1. A process for providing illumination conditions for accurate determination of Zernike tilt coefficients in the presence of third-order coma for a lithographic projection system, the process comprising: selecting an optimized illumination condition; performing a lens distortion test method using an optimized illumination condition selected from the determined illumination conditions; constructing a lens distortion map in accordance with the collected illumination conditions and calculating Zernike tilt terms a2 and a3 in accordance with the lens distortion map such that the calculated Zernike tilt terms correspond to calculations in the absence of the effects of third-order coma for field positions of interest.