人造钠信标角度非等晕性的实验研究

Experimental investigation of angular anisoplanatism for sodium beacon

为实际验证较大角度偏移(θ≈50μrad)条件下人造钠信标探测光路与科学目标光路之间经历大气湍流波前畸变的相关特性及其影响,开展了基于时序同步探测的人造钠信标角度非等晕性实验测量研究,利用单哈特曼波前传感器实现了对自然星回光点阵以及50μrad角度偏移钠信标共振回光点阵的同步测量,获得了时序同帧钠信标与自然星回光的波前序列,并从两者波前二维分布及Zernike模式的统计相关性、非等晕误差的Zernike模式统计分布特性及其对目标成像影响等方面对实验结果进行了较全面的分析.结果表明,50μrad的角度偏离导致两者波前仅低阶模式之间保持着一定相关性(如第3—9阶),而非同轴钠信标偏离望远镜接收口径的大气湍流误采样带来其探测波前与自然星波前之间部分高阶模式相关性的严重退化,致使目标成像点扩散函数Strehl比(0.31—0.22)、光学质量β(2.70—3.35)的下降,该影响不容忽视.最后,依据实验大气条件完成非等晕误差测量结果与理论计算结果的对比分析,获得了理论与实验相符的结果.

To understand the characteristics of the anisoplanatic the sodium beacon with a greater angular offset and the error resulting from different return-light experiences between science object through atmospheric turbulence, the angular anisoplanatism for sodium beacon is investigated experimentally based on the technique of synchronized range gating. The return-light spot arrays through turbulent atmosphere from the natural star and the sodium beacon with 50 μrad angular offsets are synchronously collected by using a single Hartmann wavefront sensor, consequently the synchronous turbulence-induced wavefront distortion sequences are recovered for the on-axis natural star and the off-axis sodium beacon. According to the experimental data, the temporal correlations of the wavefront distributions and decomposed Zernike modes between the on-axis natural star and the off-axis sodium beacon are discussed. By comparing the off-axis sodium beacon with the on-axis natural star, we analyse the its associated Zernike-modal variances for the off-axis sodium statistics of the acquired angular anisoplanatism error and beacon, and derive the Zernike-modal relative anisoplanatic errors as well. Furthermore, the influence of the acquired angular anisoplanatism error on the quality of imaging point spread function （PSF） is studied. The experimental results show that the existence of 50 ~rad angular deviation between the sodium beacon and the natural star causes that there are a certain correlation between just low-order Zernike modes of these two types of wavefronts （e.g. from the 3rd order to the 9th order）, but the correlations between other high- order Zernike modes of these two types of wavefronts are severely degenerated and even these modes are de-correlated, resulting from the improper turbulence probing with off-axis sodium beacon off the ray path from the natural star to the telescope aperture. The angular anisoplanatism error has a great influence on the quality of imaging PSF, which leads to a degradation of Strehl ratio of 0.31-0.22 and beam quality factor β of 2.70-3.35. Therefore, the influence may not to be ignored. At the end of this paper, according to the derived experimental turbulence coherence length and the generalized Hufnagel-Valley model, we calculate the theoretical anisoplanatic phase variance for the sodium beacon with 50 μrad angular offsets, which is in good accordance with the measured anisoplanatic phase variance. This investigation is useful in promoting our knowledge of sodium beacon angular anisoplanatism effect on turbulence probing.