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Dave,
Sorry... an ellipse by definition is a section from a cone...not a section through a cylinder.
If you do the drawings of similar major axis sized outlines of an ellipse and superimpose the cylinder section you'll see that the outline shapes are different. Based on a 100mm diagonal I used in a 12" f5 design the total loss of "efficiency" was about 10% when a cylindrical section was used.
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