three-lobe aberration

The three-lobe aberration, one of the fourth-order axial geometrical aberrations, is a parasitic aberration exhibiting three-fold symmetry. (Note that it is called a fifth-order aberration in terms of the wave aberration.) In addition to the three-lobe aberration, the fourth-order axial geometrical aberrations include the fourth-order coma aberration and the five-fold astigmatism. In the aberrations exhibiting three-fold symmetry, there is the three-fold astigmatism which is the second-order axial geometrical aberration and a low order aberration compared to the three-lobe aberration. In a two-stage hexapole Cs corrector, even when the three-fold astigmatism is corrected, a Ronchigram sometimes exhibits a three-fold symmetry pattern at its high angle areas. This pattern can be due to the residual three-lobe aberration.

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