The auto guiding system combing with sub-pixel real-time gray projection algorithm

Generally, observing astronomy target requires that astronomy telescope should be in possession of the performance of longtime and precise pointing and tracking. Especially, in modern astronomy study, astronomers hope that the performance of longtime and precise pointing and tracking should be sub-arcsec, which requires that modern astronomy telescope must be equipped with corresponding closed-loop controlling systems able to strengthen the performance of longtime and precise pointing and tracking. Auto guiding system is one of efficient closed-loop controlling systems, and it can modify the error of pointing and tracking of telescope in the process of longtime observing. Auto guiding system can be divided into two categories: the auto guiding system of full-disk solar image used to solar observing and bias auto guiding system used to night astronomy observing. This reported thesis is based on the auto guiding system of full-disk solar image.  
    The principle of the auto guiding system of full-disk solar image is as follows:Firstly, the offsets of the centre of gravity of full-disk solar image are calculated, and then are sent the controlling system of shafting of telescope. Secondly, on the basis of the offsets, the controlling system of shafting of telescope will be operated so as to decrease the offsets. However, with respect to the ground-based solar telescope, its auto guiding system of full-disk solar image often is disturbed by atmospheric turbulence and wind, resulting in that the real offsets of the centre of gravity of full-disk solar image can not obtained and the controlling system of shafting of telescope will not be well operated. Therefore, some researchers proposed corresponding methods in order to suppress the disturbances of auto guiding system of full-disk solar image from atmospheric turbulence and wind. For examples, to suppress the disturbances from atmospheric turbulence, previous researchers proposed that within a closed-loop cycle of auto guiding system, the centre of gravity of full-disk solar image is calculated N times, and then the average value of the centre of gravity of full-disk solar image is obtained so as to decrease the error of the centre of gravity of full-disk solar image N-1/2 fold. For the suppressing of wind, previous researchers proposed that increasing the gain of the controlling system of shafting of telescope and adding the controlling system of image stabilization into terminal of guiding optics of telescope are great choices. In other words, in the previous studies which focuses on how to  suppress the disturbances of auto guiding system of full-disk solar image from atmospheric turbulence and wind, the auto guiding system itself just is in charge of suppressing the disturbance of atmospheric turbulence. 
    Due to the reasons mentioned above, the authors of this paper propose a novel method which can simulatanously suppress the disturbances of auto guiding system of full-disk solar image from atmospheric turbulence and wind. The method is that modified gray projection algorithm is firstly introduced into the process of calculating the centre of gravity of full-disk solar image in order to obtain the centre of gravity of full-disk solar image of current frame and the offset between previous frame and current frame, and then the offset will be used to modify the obtained centre of gravity of full-disk solar image of current frame. Through the operations mentioned above, the disturbance from wind will be suppressed. On the other hands, the gray projection algorithm is modified in speed and resolution, which results in that the number of calculating the center of gravity is almost invariable. Therefore, the authors of this paper realize the auto guiding system able to suppress the disturbances from atmospheric turbulence and wind by itself.
    The partial member of the research group from Yunnan observatory of Chinese Academy of Science, which is directed by the researcher, Zhongquan Qu, introduces the modified gray projection algorithm into the auto guiding system of solar telescope, and proposes a new method which is able to strengthen the ability of auto guiding system to suppress the disturbance of atmospheric turbulence and wind. The method proposed by the research team is novel, and is suitable to all the solar telescopes anxious to improve the accuracy of longtime pointing and tracking.
    Auto guiding system is an efficient closed-loop controlling system used to improve the accuracy of longtime pointing and tracking of telescope. However, the accuracy of the traditional auto guiding system is disturbed by atmospheric turbulence and wind. Although current some researches have taken corresponding measures to suppress the two disturbances, there is no an efficient method able to simultaneously suppress the two disturbances only by auto guiding system itself.
    Due to the reason mentioned above, after the research team deeply analyzes the key ideals of auto guiding system. Based on similarity of structure between the algorithm of the centre of gravity and the algorithm of the gray projection algorithm, they introduce the gray projection algorithm into auto guiding system so as to obtain the centre of gravity of current frame of auto guiding system’s image and the offset between previous frame and current frame, and then the centre of gravity will be modified by the offset. After the operations, the ability of auto guiding system to suppress wind can be improved, in the meantime, the ability of auto guiding system to suppress atmospheric turbulence is almost invariable.  
    Currently, the researchers from the research group mentioned above have tested the proposed method, and corresponding experimental picture of auto guiding system is shown in the following figure. The testing conclusion shows that the proposed method can improve the ability to suppress atmospheric turbulence and wind by auto guiding system itself.  

 
Auto guiding system and telescope used to test

About Team
The research team of this paper is affiliated to the research group of Yunnan observatory of Chinese Academy of Science, which is directed by Prof. Zhongquan Qu. The two members of the team, Zhiming Song and Guangqian Liu specialize in precise controlling system and algorithm related to astromomy telescope, and the member, Zhongquan Qu, specialize in theoretical study associated with solar physics. Currently, the research group directed by Zhongquan Qu is responsible for national major scientific research instrument development project supported by national natural science foundation of China. The partial research achievements obtained by the members of the team are shown in the following: 
[1] Wang K Y, Song Z M, Sheng M L, et al. Modular real-time face detection system[J]. Annals of Data Science, 2015, 2(3): 317–333.
[2] Liu G Q, Cheng X M, Song T F, et al. The influence and control of wind loading on the one meter solar telescope servosystem[J]. Opto-Electronic Engineering, 2011, 38(6): 50–58. 
[3] Qu Z Q, Dun G T, Chang L, et al. Spectro-imaging polarimetry of the local corona during solar eclipse[J]. Solar Physics, 2017, 292(2): 37.


Article
Song Z M, Liu G Q, Qu Z Q. The auto guiding system combined with sub-pixel real-time gray projection algorithm[J]. Opto-Electronic Engineering, 2018, 45(8): 170586.
DOI:10.12086/oee.2018.170586