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The isoplanatic patch of the human eye is a key parameter for the adaptive optics system (AOS) designed for retinal imaging. The field of view (FOV) usually sets to the same size as the isoplanatic patch to obtain high resolution images. However, it has only been measured at a specific wavelength. Here we investigate the wavelength dependence of this important parameter. An optical setup is initially designed and established in a laboratory to measure the isoplanatic patch at various wavelengths (655 nm, 730 nm and 808 nm). We established the Navarro wide-angle eye model in Zemax software to further validate our results, which suggested high consistency between the two. The isoplanatic patch as a function of wavelength was obtained within the range of visible to near-infrared, which can be expressed as: θ =0.0028 λ − 0 . 74 . This work is beneficial for the AOS design for retinal imaging. [ABSTRACT FROM AUTHOR]

The interaction matrix significantly affects the correction accuracy of wavefront correctors. For diffractive liquid crystal wavefront correctors (LCWFCs), the diffraction wavefront error must be taken into account to improve the accuracy of the interaction matrix. In this paper, a tunable Zernike polynomial coefficient method is demonstrated that decreases the effect of the diffraction wavefront error on the interaction matrix. Moreover, to eliminate the effect of the random error, a least squares method is combined with the tunable coefficient method. Experimental results show that, with the combined method, the error of the interaction matrix is decreased by a factor of 2.5 compared to that when the least squares method is used alone. Furthermore, an open loop adaptive correction experiment was performed and a considerable improvement of the image resolution was obtained by the novel method. Therefore, this method is very useful for liquid crystal adaptive optics systems to acquire the high correction accuracy. [ABSTRACT FROM AUTHOR]

An experimental analysis was conducted to investigate the fitting error of diffractive liquid crystal wavefront correctors (LCWFCs). First, an experiment was performed to validate the theoretical equations presented in our previous work Cao et al., 2009 [9] . The results showed an apparent discrepancy between the theoretical and measured results for the fitting error. This difference was examined and the influence of nonlinearities and rounding errors generated by the LCWFC was analyzed and discussed. Finally, the fitting error formula of the LCWFC was modified to obtain a more effective tool for the design of LCWFCs for atmospheric turbulence correction. These results will be useful for researchers who design liquid crystal adaptive optics systems for large-aperture ground-based telescopes. [ABSTRACT FROM AUTHOR]

It has been a crucial technique to improve the dynamic response characteristics of a liquid crystal wavefront corrector (LCWFC) with optimal cell gap since the LCWFC needs at least 2π(orπ) phase modulation in adaptive optics systems (AOSs). We have given a complete process for obtaining the optimal cell gap accurately from a single photoelectric measurement, which can be conducted with a liquid crystal (LC) cell of any known thickness. This method has been analysed theoretically and confirmed experimentally by using a wedge-shaped cell; the experimental results match very well with the theoretical analysis. The response time of an optimal gap cell can be a novel evaluation method of response performance of LC materials. [ABSTRACT FROM PUBLISHER]

We present a weighted method of wavefront reconstruction for multiple laser guide stars (LGS) adaptive optics (AO) system. Unlike simple averaging method in the traditional ground layer adaptive optics (GLAO), the weighted method generates different weighting factors according to LGSs’ different positions for the reconstructed wavefront from different LGSs. And then the target aberration to correct could be obtained with summation of these wavefronts multiplied by their correspondent weighting factors. Wide-field simulations are presented and analyzed. Experiments in the lab are conducted to validate the method. The weighted method is better than the average method especially near the center area of FOV. At center of FOV, the SR is improved by about 0.2 compared to the average method and the SR decreases with enlargement of FOV. For the first time, the weighted wavefront reconstruction method effectively decreases the reconstruction error in multi-LGSs AO system. The simulation and experimental result show that weighted method is also promising to improve the imaging resolution of LGS adaptive optics systems in astronomical observations. [ABSTRACT FROM AUTHOR]

Adaptive optics systems (AOSs) rely on good estimation of a distorted wavefront. Two problems make centroid detection difficult, namely, the problem of a relatively large subaperture that brings high-order aberrations and that of a large laser guide-star spot that brings in a large photon noise. To solve these problems, we proposed an estimation method based on Poisson statistics to reduce the photon noise, and the high-order aberrations are partly separated from a spot image to well estimate the slope in the subaperture. Simulation results indicate that our method performs better than the weighted center-of-gravity (WCOG) method and correlation method (CM). Under the condition of SNR =20 and r 0 =13cm The root-mean-square values of the residual wavefront using the proposed method are approximately 10%, 50%, and 35% lower than those of the COG, WCOG, and CM, respectively. The proposed method would be highly beneficial for ground-based astronomical AO systems intended to provide moderate to high Strehl ratios at near infrared wavelengths on larger than 10m-aperture telescopes. [ABSTRACT FROM AUTHOR]

Electron multiplying charge-coupled-device (EMCCD) applied in Shack–Hartmann wavefront sensor (S–H WFS) makes the wavefront sensing more efficient for adaptive optics (AO). However when the brightness of the observed target changes in large ranges in a few minutes, a fixed electron multiplying (EM) gain may not be optimum. Thus an auto-gain-control (AGC) method based on the spots image of the S–H WFS is proposed. The designed control value is the average value of the maximum signals of all the light spots in a frame. It has been demonstrated in the experiments that the control value is sensitive to the change of the target brightness, and is stable in the presence of detecting noises and turbulence influence. The goal value for control is predetermined based on the linear relation of the signal with the EM gain and the number of photons collected in sub-apertures. The conditions of the self-protection of the EMCCD are also considered for the goal value. Simulations and experiments indicate that the proposed control method is efficient, and keeps the sensing in a high SNR which reaches the upper SNR limit when sensing with EMCCD. The self-protection of the EMCCD is avoided during the whole sensing process. [ABSTRACT FROM AUTHOR]

Abstract: To process real-time wavefront signal in our liquid crystal adaptive optics system (LCAOS), a real-time wavefront processor (RTWP) with graphics processing unit (GPU) accelerator is developed. A series of parallel acceleration algorithm is implemented to reconstruct wavefront, compute gray map and process lookup table (LUT). Especially, an algorithm by using symmetry of Zernike polynomial is designed to compute gray map and it provides a speedup of 3.16. Finally, the total computing time in our RTWP is 109μs and temporal bandwidth in our LCAOS is 45Hz; it is shown that our RTWP has the ability to real time process wavefront signal in our LCAOS. [Copyright &y& Elsevier]

Abstract: The optimal cell gap of nematic liquid–crystal wavefront corrector (LCWFC) is proved theoretically by numerical computation of the Ericksen–Leslie equation based on finite differences and confirmed experimentally. The numerical results of optimizing cell gap overcome the constraints of many approximations e.g. the small angle approximation and agree much better with the experimental data than those of analytical solution. Therefore, this method supplies a useful tool to achieve the optimal cell gap of a nematic LCWFC with high precision. Together with high birefringence and low rotational viscosity liquid crystal (LC) material effect, a nematic LCWFC with response time of sub-millisecond can be achieved in the visible spectral region. [Copyright &y& Elsevier]