This article describes the task of optical designers to achieve a better design. This is followed by some discussions on the necessity of total design that takes all the elements into account from its light source to the final image plane. Methods are given to simulate rays from a bending magnet and an undulator

surface figure errors

and thermal deformations. Some examples are given for an undulator beamline and an EUVL optical system

together with tolerance estimates of the figure error and thermal deformation.