次镜（secondary mirror）是反射式望远镜中将光线反射到较易观测位置的镜片，通常作为光学系统的第二反射元件。其结构多采用X形支架悬置于主镜前方。部分设计将次镜安装在改正镜或通过六足定位机构延长光路，采用柔性支撑结构以应对温度变化及振动的影响。卡塞格林系统中的次镜为双曲面反射镜，可将主镜反射的光线转向至镜筒后方便于观测。詹姆斯·韦布望远镜次镜由镀金铍材料制成，直径0.74米，在-220℃低温环境下工作。压电驱动变形次镜技术能实时校正镜面形变，使丽江1.8米望远镜成像分辨力达1.25倍衍射极限。

次镜

secondary mirror

A secondary mirror (or secondary) is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirrors in the form of an optically flat diagonal mirror are used to re-direct the light path in designs such as Newtonian reflectors. They are also used to re-direct and extend the light path and modify the final image in designs such as Cassegrain reflectors.