Wind buffeting of large telescopes

MacMynowski, Douglas G.; Andersen, Torben

doi:10.1364/ao.49.000625

Cited by 23 publications

(14 citation statements)

References 19 publications

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“…Modeling of the telescope image jitter due to applied wind loads 4 uses the finite element model (FEM) of the telescope structure coupled to a linear optical model, and with the main-axes mount control loops closed (roughly 0.5 Hz bandwidth used here; the final design may have a higher bandwidth) and optical guiding (0.1 Hz used here). The telescope response is predicted in the time-domain; see e.g.…”

Section: Image Jittermentioning

confidence: 99%

“…4 Compliance of the telescope structure itself plays a less important role, and the excitation of structural resonances is not important, nor is motion of M2 relative to its support structure due to pressure gradients across its surface. All of these other sources of image motion might become relatively more important if there was a higher bandwidth optical tip/tilt loop, as this guide loop would correct much of the low-frequency MCS contribution.…”

Section: Image Jittermentioning

confidence: 99%

“…3,4 Previous models of wind loading on the telescope structure for TMT have been based on a parametric model [4][5][6] informed by a combination of Gemini data, 7,8 wind tunnel testing 9 and CFD. Advances in CFD now allow the unsteady forces on the structure and optics to be computed directly, as a function of external wind speed, orientation of the telescope with respect to the wind, and with enclosure vents either open or closed.…”

Section: Introductionmentioning

confidence: 99%

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Unsteady wind loads for TMT: replacing parametric models with CFD

MacMartin

Vogiatzis

2014

SPIE Proceedings

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Unsteady wind loads due to turbulence inside the telescope enclosure result in image jitter and higher-order image degradation due to M1 segment motion. Advances in computational fluid dynamics (CFD) allow unsteady simulations of the flow around realistic telescope geometry, in order to compute the unsteady forces due to wind turbulence. These simulations can then be used to understand the characteristics of the wind loads. Previous estimates used a parametric model based on a number of assumptions about the wind characteristics, such as a von Karman spectrum and frozen-flow turbulence across M1, and relied on CFD only to estimate parameters such as mean wind speed and turbulent kinetic energy. Using the CFD-computed forces avoids the need for assumptions regarding the flow.We discuss here both the loads on the telescope that lead to image jitter, and the spatially-varying force distribution across the primary mirror, using simulations with the Thirty Meter Telescope (TMT) geometry. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are all estimated; these are then used to compute the resulting image motion and degradation. There are several key differences relative to our earlier parametric model. First, the TMT enclosure provides sufficient wind reduction at the top end (near M2) to render the larger cross-sectional structural areas further inside the enclosure (including M1) significant in determining the overall image jitter. Second, the temporal spectrum is not von Karman as the turbulence is not fully developed; this applies both in predicting image jitter and M1 segment motion. And third, for loads on M1, the spatial characteristics are not consistent with propagating a frozen-flow turbulence screen across the mirror: Frozen flow would result in a relationship between temporal frequency content and spatial frequency content that does not hold in the CFD predictions.Incorporating the new estimates of wind load characteristics into TMT response predictions leads to revised estimates of the response of TMT to wind turbulence, and validates the aerodynamic design of the enclosure.

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Section: Image Jittermentioning

confidence: 99%

Section: Image Jittermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Unsteady wind loads for TMT: replacing parametric models with CFD

MacMartin

Vogiatzis

2014

SPIE Proceedings

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“…In some cases the time frames of coupled processes are overlapping enough to justify the development of integrated models. One such case involves wind and telescope structural dynamics, as well as the corresponding control dynamics [16]. For TMT, it was thoroughly investigated, including wind buffeting of the primary mirror [17] and the close interaction between control and structure [18].…”

Section: Modeling Time Framesmentioning

confidence: 99%

Integrated modeling and systems engineering for the Thirty Meter Telescope

Angeli

Vogiatzis

MacMynowski

et al. 2011

Integrated Modeling of Complex Optomechanical Systems

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Modeling is an integral part of systems engineering. It is utilized in requirement validation, system verification, as well as for supporting design trade studies. Modeling highly complex systems poses particular challenges, including the definition and interpretation of system performance, and the combined evaluation of physical processes spanning a wide range of time frames. Our solution is based on statistical interpretation of system performance and a unique image quality metric developed by TMT. The Stochastic Framework and Point Source Sensitivity allow us to properly estimate and combine the optical effects of various disturbances and telescope imperfections.

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“…So on one hand, it is not necessary to increase the size of the single optical components in order to increase the optical performance, on the other hand, due to several sources of disturbance excitations , need arises for an opto-mechanical device to correct the difference in the optical path (optical pathway difference -OPD) between both sides to keep both light paths co-phased, which is necessary for interferometric imaging [6]. While the most important disturbance source is wind excitation [7], significant other contributors can be cooling systems and/or electrical drives [8], for example. For LINC-NIRVANA, this device is a positioncontrolled piezo drive with a travel range of 75 µm.…”

Section: Introductionmentioning

confidence: 99%

Delay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes

Böhm

Pott

Kürster

et al. 2017

IEEE Trans. Contr. Syst. Technol.

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Abstract-In ground-based astronomy, aberrations due to structural vibrations, such as piston, limit the achievable resolution and cannot be corrected using adaptive optics for large telescopes. We present a model-free strategy to estimate and compensate piston aberrations due to vibrations of optical components using accelerometer disturbance feedforward, eventually allowing the use of fainter guide stars both for the fringe detector and in the adaptive optics loop (AO-loop). Because the correction performance is very sensitive to signal delays, we present a strategy to add a delay compensation to the developed disturbance estimator, which can in principle be applied to many other applications outside of astronomy that lack observer performance due to a measurement delay or need a prediction to compensate for input delays. The ability to estimate vibration disturbances in the critical frequency range of 8 Hz to 60 Hz is demonstrated with on sky data from the Large Binocular Telescope Interferometer (LBTI), an interferometer at the Large Bincoular Telescope (LBT). The experimental results are promising, indicating the ability to suppress differential piston induced by telescope vibrations by a factor of about 3 (RMS), which is significantly better than any currently commissioned system.

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Wind buffeting of large telescopes

Cited by 23 publications

References 19 publications

Unsteady wind loads for TMT: replacing parametric models with CFD

Unsteady wind loads for TMT: replacing parametric models with CFD

Integrated modeling and systems engineering for the Thirty Meter Telescope

Delay Compensation for Real Time Disturbance Estimation at Extremely Large Telescopes

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