Shack-Hartmann wavefront sensing with extended sources

Michau, Vincent; Conan, Jean-Marc; Fusco, Thierry; Nicolle, Magalie; Robert, Clélia; Velluet, Marie-Thérèse; Piganeau, E.

doi:10.1117/12.682728

Cited by 16 publications

(22 citation statements)

References 4 publications

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“…Once the images are simulated, we use the algorithms described in [3]. However, for sake of clarity, we present results for three main methods only: thresholded center of gravity (TCoG), the correlation tracking (CORR) [6] and the Weighted Least Squares algorithm (WLS also called matched filtering) proposed by [7]. The threshold value for the normal Thresholding is 10% of the maximum [8] and the threshold for the peak finder in the correlation method is 1% at high SNR and 60% at low SNR.…”

Section: Methodsmentioning

confidence: 99%

Study of centroiding algorithms to optimize Shack-Hartmann WFS in the context of ELTs

Thomas

Lardière

Gavel

et al. 2010

1st AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes

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Abstract. Along with the expected ELTs comes the diversification of Adaptive Optics (AO) systems. Most AO are beginning to make extensive use of laser guide stars (LGS), to increase sky coverage and produce a bright beacon in order to reduce wavefront measurement errors and improve performance. However, because of the larger size of the next generation of telescopes, the elongation seen in a Shack-Hartmann Wavefront Sensor is larger, i.e up to 7" for a 30m telescope. With such an elongation, both photon and read noise will increase and distribution variations of sodium atoms in the sodium layer start to matter. In this paper, we conduct ShackHartmann simulations at the sub-aperture level made with real sodium profiles taken at Lick Observatory. We will compare focus on two methods of centroiders: the matched filter and the correlation. We then compare results with data taken form a real bench at Uvic in the particular case of NFIRAOS AO system for TMT. The goal is to better understand the impact of such variations on the final error budget for the WFS.

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Section: Methodsmentioning

confidence: 99%

Study of centroiding algorithms to optimize Shack-Hartmann WFS in the context of ELTs

Thomas

Lardière

Gavel

et al. 2010

1st AO4ELT Conference - Adaptive Optics for Extremely Large Telescopes

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“…Shift measurements of extended objects in solar AO are calculated in a two-step process (Michau et al 2006). Initially, an integer shift measurement is performed by locating the peak of a crosscorrelation of the image with a reference image (Miura et al 2009).…”

Section: Introductionmentioning

confidence: 99%

Improved shift estimates on extended Shack–Hartmann wavefront sensor images

Townson¹,

Kellerer²,

Saunter³

2015

Mon. Not. R. Astron. Soc.

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An important factor which affects performance of solar adaptive optics (AO) systems is the accuracy of tracking an extended object in the wave-front sensor. The accuracy of a center of mass approach to image shift measurement depends on the parameters applied in thresholding the recorded image, however there exists no analytical prediction for these parameters for extended objects. Motivated by this we present a new method for exploring the parameter space of image shift measurement algorithms, and apply this to optimise the parameters of the algorithm. Using a thresholded, windowed center of mass, we are able to improve centroid accuracy compared to the typical parabolic fitting approach by a factor of 3× in a signal to noise regime typical for solar AO. Exploration of the parameters occurs after initial image cross-correlation with a reference image, so does not require regeneration of correlation images. The results presented employ methods which can be used in real-time to estimate the error on centroids, allowing the system to use real data to optimise parameters, without needing to enter a separate calibration mode. This method can also be applied outside of solar AO to any field which requires the tracking of an extended object.

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“…With extended scenes we have to deal with model errors : the usual way to deal with slopes measurement in this case is to measure the correlation peak between the image of a reference subaperture and the image of the aperture where the slope has to be measured [10,11]. This assumes that both images are identical, which is a valid hypothesis in most applications where high sensitivity is not required.…”

Section: Slopes Measurement Algorithmsmentioning

confidence: 99%

“…With the wavefront maps we used, the usual maximum likelihood wavefront reconstruction method leaded to a very poor sensivitywhile MAP provided the best performance. -σ N : standard deviation of the results due to the image noise It depends also on the scene spatial power spectral density, but in this work we used favorable pictures (see [13] for a study of σ N ). Since these errors can reasonably be assumed to be independent, the error budget is: The value of each of these errors depends on design hypothesis and wavefronts.…”

Section: Theoretical Study Conclusion: Error Budget and Design Comentioning

confidence: 99%