基于制冷型320×240凝视焦平面阵列探测器，采用离轴折反射式结构设计了一种大口径长焦距红外连续变焦光学系统.系统分离轴无光焦度和透射式连续变焦两部分，通过光瞳匹配和冷屏匹配将两部分组合起来进行优化，解决了透射式连续变焦系统因材料限制不能实现大口径、共轴连续变焦系统短焦遮拦比大和离轴三反变焦系统像面移动的缺陷.系统工作波段为3.7～4.8 μm，焦距范围250～2 000 mm，变倍比为8×，F数为4，满足100%冷光阑效率.在空间频率16 lp/mm处系统的MTF值大于0.5，具有像质好，分辨率高等特点，满足设计要求.

On the basics of a cool 320×240 detector with staring focal plane array, a offaxis catadioptric middle infrared continuous zoom system with large aperture and long focal length is designed. The optical system was divided into two segments. First an offaxis aspheric nonpower system as the front system was designed, then the continuous zoom lens which matched with the nonpower system as the back system was designed. Because of the material constraint, the refractive zoom system can′t realize large aperture. Coaxal catadioptric zoom system has great block ratio. And three mirrors zoom system can′t achieve cold shield efficiency 100%. The offaxis catadioptric zoom system can account for forenamed limitation. The system worked at 3.7～4.8 μm has achieved the zoom of 250 mm to 2 000 mm and F number of 4. This system can offer a high resolution and excellent images, and its cold shield efficiency is 100%. The system satisfies the design requirements.