Orion Clear aperture telescope

So contrast is a function of aperture less the diameter of the obstruction (A-O), while the light gathering area is a function of area (πA^2-πO^2)?

("^" denotes "to the power of" in the absence of superscript)

Solpadeine anyone?

Correct John.
The guy who figured all this out was William Zmek.
He wrote it up in two issues of Sky and Telescope:July and September 1993.

He brought clarity to a very cloudy issue.

Peter.