Abstract

We present an approximate technique, based on the principles of multimode Gaussian optics and perturbation theory, for calculating the distortions that occur when an off-axis ellipsoidal mirror is placed in the path of a weakly diffracting beam. More specifically, we calculate the Gaussian-beam-mode scattering matrices of off-axis ellipsoidal mirrors. The technique can be applied when the phase errors across the surface of the mirror are small. In this case, power is only scattered into a few neighbouring modes, and simple closed-form expressions can be derived for the elements of the scattering matrix.