Aperture-plane phase change due to defocus of a Gregorian telescopeΒΆ

The equation for the phase change due to a defocus of a Gregorian telescope is the same as given in the papers for a Cassegrain telescope:

\delta p(R) = \delta z \left( \frac{1-a^2}{1+a^2} + \frac{1-b^2}{1+b^2} \right)

where a=\frac{R}{2F_1}, b=\frac{R}{2F_2}, F_1 is the focal length at prime focus and F_2 is the effective focal length at the Gregorian focus.

The derivation of this formula is as follows:

Geometry for prime focus defocus Geometry for Gregorian focus defocus

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