Stigmats using two curved Fresnel surfaces.
Any two-surfaced optical system, either refracting or reflecting, can be made stigmatic (aplanatic and anastigmatic) for object at infinity by using curved Fresnel surfaces instead of the usual conics. Such systems will also have a flat field under certain conditions and may even be distortion free. It is also shown that systems of flat Fresnel miirrors have incurable line coma.
Delano
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