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Wind loading on telescopes

In a typical telescope the top ring with the spiders (cf. fig. gif) 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 (gif) and (gif). Inserting equation (gif) in (gif) 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:

In the following sections, we present the results of measurements of the parameters U, and in various types of enclosures, in the region of the upper part of the telescope:
Measurements of wind turbulence in the dome of the ESO 3.6-m telescope.
Wind tunnel tests on a 1/50 model of the NTT enclosure.
Wind tunnel tests on a 1/64 model of the VLT cylindrical enclosure.
Wind tunnel tests on a 1/80 model of the alternative retractable enclosure for the VLT.

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.

Lorenzo Zago,, Sun Feb 26 22:57:31 GMT+0100 1995