Abstract:Based on the principle of null lens,a new method of error-separation was proposed in null testing.A mathematic model was developed to separate misalignment-induced aberration from the test result.The method was used in the simulations of an off-axis aspheric mirror.The wavefront aberration induced by misalignment is 0.0026λRMS (λ=632.8 nm) after the alignment,compared with 0.2332λRMS before the alignment,which reflects that this method has high accuracy and can improve the efficiency of the null test.