The eight distinct reflection configurations for pairs of toroidal mirrors are investigated by numerically calculating their aberration patterns for on- and off-axis source points. The best performance is obtained for a Z-shaped configuration where the two mirrors deflect in opposite directions across a common interior sagittal and tangential focus. Imaging properties can be improved significantly in some cases by replacing the toroidal elements with their elliptical or parabolic equivalents. The best overall performance is obtained with parabolic mirrors in a U-shaped configuration.
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