スリーローブ収差

three-lobe aberration

[目次：レンズ系]

4次の軸上幾何収差の一つで3回対称の寄生収差（波面収差の次数では5次）。4次の軸上幾何収差にはこの他に4次コマ収差と5回非点収差がある。対称性が3回である収差には、スリーローブ収差の他に（より低次な収差として）2次軸上幾何収差である3回非点収差がある。2段六極子型球面収差補正装置の場合、3回非点収差を補正した状態であっても、ロンチグラム図形の高角度領域に3回対称の図形が検知される場合は、このスリーローブ収差の残留が原因であることがある。

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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