Following the “First Light” for the fourth of the 8.2-m telescopes of the VLT Observatory on Paranal in September 2000, ESO scientists and engineers have just successfully accomplished the next major step of this large project.

On March 17, 2001, “First Fringes” were obtained with the VLT Interferometer (VLTI) – this important event corresponds to the “First Light” for an astronomical telescope. At the VLTI, it occurred when the infrared light from the bright star Sirius was captured by two small telescopes and the two beams were successfully combined in the subterranean Interferometric Laboratory to form the typical pattern of dark and bright lines known as “interferometric fringes”. This proves the success of the robust VLTI concept, in particular of the “Delay Line”.

On the next night, the VLTI was used to perform a scientific measurement of the angular diameter of another comparatively bright star, Alpha Hydrae (Alphard); it was found to be 0.00929+-0.00017 arcsec. This corresponds to the angular distance between the two headlights of a car as seen from a distance of approx. 35,000 kilometres. The excellent result was obtained during a series of observations, each lasting 2 minutes, and fully confirming the impressive predicted abilities of the VLTI.

This first observation with the VLTI is a monumental technological achievement, especially in terms of accuracy and stability. It crucially depends on the proper combination and functioning of a large number of individual opto-mechnical and electronic elements. This includes the test telescopes that capture the starlight, continuous and extremely precise adjustment of the various mirrors that deflect the light beams as well as the automatic positioning and motion of the Delay Line carriages and, not least, the optimal tuning of the VLT INterferometer Commissionning Instrument (VINCI).

These initial observations prove the overall concept for the VLTI. It was first envisaged in the early 1980’s and has been continuously updated, as new technologies and materials became available during the intervening period.

The present series of functional tests will go on for some time and involve many different configurations of the small telescopes and the instrument. It is then expected that the first combination of light beams from two of the VLT 8.2-m telescopes will take place in late 2001.

According to current plans, regular science observations will start from 2002, when the European and international astronomical community will have access to the full interferometric facility and the specially developed VLTI instrumentation now under construction. A wide range of scientific investigations will then become possible, from the search for planets around nearby stars, to the study of energetic processes at the cores of distant galaxies.

With its superior angular resolution (image sharpness), the VLT is now beginning to open a new era in observational optical and infrared astronomy. The ambition of ESO is to make this type of observations available to all astronomers, not just the interferometry specialists.

The full text of this Press Release (ESO PR 06/01), with 12 related photos (including a view of the ‘First Fringes’) and a video clip (ESO PR Video Clip 03/01), is available at:

In this connection, ESO has also released ESO Video News Reel No. 12 for broadcasters, cf.:

Note for Editors


For technical issues:

Andreas Glindemann European Southern Observatory Paranal, Chile Tel.: +56-55-43-5000/5240 email:

For scientific issues:

Francesco Paresce European Southern Observatory Garching, Germany Tel.: +4989-3200-6245 email:

Background information: