Toroidal mirrors are aspherical mirrors where each curvature of orthogonal two axes (horizontal and vertical one) are different.
When a spherical mirror focuses a light from a point source, there appears astigmatism. Toroidal mirrors, however, minimize such astigmatism and make it possible to focus on a small spot. The toroidals are ideal for focusing and/or aberration correction devices for spectroscopes.
A toroidal mirror has been produced and used as a prereflector in the optical system of the soft X-ray spectroscopy. The mirror reduces the astigmatism inherent in a grazing-incidence monochromator and the intensity of radiation available for experiments is appreciably increased. Undesirable higher order radiation which appears in the first order spectrum is found to be efficiently removed when the mirror is used in the high energy spectral region. Critical cut-off wavelength in a few materials is discussed.