スター収差

star aberration

[目次：レンズ系]

3次の軸上幾何収差の一つで2回対称の寄生収差（波面収差の次数では4次）。対物レンズの球面収差が大きいため普通には検知できないが、対物レンズの球面収差補正を行った後には検知される。スター収差が残留する場合、ロンチグラム図形の高角度領域に2回対称のパターンが現れる。球面収差補正装置を搭載した場合には、一般的には観察前に収差測定を行い、補正装置内の偏向素子を用いた自動収差補正機能によって各種の収差（スター収差も含まれる）が補正されるようになっている。

The star aberration, one of the third-order axial geometrical aberrations, is a parasitic aberration exhibiting two-fold symmetry. (Note that it is called a fourth-order aberration in terms of the wave aberration.) The star aberration cannot usually be detected due to the existence of a large spherical aberration of the objective lens, but is detected after correcting the spherical aberration of the objective lens. The star aberration appears as a pattern with a two-fold symmetry in high angle areas of a Ronchigram. In the case of a TEM equipped with a Cs corrector, prior to microscope observation, aberrations are measured and then the aberrations including the star aberration are corrected by the automatic aberration correction function with the use of the deflectors of the Cs corrector.

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