Reflecting optical systems desensitization: Modulation by freeform surfaces

•Sensitivity determines the achievement of the optical system's as-built performance. This paper proposes a desensitization design method, diverging from traditional design methods.•The sensitivity of a freeform optical system is directly analyzed, and we found freeform surfaces can not only improve the image quality but also reduce the sensitivity.•A desensitization design method that can modulate the sensitivity of an optical system by using freeform surfaces is proposed. Freeform surfaces have been used to achieve high imaging performance in recent years. Apart from the nominal performance, sensitivity is a vital factor for the realizability of the as-built performance. We found that freeform surfaces are not only excellent for aberration correction but also enable the achievement of desensitization. In this paper, two merit functions, FDS and SDS, are established, and a desensitization design method that performs desensitization modulation by using freeform surfaces (DMF), is proposed. This method elucidates the reasons behind freeform surfaces enabling low sensitivity and taps into the desensitizing potential of freeform surfaces. We used the Tianwen-1 High-Resolution Imaging Camera (HiRIC) optical system as the prototype and achieved a 42.14 % reduction in sensitivity by using the DMF design method we proposed.