Patent ID: 11081313

Abstract:
A corrector has a strength of a central hexapole field (ΨHP2) which is selected such that the threefold axial astigmatism (A2) vanishes and the strengths of two equal outer hexapole fields (ΨHP1,3) are selected such that the overall corrector (5) does not have a sixfold axial astigmatism (A5). The length (L) of the central multipole element (2) in relation to the lengths (L′) of the multipole elements (1 and 3) is chosen such that the axial three-lobed aberration of sixth order (D6) vanishes. A separation between the outer multipole elements (1 and 3) and round lenses (7″, 8″) further spaced apart from a symmetry plane (6) of the corrector corresponds to the focal length (f′) of those round lenses (7″, 8″) plus an additional separation (Δz) which is chosen such that the axial three-lobed aberration of fourth order (D4) vanishes for the given lengths L and L′.