Closed-form representations for third-order aberrations of two-aspherical mirror aplanats with the mirror tilt and decenter

Approximate equations are derived for imaging aberrations of two-aspherical-mirror aplanats for soft X-ray imaging. These expressions include the Seidel terms for unperturbed states and the axial coma resulting from the tilt and decenter of the mirror surface. A simple analytical method for reducing both the Seidel terms and the axial coma enables us to design novel soft X-ray imaging systems utilizing a wide field of view with reduced sensitivity to mirror misalignments. The method is illustrated by applying it to a two-mirror system for soft X-ray microscopy with a magnification m =−1/50. The design example showed an appreciable reduction in sensitivity, as compared with the existing anastigmatic designs that include a standard Schwarzschild configuration. Imaging aberrations of the novel system are also confirmed by a numerical ray tracing method.