I have been asked to write down the taylor expansion for two concentric spherical surfaces with radii r1 and r2 of a lens and mirror combination which I think is called a Mangin Mirror. It is supposed to be related to the x and y axis positions. Also, I have been asked to derive the optical path length of the incident light being reflected back to the surface of the lens with radius r1. I don't know were to start for either question. Any ideas would be much appreciated!!