Prototype of solar ground layer adaptive optics at the 1-m New Vacuum Solar Telescope
The effect of ground layer adaptive optics (GLAO) system for solar active imaging. The GLAO prototype system was integrated into the New Vacuum Solar Telescope (NVST) and saw the first light on January 12th, 2016, with the wavelength of 705.7 nm@0.6 nm. The solar active area NOAA 12480 was imaged without and with GLAO system. It can be seen that GLAO system improved the image quality in large FoV.
Adaptive optics (AO) is an indispensable technique in ground-based solar imaging which provides diffraction limited resolution. However, the traditional AO technique has only a very small corrected field of view (FoV) (~ 10 arcsec), much less than the solar active region which has the typical size of 1~3 arcmin. Multi-conjugate adaptive optics (MCAO) is the most promising technique currently developed to increase the corrected FoV.
The fact that about 60% of the turbulence strength is concentrated in the first few kilometers above the telescope has inspired the concept of ground layer adaptive optics (GLAO). A GLAO system can be seen as a simplified MCAO system. It is equipped with several wavefront sensors to measure the wavefront perturbation introduced by the ground turbulent layer and a single deformable mirror conjugated to a low altitude to compensate for the ground turbulent layer. As opposed to the MCAO, it is important to emphasize that the goal of GLAO is not to attain a near-diffraction-limited correction but to simply reduce the perturbation and stabilize the seeing over a wide FoV.
A solar GLAO prototype was developed and tested at the 1-m New Vacuum Solar Telescope of Fuxian Solar Observatory and saw the first light on January 12th, 2016. The on-sky observational results show that the solar image is apparently improved in the whole 1 arcmin FoV with the GLAO correction. The corresponding results are published in Chinese Optics Letters, Volume 14, No. 10, 2016 (L. Kong, et al., Prototype of solar ground layer adaptive optics at the 1-m New Vacuum Solar Telescope).
Prof. Changhui Rao, the leader of the solar imaging research group of the Adaptive Optics Laboratory, Chinese Academy of Sciences, said the success on the solar GLAO demonstration is an important progress on the AO technique in China, and this will lay the foundation for further development of MCAO.
Based on the GLAO progress, we will concentrate on the solar MCAO research in the next step. The large solar telescopes with MCAO system can be used to monitor the solar activity and acquire the high resolution solar data in real time for the space weather and the solar physics research.
欲穷“千里目”，有此校正波前术——地表层自适应光学技术
图片说明：地表层自适应光学（GLAO）系统工作效果对比图。2016年1月12日对太阳活动区（编号NOAA 12480）进行观测，观测波长为705.7 nm@0.6 nm，左图为系统开环（GLAO系统未工作）时太阳图像，右图为GLAO系统闭环太阳图像，可以看出，GLAO系统在大视场范围内明显改善了图像质量。
自适应光学技术作为地基太阳高分辨力观测必备的技术手段之一，可以有效校正大气湍流的影响，帮助太阳望远镜获得接近衍射极限的系统成像分辨力。然而，自适应光学技术典型的校正视场只有约10角秒左右，无法满足太阳活动区1~3角分的大视场高分辨成像观测的需求。多层共轭自适应光学技术通过对大气湍流进行分层校正，从而获得大视场范围内接近衍射极限的成像分辨力，已成为下一阶段自适应光学领域研究的热点。另一方面，由于大部分大气湍流集中在望远镜上方几公里范围内的地表层，科学家又提出了地表层自适应光学（GLAO）的概念。
地表层自适应光学可以看作多层共轭自适应光学的一个特例，其通过多个视线方向上的波前探测获得地表层大气湍流引起的波前像差，并控制共轭于地表层的变形镜进行补偿校正。不同之处在于，地表层自适应光学的目标不是实现大视场内接近衍射极限的成像能力，而是通过对地表层大气湍流引起的波前像差进行探测和校正，在大视场范围内降低大气湍流的影响，获得更加清晰稳定的图像。
中国科学院自适应光学重点实验室饶长辉研究员领导的太阳高分辨力光学成像研究小组，基于云南天文台1米新真空望远镜（NVST），成功研制了太阳地表层自适应光学系统原理样机，并于2016年1月12日首次获得太阳观测结果。该系统可以在1角分视场内明显改善太阳图像质量。相关研究成果发表在Chinese Optics Letters 2016年第10期上（L. Kong, et al., Prototype of solar ground layer adaptive optics at the 1-m New Vacuum Solar Telescope）。
饶长辉研究员认为，地表层自适应光学系统的验证成功，标志着我国自适应光学技术取得重要进展，并且为进一步开展多层共轭自适应光学研究奠定基础。
基于现有技术的积累，下一步该研究小组将着重开展太阳多层共轭自适应光学技术的研究。配备多层共轭自适应光学系统的大口径太阳望远镜，将有望获得整个太阳活动区的大视场高分辨力实时图像，为太阳物理研究和空间天气预报提供数据支撑。