In a typical telescope the top ring with the spiders
(see fig. 
) represents the largest drag
contribution to the aerodynamic torque acting on the telescope.
Therefore
measurements of the flow characteristics near the upper
part of the telescope are representative of the overall aerodynamic
action which influences the guiding performance.
The guiding rms
error will depend on the amplitude of the torque
spectra about the two axes of the telescope -
equations () and ().
Inserting equation () in () the torque
spectrum about any axis y can be expressed as:
We see that 
is a function of the static torque coefficient 
=
, and of
the three flow parameters:
, speed rms
, turbulence length scale
and 
in various types of enclosures,
in the region of the upper part of the telescope:
The measurements in the 3.6-m dome were performed by the author with
the help of F. Rigaut, who at the time was serving as a
cooperant at La Silla.
The wind tunnel studies were defined and coordinated by the author
and contracted by ESO to specialized laboratories equipped with a
boundary layer wind tunnel which allows a realistic simulation of the
wind turbulence.
