3回非点収差

three-fold astigmatism

[目次：レンズ系]

2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3次）。六極子場を使って3回非点収差をキャンセルすることができる。
現在広く用いられている2段六極子型球面収差補正装置では、3回非点場内でのコンビネーション効果を用いて対物レンズの球面収差補正を行っている。その収差補正系では、1段目の六極子で大きな3回非点収差と負の球面収差を発生させる。続いて配置されている2段目の六極子で、負の球面収差と逆向きの3回非点収差を発生させ、1段目で生じた3回非点収差を同時にキャンセルし、発生させた負の球面収差によって対物レンズの正の球面収差を補正している。

The three-fold astigmatism, one of the second-order axial geometrical aberrations, is a parasitic aberration exhibiting three-fold symmetry. (Note that it is called a third-order aberration in terms of the wave aberration.) The aberration is corrected using magnetic fields produced by hexapoles.
A widely used two-stage hexapole Cs corrector corrects the spherical aberration of the objective lens using a combination effect in the three-fold astigmatism field. In the Cs correction system, the first hexapole simultaneously produces a large three-fold astigmatism and a negative spherical aberration. Then, the second hexapole placed below the first hexapole produces a three-fold astigmatism having an opposite sense to the former and a negative spherical aberration. The three-fold astigmatism produced in the first stage is canceled by that with an opposite sense produced in the second stage. The positive spherical aberration of the objective lens is corrected by the negative spherical aberration produced in the two stages.

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