Foreword 
 
Astronomical interferometry was first imagined by Hippolyte Fizeau in
1868. He proposed to observe stars through two apertures to obtain
interferences that would give an information on the spatial intensity
distribution of the source. The resulting resolving power linearly
increases with the distance between the apertures and therefore the
interference fringes vanish for stars with a diameter proportional to the
reciprocal of the baseline of the interferometer. This idea opened the way
to a better understanding of most of celestial sources then only known as
point-like objects. This technique after being first tested by Édouard
Stéphan in 1874, proved to be efficient when Albert A. Michelson first
measured the Jupiter satellites, binary stars orbits and the first diameter
of a star other than our Sun, Betelgeuse in 1921. Stellar diameters were
measured by the Michelson group during the first half of the twentieth
century with two flat mirror-apertures fixed on a beam on top of a
telescope mount. This technique gave access to a limited baseline hence to
a limited spatial resolution. More resolution required longer baselines and
the use of separate telescopes. This challenge was overpassed by Antoine
Labeyrie in 1975 when the diameter of Vega with a 16 meter baseline
interferometer was measured. Interferometry entered into a new era.
 
Since these pioneering times, astronomical interferometry has developed and
a large variety of instruments have been built in Europe, in the United
States and in Australia. These instruments are called stellar
interferometers since they essentially measure the morphology of
stars, the orbits of binary systems or perform stellar
astrometry. Limitation to stellar physics only results from the limited
sensitivity of these instruments. The recently developed adaptive optics
technique corrects for wavefront distortions in real time allowing the use
of much bigger apertures. Similarly fringes can also be stabilized actively
and summed up to increase the quality of the signal. These two techniques
are the main features of the new generation interferometers. With a better
sensitivity fainter sources can be observed and new astrophysical domains
become reachable as for example extragalactic astronomy.
 
Access to optical interferometers has been limited until now to a small
community of experts because the instruments were essentially prototypes
with a large number of sub-systems to handle that observers needed to
understand and master in order to perform observations. New interferometers
like VLTI have been designed to be operated like regular astronomical
instruments for which technical knowledge is not mandatory. This is a very
important progress: a much wider community of astrophysicists can have their own
observations. In addition, VLTI will be among new born interferometers one
of the most powerful with four 8-m unit telescopes and at least
three 1.8-m auxiliary telescopes. The VLTI will push the boundary of
current investigations from studies on multiple stellar systems and
determination of stellar diameters to the fields of star formation,
extra-solar planets and for the first time the study of the inner part of
galaxies. VLTI will be able to probe the objects with an angular resolution
20 times better than the biggest telescopes on Earth.
 
The motivation to organize this school was the perspective that a large
fraction of astronomers are going to use the VLTI instruments, MIDI and
AMBER, to make progress in their own research fields. We are confident that
the imagination and creativity of these astrophysicists will generate many
new programs and important results in still virgin fields. One of the
challenges of this school is to convince participants that training to
interferometric observations is not very demanding and in turn will be very
rewarding. This training can be split into two parts : (i) preparation of
observations and (ii) data analysis. This first school covers the
preparation of observations. A second school of this series will focus on
data reduction and analysis with VLTI.
 
Although not difficult, the philosophy of the preparation of the
observations is somewhat specific to astronomical
interferometry. Observability of sources not only requires that the source
be properly located in the sky but also that the array of telescopes be
well configured at the time when the source is observable. It is important
that the configuration is optimized to ensure that the searched information
on the object be measured or that the imaging quality is the best. This
specificity makes the preparation more demanding than it is for usual
single-telescope observations as sources may only be observable more than a
few minutes for a given configuration. The optimization of the duty cycle
is an important feature of interferometric observations. Other items than
geometrical constraints such as computing exposure times, choosing spectral
resolution and filters, etc ..., also play a major role in the preparation
of observations.

We have selected lectures to cover most of aspects of astronomical
interferometry which are of importance for preparing observations. Lectures
include both theoretical and practical aspects of interferometry to give
participants a complete and coherent basis from the theory of aperture
synthesis to the optimization of the array and the computation of exposure
times. We have made the choice to allocate a large fraction of time for
practical sessions and put the emphasis on the use of tools developed to
prepare observations which will be available to VLTI users. Lectures also
include examples of real observations with their astrophysical analysis to
illustrate the power of the technique and give hints on how to address some
classes of problems.
 
The scientific program of the school has been divided into five main parts
which are the main chapters of this book. The first two parts are on the
theoretical and practical principles of interferometry. The third part is
on the VLTI and its instruments MIDI and AMBER. The fourth part deals with
the details of the preparation of an observation. The fifth part is
dedicated to the interferometric information with examples of astrophysical
projects based on the use of visibilities, closure phases and differential
phases or spectral visibilities. Projects and proposals to use the VLTI
presented by the participants have been added to this chapter. The last
chapter includes poster contributions presented by the participants. The
chapters also feature the exercizes of the practical work sessions.
 
Advertizing of the school has met a good success and twice more
applications than the number of possible attendees were rapidly
collected. Fifty four participants from twenty two different countries were
eventually welcomed. Most of astrophysical fields of interest were
represented. As we believe the school was useful to all, we wish to thank
the lecturers who dedicated their time to prepare their presentations, to
discuss with the participants and also to contribute to this book. We also
wish to thank the staff of Les Houches Physics Center and of the Grenoble
Observatory involved respectively in the local organization of the school
and in the very demanding organization of the practical sessions. We also
wish to thank the sponsors who made this school possible : the European
Union, the Jean-Marie Mariotti Center, the European Southern Observatory,
the Laboratoire d'Astrophysique de l'Observatoire de Grenoble and the
Centre National de la Recherche Scientifique.
 
 
 
Guy Perrin & Fabien Malbet