We propose a speckle imaging algorithm in which we use the improved form of spectral ratio to obtain the Fried parameter, we also use a filter to reduce the high frequency noise effects. Our algorithm makes an improvement in the quality of the reconstructed images. The performance is illustrated by computer simulations.
An imaging system is constructed by atmosphere turbulence and ground-based telescope when the latter is used to observe a space object. The wavefront measurement produced by adaptive optics system can be used to estimate the point spread function (PSF) of the imaging system since it contains the wavefront aberration information of the light from the object. But the detector noise of the wavefront sensor (WFS) will inevitably bring estimation error. Based on the statistical theory, a method is presented to improve the PSF estimation accuracy by eliminating the noise error from the wavefront measurement. The numerical simulation shows that the estimation error of this method could be lower than 10%. It also indicates that the higher the signal-noise ratio (SNR) of the WFS is, the more frames of the wavefront measurements are used, and the bigger the Fried constant is, the more accurate the estimation will be. The work in this paper can be applied to performance evaluation of imaging system, deconvolution of AO images, as well as photometric analysis of space object.
The wavefront sensor is used in adaptive optics (AO) to detect the atmospheric distortion, which feeds back to the deformable mirror to compensate for this distortion. While the Shack–Hartmann sensor has been widely used, the plenoptic sensor was proposed in recent years. The two different wavefront sensing methods have different interpretations and numerical consequences, though they are both slope-based. The plenoptic sensor is compared with the Shack–Hartmann sensor in a closed-loop AO system. Simulations are performed to investigate their performances under closed-loop conditions. The plenoptic sensors both without and with modulation are discussed. The results show that the closed-loop performance of the plenoptic sensor without modulation is worse than that of the Shack–Hartmann sensor when the star for observation is brighter than magnitude 7, but better when the star is fainter. The closed-loop performance of the plenoptic sensor could be improved by modulation, except for the faint star. In summary, the limiting magnitude of the astronomical AO system may be improved by using the plenoptic sensor instead of the Shack–Hartmann sensor, and the modulation of the plenoptic sensor is more suitable for the bright star.
In adaptive optics (AO) system, the detector noise is one of the main error sources of Shack-Hartmann wavefront sensor (SH-WFS). Based on the statistical analysis of the noise, a noise error estimation method is presented by using multiframe of the Hartmann spots pattern and the centroid displacements calculated from them. A numerical simulation system for wavefront measuring is built, and used to verify the validity of this method. It shows that the estimation error of this method could be lower than 2%, provided that the signal-noise-ratio (SNR) is sufficient for the WFS working normally. We studied the least frames of data that are required for the method when the SNR of the WFS is at different levels. It indicates that fewer frames are required as the SNR level is higher, and only 2 frames of data are required when the SNR level is high enough. For different types of detector noise, we have analyzed the influence of the accuracy of their prior information on the estimation error. It shows that the influence of the readout noise is strong, and the influence of the photon-noise, the dark-current noise and the sky-background noise is neglectable, since the WFS is usually exposed shortly. The work in this paper can be of certain significance in estimating the point spread function of AO system with the WFS measurements.
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