Libo Zhong

INSTRUMENT DESCRIPTION AND PERFORMANCE EVALUATION OF A HIGH-ORDER ADAPTIVE OPTICS SYSTEM FOR THE 1 m NEW VACUUM SOLAR TELESCOPE AT FUXIAN SOLAR OBSERVATORY

A high-order solar adaptive optics (AO) system including a fine tracking loop and a high-order wavefront correction loop has been installed at the 1 m New Vacuum Solar Telescope of the Fuxian Solar Observatory, in routine operation since 2016. The high-order wavefront correction loop consists of a deformable mirror with 151 actuators, a correlating Shack-Hartmann wavefront sensor with 102 subapertures of which the Absolute Difference Square Algorithm is used to extract the gradients, and a custom-built real-time controller based on a Field-Programmable Gate Array (FPGA) and multi-core Digital Signal Processor (DSP). The frame rate of the wavefront sensor is up to 3500 Hz and this is, to our knowledge, the fastest solar AO system. This AO system can work with a Fried parameter r(0), at the 500 nm wavelength, of larger than 3 cm. The first 65 modes of the Zernike aberrations can be efficiently corrected and the Strehl ratio of the corrected TiO image for the solar pore is superior to 0.75 with the Fried parameter r(0) larger than 10 cm. In this paper, the design of the system is described, and high-resolution solar observational images are presented. Furthermore, the performances of the AO system are evaluated according to the data recorded by the real-time controller.

Speckle image reconstruction of the adaptive optics solar images

Speckle image reconstruction, in which the speckle transfer function (STF) is modeled as annular distribution according to the angular dependence of adaptive optics (AO) compensation and the individual STF in each annulus is obtained by the corresponding Fried parameter calculated from the traditional spectral ratio method, is used to restore the solar images corrected by AO system in this paper. The reconstructions of the solar images acquired by a 37-element AO system validate this method and the image quality is improved evidently. Moreover, we found the photometric accuracy of the reconstruction is field dependent due to the influence of AO correction. With the increase of angular separation of the object from the AO lockpoint, the relative improvement becomes approximately more and more effective and tends to identical in the regions far away the central field of view. The simulation results show this phenomenon is mainly due to the disparity of the calculated STF from the real AO STF with the angular dependence.

Solar adaptive optics for 1m new vacuum solar telescope

Adaptive Optics (AO) has become the requisite equipment of the ground-based large solar telescope to correct the wavefront aberration induced by the atmospheric turbulence. Two generation solar AO systems, one is the 37-element loworder AO system with 2100Hz frame rate and the other is 151-element high-order AO system with 3500Hz frame rate, were successfully developed in 2013 and 2015 respectively. In this presentation, the development of the two AO systems for 1-m New Vacuum Solar Telescope (NVST) at Fuxian Solar Observatory (FSO) will be introduced and the solar high resolution observational results are presented.

Speckle transfer function for partial correction adaptive optics image reconstruction

The residual aberrations of adaptive optics (AO) images can be removed by the speckle reconstruction method. To achieve accurate photometry, the knowledge of the speckle transfer function (STF) is necessary. In this Letter, a new method is proposed to evaluate the STF for speckle image reconstruction based on the partial correction AO. The STF is divided into two sections: the low-frequency section and the high-frequency section, and two different generalized Fried parameters for the partial wavefront compensation are used to calculate the STF values of those two different sections. This method is tested with the different seeing conditions and the various correction modes. The results show that the combined STFs have satisfactory agreement with the simulated AO STFs and give more accurate results than the STFs obtained from the traditional spectral ratio method. Further discussions show that the method is more reliable when the AO is operated in low-order correction mode because the approximation of the STF and the two generalized Fried parameters have better accuracy when the pupil diameter is much larger than those generalized Fried parameters.

Progress on solar multi-conjugate adaptive optics at the New Vacuum Solar Telescope

Multi-conjugate adaptive optics (MCAO) has been proved to obtain the high resolution images with a large field of view in solar observation. A solar MCAO experiment system had been successfully developed and tested at the 1-meter New Vacuum Solar Telescope (NVST) of Fuxian Solar Observatory. It consists of two deformable mirrors (DMs), a multidirection Shack-Hartmann wavefront sensor (MD-WFS), and a real-time controller. In order to command the two DMs, five guide regions were selected from the MD-WFS to retrieve a three-dimensional measurement of the turbulent volume based on atmospheric tomography. This system saw the first light in October, 2017, and a series of MCAO-corrected high resolution sunspots images were acquired. In this presentation, the MCAO experiment system is introduced, and the observation results are presented. Furthermore, a new MCAO system based on our proposed MCAO configuration with a high order ground layer adaptive optics and low order high altitude correction will be developed for the NVST as a regularly operating instrument for scientific observations of the sun.

Comparison of correlation algorithms with correlating Shack-Hartmann wave-front images

Correlating Shack-Hartmann wavefront sensor is widely used in solar adaptive optics in which the relative shift between different subapertures by correlation algorithm is computed, and then the control voltage by wavefront reconstruction can be estimated to use for correcting the wavefront distortion induced by atmospheric turbulence. In this paper, several different correlation algorithms including Cross-Correlation Coefficient, Absolute Difference Function, Absolute Difference Function-Squared and Square Difference Function are used to estimate relative shift in correlating Shack-Hartmann wave-front sensor with the different observed solar structure such as sunspot, solar pore and solar granulation. The measurement noise RMS error is computed to compare the performance of the correlation algorithms. The results show the correlation algorithm precision is directly related to the solar structure. The measurement noise is relatively small with the relatively high contrast target, and vice versa. At the same time, the size of reference image also could influence the measurement noise, the larger size of the reference image, the smaller the measurement noise is.

The speckle image reconstruction of the solar small scale features

The resolution of the astronomical object observed by the earth-based telescope is limited due to the atmospheric turbulence. Speckle image reconstruction method provides access to detect small-scale solar features near the diffraction limit of the telescope. This paper describes the implementation of the reconstruction of images obtained by the 1-m new vacuum solar telescope at Full-Shine solar observatory. Speckle masking method is used to reconstruct the Fourier phases for its better dynamic range and resolution capabilities. Except of the phase reconstruction process, several problems encounter in the solar image reconstruction are discussed. The details of the implement including the flat-field, image segmentation, Fried parameter estimation and noise filter estimating are described particularly. It is demonstrated that the speckle image reconstruction is effective to restore the wide field of view images. The qualities of the restorations are evaluated by the contrast ratio. When the Fried parameter is 10cm, the contrast ratio of the sunspot and granulation can be improved from 0.3916 to 0.6845 and from 0.0248 to 0.0756 respectively.