以三级像差理论为基础，对反射式无光焦度系统的像差进行了分析。得出的结论为：在主、次镜均 为抛物面的前提下，反射式无光焦度系统可以同时消除四种像差。以此结论为基础，综合离轴反射系统的优点，得出了一 套离轴反射式无光焦度系统。以光束口径为φ100mm、缩束倍率β=5、主镜离轴量h=150mm的离轴反射式无 光焦度系统为例，根据系统的像差结果可以看出，三级像差理论的分析结果是正确的。

The aberration in a reflective non-power system is analyzed according to the third-order aberration theory. It is concluded that if both a primary mirror and a secondary mirror are parabolic, the reflective non-power system can eliminate four kinds of aberrations. On the basis of the above conclusion, an off-axis reflective non-power system with the advantages of an off-axis reflective system is obtained. The aberration of an off-axis reflective non-power system which has a beam aperture of φ100, a beam expanding power of β=5 and an off-axis distance of 150mm is analyzed. The analysis result shows that the third-order aberration theory is correct.