From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 


Clear aperture is the diameter of the optical assembly or reflecting
surface that contributes to the designed flow of light in an optical
system.  So clear aperture normally excludes the bevel, and any painted-out
or obstructed parts of the optic that do not contribute to the optical
result.

Sometimes, the clear aperture can be further limited by specification.
Since the edge of an optic is frequently the optically weakest part, it
might be said that the optic has a clear aperture of some diameter less
than the full polished diameter, for meeting a specific performance
specification.  One maker of paraboloidal (such a nice word!) telescope
mirrors specifies that a "clear aperture" of 98% of the mirror's
diameter will meet his promised wavefront specs -- this essentially
excludes the edge from being considered, and is an honest, pragmatic spec. 
When specifications like this are given, it is assumed that the area
outside the promised "performance clear aperture" must be masked
off, so in a way this is not really a special form "clear
aperture."

Bill T.


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