There is little effect from wind on the performance of telescopes enclosed in large, oversize domes like the CFHT or the ESO 3.6-m (cf. chapter ). In fact one of the main design criteria of those domes was the prevention of any wind loading on the telescope by leaving a large distance between the telescope tip and the slit and including movable windscreens to close those sectors of the slit that are not in the field of observation. The only wind tunnel study performed for these domes concerned static load measurements on a model of the CFHT telescope ([ENSAM]) which concluded that wind loads were indeed negligible on the telescope.
After that the MMT showed the favorable effects of wind on seeing, several studies were performed on enclosure design concepts in which the telescope volume was flushed by the wind flow. In particular some water tunnel tests were performed by Japanese and US teams searching for the best arrangement of venting openings in enclosures of hemispherical, cylindrical and various polyhedral form ([Ando], [Siriluk]).
However, these studies generally overlooked the importance of the effect of the slit induced turbulence, first put in evidence by [Zago 85] in a preliminary analysis of the wind tunnel measurements of the NTT building. While venting definitely improved dome seeing, in the context of the latest projects for 8-m telescopes it could diminish the telescope performance in two respects:
It may be useful to underline two aspects that make the subject peculiar with respect to more conventional wind loading engineering. The first aspect is the extreme smallness of critical structural deflections that must be evaluated and ultimately minimized by proper engineering. With an aimed guiding accuracy of 0.3 arcsec rms, the tip of an 8-m telescope shall keep its deflection under wind to less than 20 m rms . Therefore optical telescopes are not operated with wind speeds exceeding 80 or 100 km/hr, as in these conditions the enclosure is closed. One of the objectives of this research is the determination of the relatively low wind loads acting on a telescope in a way that is both accurate and suitable for input in parametric studies of the telescope performance. Similarly, the quality of the large primary mirrors starts deteriorating when deflections are greater than 200 nanometers, and the aberration depends strongly on the modal shape of the loading.
The second peculiar aspect is consequent to the fully automated active control of guiding and mirror support. These system have allowed a tremendous progress in maintaining the accuracy of the telescope over the exposure times of the observations but are able to compensate external loads only within a certain frequency bandwidth. Therefore one is interested in particular in the accurate characterization of the high frequency component of wind load, even when this constitutes a small part of the overall load.