Gaussian beam modes analysis for off-axis mirror with feed position displacement

In quasi-optical system, the distortion and cross polarization caused by off-axis mirrors should be paid attention to seriously. In this paper, we improve a Gaussian beam modes analysis (GBMA) originally described in [1] to apply in a more common situation, an offset mirror illuminated by any feed with position displacement. Here, we provide a number of significant expressions of the theory and validate this technique with two examples of off-axis conic surface mirrors. In order to verify the accuracy of this method, we compare the results from this approach and the physical optics (PO) in commercial software GRASP10 including the amplitude and phase in co-polarization and cross-polarization.