Astro-Dictionary: Zonal Defect

This is an optical defect that is isolated to a small part of a lens or mirror. Let’s take an example: looking at a lens as if it were a clock face, it might only affect a small area between 1 and 2 o’clock, a quarter of the way in from the edge of the lens. This can be an area that is overcorrected or undercorrected but it does not occur all the way around the lens. A shorthand term often used is that the optic has "zones".

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