The coatings used on telescope mirrors and other optical interfaces ca
n have a marked effect on an optical system's image quality. We descri
be the wave-front aberrations, particularly the defocus and on-axis as
tigmatism, that are induced by the s- and p-phase shifts of coatings.
These coating-induced wave-front aberrations are very small, particula
rly near the design wavelengths of the coatings, but they can under ce
rtain circumstances overshadow the geometric wave-front aberrations of
the system. The wave-front aberrations that are induced by reflection
-enhanced coatings on an f/1.5 Cassegrain telescope are numerically ev
aluated as an example. A theory of coating-induced on-axis astigmatism
and chromatic aberration is presented, and a simple algorithm that us
es polarization ray tracing to calculate coating-induced defocus and a
stigmatism coefficients in radially symmetric systems is provided.