Main Functions

Each VLT Unit Telescope is enclosed in a building called the enclosurewhose main functions are:

• In the closed position, to protect the telescope and its instrumentation against adverse weather conditions, dust, lightning and lightning electromagnetic pulse and to preserve its thermal environment.
• In the open position, to allow the telescope a free field of view by means of a large slit in a rotating dome. At the same time the enclosure provides wind protection and the right amount of ventilation and air circulation in the enclosure to create optimum observing conditions for the telescope during night-time astronomical observations

In addition to these main functions, each enclosure provides:

• a circular floor in continuation of the fork base azimuth platform as required for the main mirror transport and Cassegrain instruments
• a large access door for the 8.2 m diameter main mirror cell on its carriage
• platforms allowing access to all telescope levels where instruments are to be installed and operated
• handling facilities for the lifting and transporting of instruments onto the telescopes
• access to platforms and floors

Design

The enclosure is designed as a building of cylindrical shape, constructed as a steel structure with both roof and facades covered with weather-tight and thermally insulating cladding panels.In the adjacent 3-dim picture the dome is displayed. Also look at theVLT sectional drawings, front view andside view. Each enclosure consists of: A fixed cylindrical part rising from 5.144 m (above the nominal 0.0 Telescope Area level) up to approximately the level of the Nasmyth platforms of the telescopes. This fixed part of the enclosure surrounds the lower part of the telescope and has a floor at the same level as the telescope azimuth platform. At this level the large main mirror door is situated, as well as some large ventilation doors. The fixed part has a second floor, the Nasmyth access area, situated at the same level as the Nasmyth platforms of the telescope. The fixed part of the enclosure furthermore contains the lift operating between levels -4.25 m and +11.044 m and staircases between fork base floor and Nasmyth access area. Furthermore, on the fixed part a circumferential walkway to access and maintain the rotation system is installed. An upper rotating part called the dome, of a cylindrical shape with a 9.50 m wide observing slit. The observing slit can be closed by two doors. The observing slit is equipped with a windscreen with variable permeability, and louvers are installed in the walls around the circumference of the dome. The dome has the same rotation axis as the telescope azimuth axis and is able to rotate independently of the telescope, without any physical interference at any position of the telescope. The dome includes a crane for a variety of handling tasks, and ladders and access platforms as required for the maintenance of all mechanisms.

Thermal and Flow Control

Given the meteorological forecast for the coming night, the operational model of the telescope will enable the control system to optimize the observing conditions.The decision criteria are:

• Temperature differences with the ambient air inside the enclosure have to be minimized;
• Maximum flushing has to be ensured
• Wind buffeting on the telescope and the primary mirror has to be kept within an acceptable range

The first two requirements aim at minimizing mirror and dome seeing due to temperature inhomogeneities, while the third one is connected to dynamic pressure fluctuations which cause image degradation.

To be able to satisfy these requirements, a number of passive and active systems are available.

Active Thermal Control

Active thermal control in the VLT is effected by:

• Air conditioning the inner volume of the enclosure during the day when the dome is closed. The cooled air is distributed and mixed homogeneously in the entire volume of the enclosure by means of air exhausts distributed along the entire inner surface, including the top part of the dome when the dome itself is locked in the parking position.

  The air conditioning system consists of a secondary chilled water circuit which feeds a number of air treatment units. The secondary chilled water circuit is interfaced by means of a mixing bottle to the central (primary) chilled water circuit of the VLT Telescope Area. The air cooling control system is integrated in the general enclosure control system.

  The heat load of the interior of the closed enclosure has been evaluated considering all components, namely:

  • The heat transfer due to input and infiltration of external air into the enclosure. The latter, in particular its seals and connections, are designed with the aim of achieving an internal maximum air renewal rate of 1 volume per hour in the closed enclosure, taking into account the slight pressurization requirement of ~ 5 mm H2O.
  • The conductive heat transfer from the external surface.
  • The heat generation of occupants.
  • The heat generation of lighting and motors in the enclosure.
  • An additional heat load from the telescope.
• Active cooling of heat sources inside the telescope enclosure (electronic cabinets ...)
• The mirror back plate cooling system

Passive Thermal Control

Passive thermal control in the VLT is effected by:

• Minimizing by adequate insulation and seals the heat input both from the external environment and from the lower enclosure basement.
• Appropriate coating for radiative exchange with the cold sky

A model-based optimization method is usedfor thermal control of the telescope. Given the meteorological forecast for the coming night, the system adjusts the air conditioning temperature set point and the mirror back plate cooling system such that we can optimize the seeing conditions during the night. The procedure is repeated continuously using temperature sensor readings as initial values in the model.

Air flow control

Air flow inside the enclosure at night is determined by the following:

• the ventilation doors in the fixed part of the telescope enclosure:
  • large mirror door (10 m width, 4.2 m height)
  • four smaller doors on the rest of the periphery (each 5 m width, 4.2 m height
• the louvers on the rotating part of the enclosure (representing about 150 square meters)
• the windscreen placed behind the vertical part of the large slit doors

Extensive wind tunnel tests and computational fluid dynamics simulations have been performed to determine the optimal air flow control strategy in order to satisfy the requirements.

Below you can see the VLT in the wind tunnel. The 1:64 model was used to measure the pressure fluctuations on the primary mirror, turbulence intensities near the secondary mirror unit, wind speeds in selected locations,and flushing times, all this for a large number of opening configurations and wind directions. Flow visualizations using a special oil mixture were recorded on tape for later use during telescope commissioning for optimization.