Patent ID: 6861651

Claim:
An electron-optical corrector for eliminating third order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberrations of the first degree, the corrector having a construction which is symmetrical about the central plane in the direction of the linear optical axis, wherein a first hexapole of a first length is first positioned in the direction of the beam path, followed by a first circular lens, a second hexapole of a second length, followed by a second circular lens, which is followed by a third hexapole with the same strength as the first hexapole and twice the first length of the first hexapole, the separation of the first circular lenses of the same strength being 2f of the focal length, the second hexapole being positioned in the principal focus between the first and second circular lenses, and the distance of the circular lens from the first hexapole being chosen such that the inner plane of the first hexapole comes to lie in the front principal focus of the circular lens that is positioned downstream, and the center of the second and third hexapoles is positioned in the focal plane and this is finally followed by further elements of the corrector, which are set up symmetrically to the center plane of the third hexapole.