combination aberration

If two thin optical elements (lenses, multi-poles, etc.) are placed at a free space, higher order aberrations than those inherent to the elements can arise due to a synergetic effect of the inherent aberrations. The higher order aberrations induced are called "combination aberrations." A thick hexapole spherical aberration corrector produces a third-order negative spherical aberration by a self-conbination effect of two three-fold astigmatism fields (second-order aberration). With the use of this third-order negative spherical aberration, the third-order positive aberration of the objective lens can be corrected. In the case of the combination between a spherical aberration (Cs) corrector and an objective lens with a free space (incomplete image transfer) between them, a positive or negative fifth-order spherical aberration is induced by a cross combination between the third-order negative aberration of the Cs corrector and the third-order positive aberration of the objective lens. Using the fifth-order spherical aberration produced, the residual fifth-order spherical aberration of the system can be removed.

Related Term(s)