fifth-order spherical aberration

Among the aberrations of rotationally symmetric magnetic and electrostatic lenses, the aberration proportional to α⁵ (α: an angle between an electron beam and the optical axis) is called "fifth-order spherical aberration." Not only the third-order spherical aberration, but also the fifth-order spherical aberration inevitably exists in an electron lens. The term, "spherical aberration," normally indicates the third-order aberration proportional to α³, and a symbol Cs is used as the spherical aberration coefficient. On the other hand, a symbol C₅ is normaly used as the fifth-order spherical aberration coefficient. In recent years, the third-order spherical aberration has been corrected, thus the fifth-order spherical aberration has been needed to be considered. Fortunately, the use of a Cs corrector with the two-stage hexapoles and transfer lenses between them makes possible the value of C₅ variable, thus C₅ = 0 being available.

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