In a typical telescope the top ring with the spiders (cf. fig. ) represents the largest aerodynamic drag contribution to the torque. 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:
Thus is a function of the (static) torque coefficient = , and of the three flow parameters:
The measurements in the 3.6-m dome were performed by the author with
the help of F. Rigaut, who at the time was cooperant at La Silla.
The wind tunnel studies were defined and coordinated by the author
and contracted by ESO to specialized laboratories.