Device for correcting third-order spherical aberration in a lens, especially the objective lens of an electronic microscope

An apparatus for correcting a third-order spherical aberration in a lens, comprising:

• an objective lens;
• a correction device positioned in a downstream direction of a propagation of optical rays, said correction device comprising a first hexapole and a second hexapole with a round-lens doublet being arranged between said first hexapole and said second hexapole, said round-lens doublet comprising a first round lens and a second round lens of equal focal lengths, with said first round lens and said second round lens positioned with a distance between one another equal to two focal lengths of said equal focal lengths, said first round lens being a distance of one said focal length from a center plane of said first hexapole and said second round lens being a distance of one said focal length from a center plane of said second hexapole;
• and,
• a third round lens and a fourth round lens positioned between said objective lens and said correction device with said third round lens having a different focal length than it said fourth round lens, said third round lens being closer to said objective lens than said fourth round lens and said fourth round lens being closer to said correction device than said third round lens, with a distance between said third round lens and an aberration-free plane of said objective lens being equal to a focal length of said third round lens, a distance between said fourth round lens and an aberration-free plane of said correction device being equal to a focal length of said fourth round lens, and a distance between said third round lens and said fourth round lens being equal to a sum of said focal length of said third round lens plus said focal length of said fourth round lens.