  Wavefront aberrations are one of the largest uncertainty factors in present
atom interferometers. We present a detailed numerical and experimental analysis
of this effect based on measured aberrations from optical windows. By placing
windows into the Raman beam path of our atomic gravimeter, we verify for the
first time the induced bias in very good agreement with theory. Our method can
be used to reduce the uncertainty in atomic gravimeters by one order of
magnitude resulting in an error of less than $3\times 10^{-10}\,g$ and it is
suitable in a wide variety of atom interferometers with thermal or ultra cold
atoms. We discuss the limitations of our method, potential improvements and its
role in future generation experiments.
