Synthesis

The main characteristics of the wind flow acting on a telescope in different types of enclosures have been investigated. The main conclusions of this study are:

• In conventional domes the turbulence created by the slit edges will generally not disturb the telescope's guiding. In the cases where the dome is oversized with respect to the telescope the fluctuations of dynamic pressure on the telescope are negligible and windscreens are not required. Only if the dome is fitted very tightly on the telescope there will be occasionally periods and orientations in which a windscreen will be needed.

• In the new types of through-flow enclosures, when no windscreen is set across the slit there is a critical range of azimuth angles between 20 and 40 in which the turbulence created by the slit is much larger than the one present in the free wind flow. In that range the absolute turbulence intensity on the telescope is higher for the NTT ( 0.45) than for the cylindrical enclosure ( 0.25), possibly because of the amplifying effect of the parallel walls. For azimuth angles and the turbulence intensity remains around 0.1 .

• Windscreens with a permeability of 50% decrease by about half the mean speed but only marginally the turbulence intensity. Windscreens with a permeability of 20% decrease the mean speed to about 20 % and limit also the turbulence intensity to about 0.1 .

• The turbulence length scale of the flow acting on the telescope is related to the slit width with factors varying from 0.4 to 1 in the different configurations without windscreen, for azimuth 0 when the telescope is facing the wind. Increasing the azimuth angle, the ratio length scale/slit width decreases respectively to below 0.2 in both enclosures, with large variations depending on the setting of the louvers allowing or denying through-flow in the enclosure. The low values of length scale will cause the wind spectrum at the telescope to have a peak in the range of 1 to 10 Hz (see equation ()), where the amplification of the fluctuating term of the dynamic pressure with respect to the free wind flow conditions reaches one order of magnitude. As these frequencies will generally be close to or above the bandwidth of the telescope servo system, maintaining a good guiding performance of the telescope will require the use of a 20% windscreen to limit the pressure fluctuations by dramatically reducing the mean flow speed (see equation ()).