基于多目标图像清晰度评价的LAMOST光谱仪离焦诊断方法

研究表明，当温度产生一定变化时，会导致大天区面积多目标光纤光谱天文望远镜（LAMOST）光谱仪的内部器件产生热诱导变形，引起CCD靶面上的图像离焦模糊，降低了天文数据测量精度，也大大增加了16台光谱仪的日常维护难度。基于LAMOST运行前拍摄的定标灯谱数据，提出了一种基于多目标图像清晰度评价的离焦诊断方法。该方法通过对不同离焦量下的定标灯谱图像进行分析，提取了一定数量的光斑的半高全宽（FWHM）及其总体分布情况，建立多目标图像清晰度与系统离焦量之间的离焦函数模型，实现对LAMOST成像像质的离焦诊断，为后续智能化主动补偿技术的实现提供了技术支撑。介绍了LAMOST光谱仪的工作原理及结构，给出了光谱仪对调焦精度的需求；详细介绍了光斑FWHM值的测量原理与多目标图像清晰度评价函数的构建方法。与传统图像清晰度评价函数的对比结果显示，文中方法具有更高的清晰度对比率与精度，对定标灯谱图像的离焦诊断误差在10 μm以内，有效降低了人为局部诊断带来的误差，提高了16台光谱仪系统一致性，有望提高LAMOST日常运行效率与光谱仪的长期稳定性。

Defocus diagnosis method of LAMOST spectrograph based on multi-object image definition evaluation

The research demonstrates that when the temperature changes a lot, the internal components of the large sky area multi-object fiber spectroscopic telescope (LAMOST) spectrographs will produce thermal-induced flexure, which will cause the image blurring of the CCD. Such image blurring reduces the measurement accuracy of astronomical data and greatly increases the difficulty of the daily maintenance of the 16 spectrographs.Therefore, a defocus diagnosis method for multi-object image definition evaluation is proposed based on the calibration lamp spectrum taken by LAMOST. By analyzing the spectrum image under the different defocusing amounts, the half-height full-width (FWHM) and its distribution of spots are obtained to establish the function model between the multi-target image definition and the defocusing amount of the system. It can realize the fast defocus diagnosis of the spectrum image quality of LAMOST and provide a reference for the realization of intelligent active compensation technology of LAMOST spectrograph. In this paper, the principle and structure of the LAMOST spectrograph are introduced, and the demand for the focusing precision of the spectrograph is given. And the measurement principle of FWHM value and the construction method of the multi-object image definition evaluation function are introduced in detail. Compared with the traditional image definition evaluation function, the results show that the proposed method has a higher definition ratio and accuracy. And the defocus diagnosis error of the spectral image is less than 10 μm, which effectively reduces the error caused by artificial diagnosis and improves the consistency of the 16 spectrographs. This method can also improve the daily operation efficiency and long-term stability of the LAMOST spectrographs.