Wind buffeting effects on the Gemini 8m primary mirrors

One of the critical design factors for large telescopes is control of primary mirror distortion caused by wind pressure variations. To quantify telescope wind loading effects, the Gemini Observatory has conducted a series of wind tests under actual mountaintop conditions. During commissioning of the southern Gemini 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. Five-minute data records were made for 116 different test conditions, with a data-sampling rate of ten per second. These data sets have been processed to provide pressure maps over the surface of the mirror at each time instant. From these pressure maps, the optical surface distortions of the primary mirror have been calculated using finite-element analysis. Data reduction programs have been developed to enhance visualization of the test data and mirror surface distortions. The test results have implications for the design of future large telescopes.