Geometrical aberration of a generalized Wolter type I telescope.
Using two hyperboloids, we can control spherical aberration and coma of an imaging x-ray telescope. The coma-free condition is satisfied only at a particular radius of the first mirror. The best performance is obtained when we allow the coma-free condition at the outer edge and cancel about half of the defocused image for the largest height of an object by introducing spherical aberration.
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Nariai