Design method for freeform reflective-imaging systems with low surface-figure-error sensitivity

Freeform surfaces are difficult to manufacture due to their lack of rotational symmetry. To reduce the requirements for manufacturing precision, a design method is proposed for freeform reflective-imaging systems with low surfacefigure-error sensitivity. The method considers both the surface-figure-error sensitivity and optical specifications,which can design initial systems insensitive to surface figure errors. Design starts with an initial planar system;the surface-figure-error sensitivity of the system is reduced during construction. The proposed method and another that is irrelevant to figure-error sensitivity are used to design a freeform off-axis three-mirror imaging system.Comparison of the sensitivities of the two systems indicates the superiority of our proposed method.