In the previous section we have found a general, if approximate relationship between the average rms of pressure fluctuations on a primary mirror and the optical aberrations caused by dynamic deformations. It is also of interest to establish correlations between the wind speed measured on the mirror and the pressure fluctuations so that the ventilation of the enclosure can be planned optimally.
In fact the wind has a twofold effect on the mirror: on the one hand
it affects negatively the optical performance by deforming the mirror,
on the other hand it has a positive effect as it decreases mirror
seeing
. We will present here some wind
tunnel measurements taken for the VLT cylindrical enclosure.
Figure: Photograph of test model