Modeling wind-buffeting of the Thirty Meter Telescope

Abstract

The Thirty Meter Telescope project is designing a 30 m diameter ground-based optical telescope. Unsteady wind loads on the telescope structure due to turbulence inside the telescope enclosure impact the delivered image quality. A parametric model is described that predicts the optical performance due to wind with sufficient accuracy to inform relevant design decisions, including control bandwidths. The model is designed to be sufficiently computationally efficient to allow rapid exploration of the impact of design parameters or uncertain/variable input parameters, and includes (i) a parametric wind model, (ii) a detailed structural dynamic model derived from a finite element model, (iii) a linear optical response model, and (iv) a control model. Model predictions with the TMT structural design are presented, including the parametric variation of performance with external wind speed, desired wind speed across the primary mirror, and optical guide loop bandwidth. For the median mountaintop wind speed of 5.5 m/s, the combination of dome shielding, minimized cross-sectional area, and control results in acceptable image degradation.

Wind loads on ground-based telescopes

“…While the enclosure surrounding the telescope provides a significant reduction in wind speeds, the residual wind loads that result from large-scale flow structures and turbulence inside the enclosure may still lead to significant vibration. 1,2 Estimates of the wind loads are required early in the design process, influencing the design of the enclosure, telescope structure, and control system. The wind parameterization herein reflects our best current understanding of the wind environment inside a roughly hemispherical telescope enclosure, informed by three separate sources of data.…”

“…The optical consequences due to unsteady forces result primarily from the motion and deformation of M1 and M2 and can be computed from an integrated model that predicts the actively controlled structural response. 2 The performance is less sensitive to the motions of M3 and Nasmyth platforms, and forces on these surfaces are not estimated. Wind loads on the dome could also be transmitted to the structure, although this effect is neglected herein.…”

Wind buffeting of large telescopes

“…The model summarized below originates from a generic set of simulation tools first applied for the proposed Euro50 extremely large telescope [17,18] and then later expanded and applied to the European Extremely Large Telescope (E-ELT) [8]. A few specific aspects have been explored in more detail in a separate integrated model described in [9,10]; these include the effect of the guide loop, and a more detailed mount control design to maintain bandwidth with lower structural damping.…”

“…This is particularly relevant as telescopes get larger, while the desired performance gets more stringent. The current design efforts for extremely large telescopes have resulted in substantial progress in understanding the wind environment within the telescope enclosure [3][4][5][6][7] and in detailed integrated modeling to understand the effects of wind on performance [8][9][10].…”

Segmented Deformable Mirrors for Ground Layer Adaptive Optics

“…MacMynowski [7] suggests that the wind speed at the secondary mirror is ¼ the wind speed at the site. Under these conditions, the power spectrum due to wind and reaction forces tracking the atmospheric wavefront are shown in figure 3 for a wind speed at the site of 15 m/s.…”