Toroidal reflector
A toroidal mirror is a form of parabolic reflector which has a different focal distance depending on the angle of the mirror. The curvature is actually that of an elliptic paraboloid with 
. If the shape were that of a toroid, the mirror would exhibit spherical aberration.
Toroidal mirrors are used in Yolo telescopes and optical monochromators. In these devices, the source and detectors of the light are not located on the optic axis of the mirror, so the use of a true paraboloid of revolution would cause a distorted image.
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