GSMT Assembly AURA New Initiatives Office

Technical Studies: Wind Loading

The Gemini South Wind Load Test

Wind loading will be a key factor in the structural design of the Giant Segmented Mirror Telescope (GSMT). Large telescopes are particularly susceptible to wind loading - as telescopes get larger, the structural deflections caused by the wind loading increase and the resonant frequencies of the telescope structure decrease. The lower resonant frequencies are closer to the peak frequencies of the wind velocity spectra, which increases dynamic amplification of the disturbance.

Wind loading of the telescope structure can affect pointing and tracking, and it can also produce uneven pressure loading that distorts the figure of the primary mirror. However, some wind flow across the telescope is desirable to avoid problems with thermally-induced local seeing. The benefits of ventilation must be balanced against the problems caused by wind buffeting. To understand how wind buffeting affects the performance of large telescopes, we need measurements conducted under actual mountaintop conditions at as large a scale as possible.

During the integration of the second Gemini 8-meter Telescope in 2000, there was an opportunity to conduct a series of wind tests while the dummy primary mirror was in the telescope. Ultrasonic anemometers were installed above the dome, behind the secondary mirror assembly, and at three points around the primary mirror. The surface of the dummy primary mirror was instrumented with pressure sensors (24 for the first series of tests, then 32 for the second series). During the first series of tests, several dozen sensitive accelerometers were placed at various points on the telescope structure by our collaborators from the Modal Analysis & Controls Laboratory of the University of Massachusetts Lowell. Simultaneous measurements of telescope acceleration, wind velocity and pressure on the primary mirror were recorded at 10 Hz or faster. The locations of the anemometers and pressure sensors are indicated in the figure below.

Locations of anemometers and pressure sensors

For these tests the Gemini Telescope was a particularly good test bed for the following reasons:

Since the tests were conducted, the data have been analyzed in several ways. The following sections contain results from these analyses, and more results will be posted in this section in the near future.


 Wind buffeting effects on the Gemini 8m primary mirrors

Hex segment bullet Gemini South Wind Tests, May 2000

Hex segment bullet Structure Functions

Hex segment bullet Animations

Hex segment bullet Wind Pressure and Wind Velocity

Jennifer Purcell / / February 19, 2002