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 (mirrors C and E in the diagram). 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.