Appendix 5.5.d Characterization of Wind Loading of Telescopes

Ground-based telescopes operate in a turbulent atmosphere that affects the optical path across the aperture by changing both the mirror positions (wind induced vibrations) and the air refraction index. Although the characteristics of the atmosphere are well understood in the inertial range, the validity of the homogeneous, isotropic field assumption is questionable inside the enclosure and in the close vicinity of the structure. To understand the effect of wind on an actual telescope, we conducted extensive wind measurements at the Gemini South Telescope. Simultaneous measurements were made of pressures at multiple points on the mirror surface, as well as wind velocity and direction at several locations inside and outside the dome. During the test we varied the dome position relative to the wind, the telescope elevation angle, the position of windscreens in the observing slit, and the size of the openings in the ventilation gates. The data sets have been processed to provide the temporal and spatial characteristics of the pressure variations on the primary mirror in comparison to the theory of atmospheric turbulence. Our investigation is part of an effort leading to the development of a scalable wind model for large telescope simulations, which describes the forces due to air turbulence on the primary mirror and telescope structure reasonably well even inside an enclosure.