介绍了一种基于步进电机(无位置检测传感器)和像质反馈的广角天文望远镜自动调焦系统, 该系统由步进电机位置开环控制系统和图像清晰度评价系统组成。基于天文图像的特点, 提出利用天体点源星像的半高全宽(FWHM)作为图像清晰度的评价参数;为了提高系统精度, 对定位控制系统进行了误差分析;测定了调焦当量、光学焦深、系统回差、步进脉冲当量等系统关键机构参数,进而高精度地修正了系统定位误差;分析和测定了温度变化和像质变化的相关性, 明确了调焦方向和温度变化之间的定性关系;设计了焦深边缘定位策略, 实现了快速调焦。该系统具有“容错”和“自学习”功能, 具有较高的自动化和智能化程度。将系统设计应用于SVOM天文卫星地基广角相机阵［1-2］(GWAC), 实际测试和天文观测表明, 调焦系统的定位精度为±3.0 μm, 调焦精度为0.05 pixel,且一次调焦过程即可获得最佳像质, 调焦效率较高, 系统运行也很稳定, 基本满足天文观测的需求。

An auto-focusing system based on a step motor (without position detection sensor) and image quality feedback for an astronomical telescope was introduced, which consists of a step motor position open-loop control subsystem and an image definition evaluation subsystem. Based on the characteristics of astronomical images, the Ful Width at Half Maximum( FWHM)of point source images of stars was used as image definition evaluation parameters. In order to achieve high efficiency of focusing process, the error of the position control system was analyzed. Then, the focusing equivalent, focal depth, system backlash and the step pulse equivalence were measured to correct the system position errors effectively. By analyzing and measuring the correlation between temperature change and image quality change, the qualitative relationship between focusing direction and temperature change was obtained. Moreover, the focal depth edge positioning policy of the focal plane was designed and a fast process focusing was achieved. The system has fault-tolerant function and self learning ability, so it shows higher levels on automation and intelligence. The design above mentioned was applied to a Ground-based Wide-Angle Camera(GWAC) array of SVOM astronomy satellite. Measurement results and astronomical observations show that positioning accuracy of the auto-focusing system is ±3.0 μm and the focusing accuracy is 0.05 pixel. As it can obtain the best image quality only by one focusing process, it has higher focusing efficiency while containing stable running.