Optical path difference, δ(x, y; d_z), function of a Gregorian telescope. The red light ray is in focus and the orange is out of focus, given a certain axial offset, d_z. Blue lines and circles are zoom-in sections. The top right dashed line represents the subreflector offset in the z_f-axis. The diagram on the left-hand side corresponds to the primary reflector and its axial defocus. The right-hand side image is the continuation of the same light ray, A, reflected on the primary reflector, which reaches the second focus of the ellipse. The axial offset is the same for both foci. The angle γ₂ exists between the points $p_2-g-{p^{\prime }}_2$.