The favorable experience with the MMT and the NTT influenced fundamentally the design of the next generation of large telescope in the 8-meter class. Now natural ventilation was the word to be followed in designing the new enclosures.
In 1984, at the start of the ESO VLT project, the work on the definition of the telescope enclosures started with the objective to study and design a fully retractable type of enclosure, in which the telescope would be largely exposed to the undisturbed wind flow during observations. Beside the reduction of local seeing, another main design driver was the objective to drastically lower the cost of the telescope enclosures and other infrastructures to less than 25% of the overall project cost, so that the entire VLT project could be realized within a budget frame compatible with what it was felt were the financial possibilities of the ESO organization.
Figure: The first artist's view of the VLT (1984).
Thus the very first artist's view of the VLT dating from 1984
(fig. 
) saw the four 8-m unit
telescopes completely exposed during observations and protected during
the day by movable roll-on/off shelters. A large windscreen
had the function of limiting the wind loads on the telescope.
For many reasons this first design was not very practical;
however it illustrates well the objectives that were set for the
enclosure with respect to the atmospheric environment:
the enclosure should allow on the telescope as much wind flow
as required for eliminating local seeing, while limiting the amplitude
of wind buffeting to levels acceptable for the optimum guiding
performance of the telescope.
Figure: The retractable enclosure which was envisaged for the VLT.
Eventually this design evolved to the retractable dome enclosure
represented in fig. .
The fixed base of the enclosure is made of a metal space frame
ring-shaped structure and supports the rotating part on a number of
roller bearings.
The upper rotating part is made of an approximately cylindrical
panel clad space frame, which constitutes a wind shielded recess
in which the lower
part of the telescope is contained, topped by a retractable
hemispherical dome consisting of overlapping shell sections.
During observations the enclosure leaves the upper part of the
telescope essentially in open air.
The problem of achieving a good guiding accuracy in open air was
overcome by the design of a servo-controlled tilting secondary mirror.
The effects of wind buffeting on the primary mirror, however, were not
fully quantified until quite late in the project development. Although
a possible criticality of this aspect had been recognized at a earlier
stage, which led to provide the cylindrical recess for the lower part of
the telescope (cf. fig. ), it was thought that the
active mirror support system could be designed
capable of dynamic corrections
up to a frequency of 1 Hz. Unfortunately this objective appeared not
to be achievable ([Noethe & Zago 93]).
This issue led to a redesign of the VLT enclosure in form of a more
conventional building (fig. ).
This choice brings again in the foreground also for the VLT
the issues of local seeing.