available on the web. Media representatives who would like to participate in this event and who want further details should contact the Nova Information Centre (e-mail:; Tel: +31-20-5257480 or +31-6-246 525 46).

VLTI-related videos now available

In conjunction with the Inauguration of the NEVEC Centre (Leiden, The Netherlands) on May 26, 2000, ESO has issued ESO Video News Reel No. 9 (May 2000) ("The Sharpest Vision - Interferometry with the VLT").

Tapes with this VNR, suitable for transmission and in full professional quality (Betacam, etc.), are now available for broadcasters upon request; please contact the ESO EPR Department for more details.

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About the Release

Release No.:eso0020
Legacy ID:PR 14/00
Name:Interferometry, Paranal, Very Large Telescope, Very Large Telescope Interferometer, VLT Unit Telescopes
Type:Unspecified : Technology : Observatory
Unspecified : Technology : Observatory : Telescope
Facility:Very Large Telescope Interferometer


The VLT Array on the Paranal mountain
The VLT Array on the Paranal mountain
VLT interferometer principle
VLT interferometer principle


Video News Release 9: The Sharpest Vision — Interferometry with the VLT (eso0020a)
Video News Release 9: The Sharpest Vision — Interferometry with the VLT (eso0020a)