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The secondary mirror is an ellipse, (oval like shape), and not a circular shape like your main mirror. When viewed at 45 degrees, this elliptical shape then appears to have the profile of a circle.
An ellipse has a long axis (major axis) and a short axis (minor axis) whereas a circle has just the one diameter.
I think it was Kepler who first proposed that the Planets orbit the Sun in elliptical orbits and not circular orbits?
So, you need to measure the short axis (minor axis).
Cheers
Dennis
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