Collaboration between an astronomy laboratory and a French industrial firm: the first instrument for the Very Large Telescope Interferometer

The "Département
de Recherche Spatiale" (Department of Space Research – a joint CNRS
and Observatoire de Paris unit) is soon to deliver the first instrument
for the Very Large Telescope Interferometer (VLTI)* to
the European Southern Observatory (ESO). This instrument, VINCI, will
serve to test and validate the combining of the beams coming from the
various telescopes of the VLTI. Optical fibers made of fluoride glass
are used in this instrument. These fibers are developed by "Le Verre
Fluoré," a company specialized in this type of material./p>

VINCI (VLT INterferometer Commissioning
Instrument) is a "first light" instrument for the VLTI. It will
make it possible to test and to validate the infrastructure as a whole,
and it will combine the telescopes to be installed on the Cerro Paranal
site in Chile. This combining of the light beams coming from the various
telescopes will take place via optical fibers made of fluoride glass.

VINCI is a functional copy (adapted to suit the context of the VLTI) of
FLUOR. That instrument was developed by the "Département de
Recherche Spatiale" (DESPA) of the Observatoire de Paris. At Kitt
Peak in 1991, it performed the first coherent combining via optical fibers
of two independent telescopes. It has been operating for 5 years now as
a focal instrument for the IOTA (Infrared Optical Telescope Array) interferometer
on Mount Hopkins in Arizona (in collaboration with the Harvard Smithsonian
Center for Astrophysics), where it has yielded a rich harvest of scientific
data.

The heart of VINCI (known as "MONA") is a fiber optics beam
combiner unit which is the fruit of a 10-year partnership between a laboratory
(DESPA) and a small business (Le Verre Fluoré). Since VINCI will
be working in the near-infrared domain (from 2 µm to 2,5 µm),
it is necessary to use special fibers made of fluoride glass, a material
developed by Professor Marcel Poulain and his team at the "Laboratoire
des matériaux photoniques" (Photonic Materials Laboratory)
of the Université de Rennes 1. The fibers are of the "mono-mode"
or "single-mode" type. The core of the fiber is very small,
with a diameter of 6.5 µm. All of the light collected by the 50-m2
mirror of the telescope is concentrated into the fiber core. To achieve
this, adaptive optics must be used and DESPA has contributed greatly to
the development of this technique for astronomy. These fibers guide the
light while "cleaning" it by removing errors generated by atmospheric
turbulence, thereby improving the quality of the observations considerably
(by a factor of 10). This unique property has been demonstrated in the
sky by DESPA with the FLUOR experiment. Following that work, it was decided
to use fibers or equivalent systems for the instruments of all of the
interferometers currently under construction.

"Le Verre Fluoré" is the only company in the world that
has the know-how to manufacture the fiber optics components made of fluoride
glass that serve to combine light beams. The development of these systems
was supported by the French Ministry of Research and by the Region of
Brittany. Its use by DESPA in FLUOR, which served as a test bench, has
enabled this technology to mature, and it is now well mastered.

In the future, it is possible to imagine kilometer arrays of giant telescopes
interconnected by optical fibers. The use of the filtering properties
of the fibers is also vital to the viability of the future space interferometer
project Darwin, which is devoted to research into extra-solar planets
similar to the Earth, and for which a program is in progress to develop
fibers having very long wavelengths (10 µm to 20 µm).

The VINCI instrument is being produced in partnership with the ESO, which
is financing it, and with the following laboratories and institutes:
– the DESPA of the Observatoire de Paris is providing the expertise, and
is designing, producing, and integrating all of the opto-mechanics (except
for the camera) and the electronics;
– the control software is being produced by the Observatoire Midi-Pyrénées;
– the camera (LISA) is being produced by the Max Planck Institut für
Extraterrestrische Physik in Garching; and
– the ESO is providing the overall observational control software and
is handling final integration.

The "Département de Recherche Spatiale" delivered VINCI
to the ESO on September 27, and is currently re-assembling it in the ESO
laboratories in Garching near Munich. The instrument will remain in Germany
for three months in order to be integrated with the camera, and to undergo
additional tests, before being shipped to Chile at the beginning of next
year to prepare for the first observations of the sky.

You can also consult the ESO server: http://www.hq.eso.org/eso-homepage.html

*The
Very Large Telescope Interferometer will be made up of 4 large main telescopes
that are 8.2 meters in diameter (the construction of the four telescopes
is now complete) and of 3 auxiliary telescopes that are 1.8 meters in
diameter. The auxiliary telescopes will be mounted to run on rails, and
they will be positionable on 30 observation stations. This array of telescopes
should, as of its coming into service in 2001, make it possible to:
– discover new extra-solar planets that are comparable to Jupiter;
– detect low-mass stars such as brown dwarfs;
– observe star-forming regions and understand better the physical processes
that give birth to stars;
– analyze the atmospheres of nearby stars;
– see the objects making up the core of our Galaxy, and detect any black
holes in the active nuclei of galaxies.