A large part of the present work was performed in the framework of the a project for a new generation telescope named the Very Large Telescope (VLT). It is then useful to illustrate briefly this project and by the same occasion to introduce an essential nomenclature of the terms and concepts relative to large optical telescopes which are used throughout this report.
The European Southern Observatory (ESO) is an intergovernmental organization constituted by eight European countries, including Switzerland, for the purpose of providing and managing astronomical observatories in the southern hemisphere. ESO owns the observatory of La Silla, in the Chilean Andes, which includes 15 telescopes of different size and characteristics, among which is the 3.5-meter New Technology Telescope (NTT) which is deemed to be the best telescope presently in operation.
In 1984 ESO started the VLT project for a new observatory on the site of Cerro Paranal, which will be constituted by four unit telescopes, each with a primary mirror with a diameter of 8 meters. An artist's view of the future observatory is shown here below.
Figure:Artist's view of the VLT observatory
The telescope and its main components are schematically illustrated
in figures 
and .
The mounting is altitude-azimuth.
The primary mirror is only 17 cm thick and
is supported by a so-called active optics
system which consists of 150 combined hydraulic-electromechanic
actuators capable of correcting the mirror shape for gravity and other
low frequency deflections by applying controlled forces.
The secondary mirror is provided with a fast tilt control called
field stabilization for
precision guiding. For observations in the infrared wavelength range,
a chopping motion of the secondary mirror allows astronomers
to measure
the difference between the signal and sky background, but in this case
the field stabilization may not be as effective.
The telescope is protected by an enclosure of cylindrical shape
(fig. ). The upper part co-rotates with the
telescope and has a large upside-down-L shaped slit, closed
with two sliding doors.
The two slit doors are supported on two
protuberances of the dome, which also integrate a set of pneumatically
activated bars that constitutes a wind screen with different levels
of wind permeability across the slit.
In the lower fixed part there are
5 large doors (one 10x4m and four 5x4m) while in the rotating upper
part there are about 150 1.5x0.5m openings equipped with louvers.
Figure: The VLT unit telescope in its enclosure
Figure: Main components of the VLT 8-m unit telescope
