Gemini Preprint #78

Comparison of Pressure Measurements and Operating Data for Wind Excitation of Telescope Structures

David R. Smith, Keith Weech, Johann Teutsch, Peter Avitabile
Modal Analysis and Controls Laboratory, University of Massachusestts Lowell, Lowell MA 01854

Geoff Gwaltney
Keweenaw Research Center, Michigan Technological University

Michael Sheehan, Myung Cho
Gemini Observatory


The performance of large, ground-based optical telescopes is limited by the quality of the image that can be delivered to the final detector (or camera). The telescope and its protective dome create atmospheric variations due to thermal mismatch with the local environment. This "dome seeing" effect reduces performance because it results in a blurring of the incoming image. This effect can be minimized by allowing bulk airflow across the mirror, but this comes at the price of wind buffeting of the structure, which also reduces performance. Operation of a world-class telescope requires an understanding of this performance trade-off in order to assure the users of the facility the best possible data.

The current stage of construction of the Gemini South 8m (26 ft) telescope in Chile provided a unique opportunity to make simultaneous measurements of wind pressure and structural response. Further, the Gemini dome design allows for a wide range of flow configurations, from nearly enclosed to almost fully exposed to the wind. We measured the operating response of surface pressures on the primary mriror cell as well as more than 70 channels of accelerometers on the telescope structure. The data were taken in a variety of dome configurations and relative wind directions.

Since the wind pressure measurements will not be available during telescope operation, an understanding of the relationship between these pressures and other measurable quantities is essential to real time monitoring of the wind buffeting of the mirror. In this paper, we present the comparison of the wind pressure measurements with the structural response. Further, we present comparisons between the wind pressure measurements and the wind speed at various locations inthe dome.

Appeared in Sound & Vibration, June 2001.

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Ruth A. Kneale / / November 2, 2001