Dynamic analysis of TMT

Abstract

Dynamic disturbance sources affecting the optical performance of the Thirty Meter Telescope (TMT) include unsteady wind forces inside the observatory enclosure acting directly on the telescope structure, unsteady wind forces acting on the enclosure itself and transmitted through the soil and pier to the telescope, equipment vibration either on the telescope itself (e.g. cooling of instruments) or transmitted through the soil and pier, and potentially acoustic forces. We estimate the characteristics of these disturbance sources using modeling anchored through data from existing observatories. Propagation of forces on the enclosure or in support buildings through the soil and pier to the telescope base are modeled separately, resulting in force estimates at the telescope pier. We analyze the resulting optical consequences using integrated modeling that includes the telescope structural dynamics, control systems, and a linear optical model. The dynamic performance is given as a probability distribution that includes the variation of the external wind speed and observing orientation with respect to the wind, which can then be combined with dome seeing and other time-or orientation-dependent components of the overall error budget. The modeling predicts acceptable dynamic performance of TMT.

Wind buffeting of large telescopes

“…The frequency-dependent spatial decorrelation to predict the force spectrum can be obtained assuming von Karman pressure statistics and frozen turbulence. This derivation previously appeared in [10], and is similar to that in [4] but more directly applicable to the assumptions made herein regarding the geometry and wind speed characteristics.…”

“…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.…”

“…A thorough analysis of wind loads on all of the upper members of the telescope has been conducted [10], including the effects of reduced wind speed further from the aperture, as well as reduced moment arm. The resulting wind spectrum is almost identical to the simpler model developed herein, as shown in Fig.…”

“…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].…”

“…The wind also results in image degradation due to optics misalignment (typically small) and from the motion of the primary mirror (M1) segments caused by forces on M1. While this latter response may be significant [10], we focus on image jitter.…”