应用天文研究组知识库

Objective As a precise optical instrument, the telescope is subject to changes in its focal position due to atmospheric disturbances, temperature changes and installation errors. If realtime focusing is not performed, the image may be distorted, which will seriously affect the tracking and measurement effects of the telescope. With the improvement of the level of intelligence, the automatic focusing technology is applied to the focusing of the telescope system. The image sharpness evaluation algorithm is the key to the decision of the focus position of the telescope's automatic focusing technology, and the performance of the algorithm directly determines the accuracy of the automatic focusing. The traditional telescope focusing evaluation algorithm is implemented on the basis of statistical analysis, and it is difficult to ensure real-time performance and noise immunity for astronomical images. Most of the existing telescope focusing evaluation algorithms have relatively low performance, and it is difficult to extract target features from the captured images for highspeed moving targets. Moreover, it is often impossible to evaluate the engineering level of algorithms and hardware due to insufficient hardware experiments of the system. In order to solve the above problems, in this study, we propose a half-flux diameter real-time autofocusing sharpness evaluation algorithm with improved centering accuracy (HFD-ICA). The algorithm has low cost, high real-time performance and good stability, and is suitable for most Focusing of the telescope system. We hope that this method will help improve the performance of telescope autofocusing and provide references for research in related fields. Methods In this study, we first denoise the acquired raw image sequence (defocus-focusdefocus) using the anisotropic diffusion method. Then, the denoised image is binarized by the Qtsu threshold method, and the target star is extracted from the background. Next, on the basis of binarization, the pixels adjacent to the target are clustered, and the boundary of the target is calculated to obtain the target region of interest (ROI). According to the determined ROI domain, the improved intensity-weighted centroid method (improved IWC) is used to iteratively calculate the centroid of the star image until the centroid reaches sub-pixel accuracy. After the centroid is determined, the half flux diameter (HFD) value of the star image is measured by the HFD-ICA method, and the hyperbolic fitting method is used to further process these values. The V-shaped curve that guides the focusing of the telescope can be drawn, and then the focus position of the telescope can be determined. Result and Discussions The HFD value measured by the proposed algorithm is V-shaped with the focus position, and the shape of the V curve represents the characteristics of the optical system consisting of the focus, telescope and camera (Fig. 13). The focusing accuracy of the HFD-ICA algorithm is high, and its fixed focus rate is equivalent to that of highprecision astronomical image processing software IRAF, reaching 98% (Table 1). The antinoise performance test of the algorithm shows that after adding a small amount of noise, the gray value around the star point changes, which interferes with the processing performance of the algorithm, and the precise focusing rate of the algorithm will be affected to a certain extent. In comparison, the anti-noise performance of HFD-ICA is the best (Table 2). Further, compared with other algorithms in terms of operation time, the HFD-ICA algorithm has faster calculation time and the best real-time performance. Compared with the HFD method, the real-time performance is improved by about 3 times. The Full Width at Half Maximum method takes a lot of time because it needs curve fitting during measurement. The average processing time reaches 32. 4s, which is 6. 89 times that of HFD-ICA. The IRAF software with relatively high processing accuracy has an average processing time as high as 45. 7s, which is nearly 10 times longer than the HFD-ICA method (Table 3). Conclusions This paper mainly studies the image sharpness evaluation algorithm of automatic focusing of telescopes. Through a series of experiments, it is verified that the HFDICA method is stable, efficient, and robust, and can handle those image frames that are seriously out of focus when the HFD-ICA method guides the automatic focusing of the 1. 2m telescope system. Compared with the HFD method without improved centering accuracy, the algorithm has improved performance, and its precise focusing rate is comparable to that of the high-precision astronomical image processing software IRAF, both reaching 98%. The average processing time of the algorithm in the process of guiding focusing is only 4. 7s, which is 1/10 of that of IRAF, which meets the real-time requirements of the focusing system. Compared with the system's original manual focusing, this study improves the system's average focusing efficiency by roughly 37%. To a certain extent, it lays the foundation for the fully automated observation of future stations, and also provides a reference for the automatic focusing of other telescope systems.