First Scientific Results with Combined Light Beams from Two 8.2-m Unit Telescopes

It started as a preparatory technical experiment and it soon developed
into a spectacular success. Those astronomers and engineers who were
present in the control room that night now think of it as the
scientific dawn of the Very Large Telescope Interferometer (VLTI).

On October 29, 2001, ANTU and MELIPAL, two of the four VLT 8.2-m Unit
Telescopes at the ESO Paranal Observatory, were linked for the first
time. Light from the southern star Achernar (Alpha Eridani) was
captured by the two telescopes and sent to a common focus in the
observatory’s Interferometric Laboratory.

Following careful adjustments of the optical paths, interferometric
fringes were soon recorded there, proving that the beams from the two
telescopes had been successfully combined “in phase”. From an analysis
of the observed pattern (the “fringe contrast”), the angular diameter
of Achernar was determined to be 1.9 milli-arcsec. At the star’s
distance (145 light-years), this corresponds to a size of 13 million
km. The observation is equivalent to measuring the size of a 4-metre
long car on the surface of the Moon.

This result marks the exciting starting point for operations with the
Very Large Telescope Interferometer (VLTI) and it was immediately
followed up by other scientific observations. Among these were the
first measurements of the diameters of three red dwarf stars
(“Kapteyn’s star” – HD 33793, HD 217987 and HD 36395), a precise
determination of the variable diameters of the pulsating Cepheid stars
Beta Doradus and Zeta Geminorum (of great importance for the
calibration of the universal distance scale), as well as a first
interferometric measurement of the core of Eta Carinae, an intriguing,
massive southern object that may possibly become the next supernova in
our galaxy.

This milestone is another important step towards the ultimate goal of
the VLT project – to combine all four 8.2-m telescopes into the most
powerful optical/infrared telescope system on Earth. When ready, it
will be able to reveal at least 15 times finer details in astronomical
objects than what is possible with any existing, single ground-based

Notes for editor

For further information, please contact:

For technical issues:

Andreas Glindemann

European Southern Observatory

Garching, Germany

Tel.: +49-89-3200-6590


For scientific issues:

Francesco Paresce

European Southern Observatory

Garching, Germany

Tel.: +4989-3200-6297


Peer reveiwed publication and references

Read the details in ESO Press Release 23/01, now available at:

with five photos and a “scientific appendix” in which the first
results are expounded in more detail.

Reference URL :