A method for forming an optical element deposits a first thin film coating to form a first coated surface on a first transparent substrate that is substantially flat, wherein, for incident light at an angle of incidence exceeding 5 degrees, the first multilayer thin film coating transmits a first wavelength band and reflects a second wavelength band. The first coated surface has a first surface form, and further has a second surface opposite the first coated surface with a second surface form. The second surface of the first substrate bonds to a third surface of a second transparent substrate to form the optical element. The third surface has a third surface form prior to bonding and the first coated surface has a fourth surface form after bonding. The difference between the fourth and the third surface forms is less than the difference between the first and third surface forms.