  Deflecting structures are used now manly for bunch rotation in emittance
exchange concepts, bunch diagnostics and to increase the luminosity. The bunch
rotation is a transformation of a particles distribution in the six dimensional
phase space. Together with the expected transformations, deflecting structures
introduce distortions due to particularities - aberrations - in the deflecting
field distribution. The distributions of deflecting fields are considered with
respect to non linear additions, which provide emittance deteriorations during
a transformation. The deflecting field is treated as combination of hybrid
waves HE 1 and HM_1. The criteria for selection and formation of deflecting
structures with minimized level of aberrations are formulated and applied to
known structures. Results of the study are confirmed by comparison with results
of numerical simulations.
