Closed-Loop Active Optics: Its Advantages And Limitations For Correction Of Wind-Buffet Deformations Of Large Flexible Mirrors

Wilson, R. N.; Noethe, L.

doi:10.1117/12.960834

Cited by 6 publications

(5 citation statements)

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“…But in the extended active bandpass with 1/30 Hz < / WB < 10 Hz there is a central problem of detection and measurement of the WB effects on the mirror. This was discussed in detail by Wilson and Noethe ( 1989) and is a consequence of the isoplanatic angle of the atmospheric seeing. In the bandpass A of Fig.…”

Section: Introductionmentioning

confidence: 95%

“…The bandpass B of Fig. 1, roughly continuing from the upper limit of A at 1/30 Hz to somewhere between 1 and 10 Hz, was termed the extended áctive optics bandpass specifically in connection with the wind-buffeting correction (WBC) of large thin primaries (Wilson and Noethe 1989). The temporal frequencies and spatial modes of wind-buffeting deformations induced by typical wind gust spectra will depend not only on the direction and power spectrum of the wind itself, but also on a complex interplay of the telescope enclosure (with or without windscreens), the obstruction of the telescope mechanics, the thin mirror block as a low-pass filter, and the damping effects of axial and radial supports or other constraints of the cell.…”

Section: Introductionmentioning

confidence: 99%

“…But it is useless for the field center where the observation takes place. Theoretically, a separation of the effects of WB, which are constant over the field, and the effects of the atmosphere, which vary over the field, can be made statistically using a number of image analyzers (Wilson and Noethe 1989). But in practice, this technique is too inefficient as the separation gain only goes as p 1/2 , where p is the number of image analyzers.…”

Section: Introductionmentioning

confidence: 99%

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Active correction of wind-buffeting deformations of thin telescope primaries in the extended active optics bandpass

Wilson¹,

Franza²,

Noethe³

et al. 1993

PASP

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Wind buffeting can produce deformation of thin meniscus primaries, above all for large sizes. At low temporal frequencies, below 1/30 Hz, these deformations can be monitored and corrected by the normal closed-loop active optics system using a natural star in the field as reference. However, the temporal frequency bandpass of wind-buffeting deformation can extend into the extended active optics bandpass ( 1/30 to about 10 Hz). In this bandpass, a natural star near the edge of the field gives wrong information for the field center because of overlap with the atmospheric bandpass and the limitation of the isoplanatic angle. An independent detection system for this bandpass is therefore required. This paper reviews the possibilities for an independent detection system. We conclude that the most promising are a group of optical detection systems operating from the front face of the primary. The application is considered for the ESO VLT 8 m primaries with an aspect ratio of 47. Some of the front surface detection methods discussed may also be highly relevant to the active control of radio telescopes. The front detection systems can also detect the influence of "tube seeing" in a functional arrangement, i.e., on-line during observing. Correct measurement of the errors induced by wind buffeting of primaries is an essential prerequisite to their active correction. The design of actuators to achieve this correction is a separate technical issue not treated in this paper.

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Section: Introductionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Active correction of wind-buffeting deformations of thin telescope primaries in the extended active optics bandpass

Wilson¹,

Franza²,

Noethe³

et al. 1993

PASP

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“…However, turbulence in the direction of the guide star is uncorrelated with the turbulence over most of the seeing-limited telescope field of view, and thus wave-front information provides a noisy estimate of mirror deformations. 7,8 The control matrices for relative displacement sensors for large segmented-mirror telescopes have been developed by Chanan et al, 9 from which the noise propagation characteristics have been obtained. In this paper we provide a quantitative assessment of the resulting contribution to both the seeing-limited and the diffraction-limited error budgets of the telescope as a function of the bandwidth of active control.…”

Section: Introductionmentioning

confidence: 99%

Measurement accuracy in control of segmented-mirror telescopes

MacMartin¹,

Chanan²

2004

Appl. Opt.

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Design concepts for future large optical telescopes have highly segmented primary mirrors, with the out-of-plane degrees of freedom actively controlled. We estimate the contribution to errors in controlling the primary mirror that results from sensor noise and, in particular, compare mechanical measurements of relative segment motion with optical wave-front information. Data from the Keck telescopes are used to obtain realistic estimates of the achievable noise due to mechanical sensors. On the basis of these estimates, mechanical sensors will be more accurate than wave-front information for any of the telescope design concepts currently under consideration, and therefore supplemental wave-front sensors are not required for real-time figure control. Furthermore, control system errors due to sensor noise will not significantly degrade either seeing-limited or diffraction-limited observations.

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“…Active control algorithms have been optimized for large ground telescopes and are commonly used to compensate for manufacturing errors, gravitational and thermal distortions, and low-frequency errors induced by wind. [1][2][3][4][5] By comparison, active control algorithms for space telescopes are less mature. The baseline control scheme for the first large active optical/infrared space telescope, the James Webb Space Telescope (JWST), is conceptually simple, consisting of measuring the wavefront error (WFE) every 2 days and using these measurements to apply corrections every 2 weeks as needed.…”

Section: Introductionmentioning

confidence: 99%

Improving active space telescope wavefront control using predictive thermal modeling

Gersh-Range

Perrin

2014

J. Astron. Telesc. Instrum. Syst

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Abstract. Active control algorithms for space telescopes are less mature than those for large ground telescopes due to differences in the wavefront control problems. Active wavefront control for space telescopes at L2, such as the James Webb Space Telescope (JWST), requires weighing control costs against the benefits of correcting wavefront perturbations that are a predictable byproduct of the observing schedule, which is known and determined in advance. To improve the control algorithms for these telescopes, we have developed a model that calculates the temperature and wavefront evolution during a hypothetical mission, assuming the dominant wavefront perturbations are due to changes in the spacecraft attitude with respect to the sun. Using this model, we show that the wavefront can be controlled passively by introducing scheduling constraints that limit the allowable attitudes for an observation based on the observation duration and the mean telescope temperature. We also describe the implementation of a predictive controller designed to prevent the wavefront error (WFE) from exceeding a desired threshold. This controller outperforms simpler algorithms even with substantial model error, achieving a lower WFE without requiring significantly more corrections. Consequently, predictive wavefront control based on known spacecraft attitude plans is a promising approach for JWST and other future active space observatories. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

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Closed-Loop Active Optics: Its Advantages And Limitations For Correction Of Wind-Buffet Deformations Of Large Flexible Mirrors

Cited by 6 publications

References 0 publications

Active correction of wind-buffeting deformations of thin telescope primaries in the extended active optics bandpass

Active correction of wind-buffeting deformations of thin telescope primaries in the extended active optics bandpass

Measurement accuracy in control of segmented-mirror telescopes

Improving active space telescope wavefront control using predictive thermal modeling

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