Back in 1984, at the start of the VLT project, the work on the definition of the VLT enclosures started with the objective to study and design a type of fully retractable enclosure, in which the telescope would be largely exposed to the undisturbed wind flow during observations. In the end, however, the severe constraints on the allowable wind speed on the telescope imposed by the lack of intrinsic rigidity of the primary mirror led to the design of a more conventional cylindrical enclosure.
This design consists of a basement in concrete with a height of about 5 meters from the ground level, which supports a steel structure that encloses and provides access to the telescope. The fixed part of the metal structure rises up to the level of the telescope Nasmyth platform. Near the level of the primary mirror there are some large ventilation openings which, together with the mirror door, may allow some natural ventilation also on the primary mirror.
The rotating part does not include any accessible floors and has essentially the same function as a conventional observatory dome, from which it differs for its optimized structural layout and the presence of large upside down L-shaped slit doors. The cylindrical shape of the "dome" also allows the easy installation of an internal crane. 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 dome itself, a large numbers of louvers may provide natural ventilation in the entire telescope volume.
Figure: Schematic cross-section of the VLT enclosure.
A schematic cross-section of the enclosure that illustrates the
different access levels is shown in the fig. 
.
Access for personnel to the enclosure will be either via the underground
utility tunnels that connect all the buildings of the VLT
Telescope Area or
a normal door at the ground floor level. Staircases and a lift
will connect all floors of the enclosure.
A large external door at ground level will allow entry of trucks with
large instruments and goods in the so-called ground access room, which covers
a quadrant of about 90
at ground level.
The rest of the ground floor is made of exposed radial walls which support
the metal structure surrounding the telescope room.
At the underground floor a 360
ring of rooms will surround the
telescope pier and give access to the pier interior where
the Coudé instrument will be located.
Miscellaneous service equipment will be installed in these rooms, such as
the oil pumps for the telescope bearings, but ample space will remain
for storage of users' items,
which may quite useful particularly during the installation of
instruments. Large equipment items will be lowered down to the underground
floor by the dome crane, through a hatch in the floor of
the ground access room.
The crane, installed radially just under the roof of the dome, will be the main handling tool for all maintenance operations inside the enclosure and, by rotating the dome, will be able to serve the whole telescope volume except the central region. This crane will be used to mount the Nasmyth adapters and instruments, install and remove the secondary mirror unit (with the telescope in horizontal position), as well as lift the tertiary mirror unit before the exit of the primary mirror cell on its way to the aluminisation plant. Another hatch will allow the crane to pick up loads from the ground access room into the telescope room.
Inside the main telescope volume there will be two floor levels. The
fork base floor, about 5.2 m above the ground level,
will constitute a continuous surface with the rotating
azimuth platform of the telescope. Cassegrain instruments will be mounted
and accessed on this floor. The upper floor level, 11 m from the ground
level, will be
continuous with the Nasmyth platforms of the telescope over a 90
quadrant: this area will be the main access way to the
Nasmyth instrument for personnel and small equipment. Only a narrower
circular walkway
will run all along the inner wall of the enclosure, permitting maintenance
access to the dome rotation drives and wheels.
During the day and in general when the enclosure is closed, the thermal control system will bring and keep all internal surfaces inside the telescope room at a set temperature close to the predicted value for the coming night. This thermal conditioning will be achieved by air cooling and mixing: in order to achieve the desired heat transfer rates with all surfaces the mixing rate may be set at up to 10 volumes/hour. Therefore daytime users of the enclosure should expect to find a somewhat (literally) cool working environment, while the noise of fans and air treatment units should not exceed the level usual for rooms equipped with individual air conditioning equipment.
The enclosure will include a variety of programmable mechanisms for dome rotation, opening, louvers, etc. All these systems will be managed by a network of computerized controllers linked both to the Telescope Control System network and to the Building Management System (which monitors and administers all service supplies on the site). Therefore the observer will not only be able to operate from his/her control station all the enclosure mechanisms linked to telescope operation (such as dome rotation and slit opening) but will also be able to query at any time the status of all active components of the enclosure.