Two-Mirror Offner Compensator

Hi all,

in the book "Optical Shop Testing" from D. Malacara is a chapter about the
Two-Mirror Offner Compensator. Page 440 in the second edition, page 468 in
the third edition.

It seems Eta_C is the focal ratio of the mirror under test, which is defined
as (focal_length / diameter).
(2 * Eta_C) is the focal ratio of the light cone in the center of curvature.
That's written 2 pages earlier.

Eta_N is the focal ratio at the input of the compensator, where the
spherical wave enters.

My question: How is Eta_N defined? Let's assume the result for Eta_N is 3.
Does that mean
a) that I see a f/3 mirror when I look into the compensator or
b) that I see a f/1.5 mirror, because then the ratio (radius_of_curvature /
diameter) is 3 ?

I assume the latter is correct, but I'm not sure.

Thanks,
Michael


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