Design study of two-aspherical-mirror anastigmat with reduced sensitivities to misalignments: Correction of higher-order aberrations

A practical design method for correcting fifth and higher order aberration of two-aspherical-mirror optics was developed and applied to an imaging objective for soft-X-ray microscope. A sixth and eighth order deformation was applied to the both mirrors, and the deformation coefficients were optimized numerically to minimize a blur on the image plane. With this method, fifth or higher-order aberrations, which have remarkable effect on a fast soft-X-ray optics of a numerical aperture over 0.1, can be corrected without modifying third-order-aberration coefficients. A third-order aspherical design with a large misalignment tolerance was optimized to have a 50x magnification and a numerical aperture of 0.25, which resulted in a spatial resolution higher than 20 nm with tolerating a mirror decenter of 1μm.