High-resolution Reconstruction Techniques Of The Solar Images

High-resolution observations of the Sun are required in the fields of space weather prediction and the solar physics investigation. The atmospheric turbulence above the Earth decreases the angular resolution of the celestial object. The adaptive optics(AO) techniques can remove the aberrations and produce the diffraction limited images. However, due to the limitations of hardware,the correction ability of the adaptive optics(AO) system is partial. As well as due to the anisoplanatism effect of the atmosphere, the effective compensation field of the traditional AO system is only tens of arc sec at most. Except of the Multi-Conjugate Adaptive Optics(MCAO) technique, the post-facto image processing techniques are commonly used to obtain the high-resolution solar image at large field, and the combination of the traditional AO correction and the image processing technique is the main method. In this paper, the reconstructions of the solar images based on the speckle imaging technique are investigated. The main contents include the followings:Firstly, the theory of the speckle imaging technique and the implementation steps of the reconstruction are displayed, followed by the investigations of the effects of the static aberrations on the accuracy of the speckle image reconstruction. The results show that the speckle interferometry is insensitive to static aberrations provided the aberrations are approximately constant over the individual correlation cells of the atmospheric turbulence. For the effects of the static aberration on the accuracy of the Fourier phase reconstruction, some typical aberrations are analysed. It is found that, only the coma has residual phase information left in the object phase recover process among defocus, astigmatism, coma and spherical aberration in ideal case. However, the limited number of the object’s speckle images led to the phase information which used to recover the object’s phases contains the atmospheric aberrations and the decrease the amplitudes of the transfer function increase the phase error. To deal with this phenomenon, the aberration balancing method is proposed in this section, and both the theoretical analysis and the simulations showed that this method can effectively compensate the influences of the static aberration and increase the accuracy of the phase reconstruction.Secondly, the speckle image reconstruction process of the AO corrected images is investigated. Based on the introduction of the AO system correction characteristics, the existed methods are displayed and analysed. The reconstruction of the AO images showed that the reconstruction can further improve the angular resolution of the image. Closely investigated the results show that the accuracy of the reconstruction is field dependent, the relative improvement of the contrast in the lock field is less than other field. This mainly due to the disparity from the Kolmogorov spectrum of the statistical information of the residual phase error after real-time AO correction, the theoretical analysis and the simulations show that the theoretical speckle transfer function(STF) are not appropriate to obtain the high accuracy object power spectrum especially at the locked field of the AO system. It is needed to find the new STF.Finally, a new method to calculate the STF after AO correction is proposed. Based on the analysis of the effects of the AO correction on the point spread function and statistical information of the residual phase error, two generalized Fried parameters are introduced which are equivalent to the coherence length of the atmosphere at the meaning of the width of the halo of the PSF and the probability distribution of the Strehl ratios. Those generalized Fried parameter are used to calculate the amplitude of the STF at the high frequency section and the low frequency section respectively. The simulations at diverse seeing conditions and different correction abilities show that the new STF match the real AO STF well and the accuracy of the reconstructions by this STF higher than the results obtained from the traditional theoretical STF. The reconstructions of the solar images obtained by the 37-elements AO system confirmed the effectiveness of the new STF.In total, these results and innovations at the field of high-resolution solar image reconstruction are important guidance for the theoretical investigations and the engineering applications of the fine structures imaging at the solar active regions, and the observations of the evolution of the structures at solar atmosphere to investigate the dynamic derivation of the solar flare, the coronal mass ejection and the solar eruption. The results are also useful for the night-time imaging observations.