One of the most difficult problems on the way to sub-Ångstrom resolution at a medium accelerating voltage of about 200 kV, the correction of the spherical aberration, has been solved. Already more than fifty years ago, Scherzer showed that the two major axial aberrations, the chromatic and the spherical aberration, can be corrected by a suitable combination of electrostatic and/or magnetic multipole elements. Due to the availability of high-coherent electron sources with a small relative energy width DE/E the necessity of correcting the chromatic aberration can be avoided.

Our solution for spherical aberration correction follows a suggestion by Rose. It is based on two electromagnetic hexapoles and four additional lenses. The principle by which correction is achieved is based on the fact that the primary aberrations of second order of the first hexapole are compensated by the second hexapole element.