Extended object wavefront sensing based on the correlation spectrum phase

Knutsson, Per; Owner-Petersen, Mette; Dainty, Chris

doi:10.1364/opex.13.009527

Cited by 19 publications

(8 citation statements)

References 7 publications

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“…That is, because of the assumption of periodicity implicit in finite-size Fourier transforms, for large shifts, structures in one image are not matched with structures at the shifted location but with structures shifted in from the opposite side of the image. Knutsson et al (2005) derived a method based on the Fourier spectrum of the correlation function (correlation spectrum phase). It is supposed to give results similar to the CFF method but with some accuracy sacrificed for speed.…”

Section: Correlation Algorithmsmentioning

confidence: 99%

Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors

Löfdahl

2010

A&A

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Context. Solar Shack-Hartmann wavefront sensors measure differential wavefront tilts as the relative shift between images from different subapertures. There are several methods in use for measuring these shifts. Aims. We evaluate the inherent accuracy of the methods and the effects of various sources of error, such as noise, bias mismatch, and blurring. We investigate whether Z-tilts or G-tilts are measured. Methods. We test the algorithms on two kinds of artificial data sets, one corresponding to images with known shifts and one corresponding to seeing with different r 0 . Results. Our results show that the best methods for shift measurements are based on the square difference function and the absolute difference function squared, with subpixel accuracy accomplished by use of two-dimensional quadratic interpolation. These methods measure Z-tilts rather than G-tilts.

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Section: Correlation Algorithmsmentioning

confidence: 99%

Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors

Löfdahl

2010

A&A

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“…11 The "phase correlation" technique estimates the displacement of the correlation figure in the Fourier domain, thereby considerably reducing the computational cost of the correlation; [12][13][14] however, it has been reported to be very susceptible to noise 12 and is only adequate for small displacements between live and reference images due to the Fourier phase wrapping around the −π to π range, which leaves no reliable phase values when the displacement is of the order of half the FoV, the 2D-DFT is the same size as the FoV and in the presence of noise. 13 In contrast, the "periodic correlation" technique, consisting of obtaining the correlation figure in the Fourier domain with 2D-DFTs of the same size as the live image, has reported to be adequate only when the image quality is good, due to spatial aliasing. 12 Table 1 shows the results of computational requirement estimations for an example of SHWFS with 80 × 80 subapertures, 16 × 16 detector pixels per subaperture, the imposed condition of finishing the complete centroids computation in 100 μs, and for the four centroiding methods.…”

Section: Comparison Of Weighted Fourier Phase Slopementioning

confidence: 99%

Simulations and laboratory performance results of the weighted Fourier phase slope centroiding algorithm in a Shack–Hartmann sensor

Chulani

Rodríguez-Ramos

2018

Opt. Eng.

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The performance of the "weighted Fourier phase slope" centroiding algorithm at the subpupil image of a Shack-Hartmann wavefront sensor for point-like astronomical guiding sources is explored. This algorithm estimates the image's displacement in the Fourier domain by directly computing the phase slope at several spatial frequencies, without the intermediate step of computing the phase; it then applies optimized weights to the phase slopes at each spatial frequency obtained by a Bayesian estimation method. The idea was inspired by cepstrum deconvolution techniques, and this relationship is illustrated. The algorithm's tilt estimation performance is characterized and contrasted with other known centroiding algorithms, such as thresholded centre of gravity (TCoG) and cross correlation (CC), first through numerical simulations at the subpupil level, then at the pupil level, and finally at the laboratory test bench. Results show a similar sensitivity to that of the CC algorithm, which is superior to that of the TCoG algorithm when large fields of view are necessary, i.e., in an open-loop configured adaptive optics system, thereby increasing the guide star limiting magnitude by 0.6 to 0.7 mag. On the other side, its advantage over the CC algorithm is its lower computational cost by approximately an order of magnitude.

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“…To accelerate the computations, shifts of these maxima could be estimated by two-di mensional mutual inte nsity spectra at s ub-apertures of current and reference frames [9]. After finding the integer coordinates , m m i j of t he maximum points of mutual correlation function, their values are adjusted using the quadratic approximation…”

Section: Algorithms For Local Wavefront Slopes Estimationmentioning

confidence: 99%

Development of adaptive optics elements for solar telescope

et al. 2012

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The devices and components of adaptive optical system ANGARA, which is developed for image correction in the Big solar vacuum telescope (BSVT) at Bayk al astrophysical obse rvatory are descri bed. It is shown t hat the use of modernized adaptive system on BSVT not only reduces the turbulent atmospheric distortions of image, but also gives a possibility to improve the telescope de veloping new methods of solar observations. A high precision Shac k-Hartmann wavefront (WF) sensor has been de veloped on the basis of a low-aperture off-axis diffraction lens array. The device is capable of measuring WF slopes at array sub-apertures of size 640X640 μm with an error not ex ceeding 4.80 arc.sec. Also the modification of this sensor for adaptive system of solar telescope using extended scenes as t racking objects, such as sunspot, pores, solar granulation and limb, is presented. The software package developed for the proposed WF sensors includes three algorithms of lo cal WF slopes esti mation (modified centroids, normalized cross-correlation and fast Fourier-demodulation), as wel l as t hree methods of WF reconstruction (modal Zernike polynomials expansion, deformable mirror response functions expansion and phase unwrapping), that can be selected during operation with accordance to the application.

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Extended object wavefront sensing based on the correlation spectrum phase

Cited by 19 publications

References 7 publications

Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors

Evaluation of image-shift measurement algorithms for solar Shack-Hartmann wavefront sensors

Simulations and laboratory performance results of the weighted Fourier phase slope centroiding algorithm in a Shack–Hartmann sensor

Development of adaptive optics elements for solar telescope

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