This is the depth of the optical center of the mirror with
respect to the edges. The distance "s" in the following
figure:
It is best calculated by using the formula that describes the shape. (I.e. for Newtonians the parabolic or the circle) The value of the sagitta is equal to the height of shape at the edge of the mirror. In the formulas presented on the pages "Paraboloidal" and "Sphere" use the value of half the mirror diameter for "r".