An objective lens for an electron microscope with compensated axial chromatic aberration

Abstract

The resolving power of transmission microscopes is limited by diffraction and by spherical and chromatic aberrations of the objective lens.To compensate the axial chromatic aberration, electric quadrupole fields may be used (1,2). We therefore combined a condenser-objective lens of 2 mm focal length with an electron-optical aplanator (3). Earlier experiments showed that the axial chromatic aberration of the aplanator can be compensated by counteracting electric and magnetic quadrupole fields (4,5). High values of these correcting fields yield a negative chromatic error of the aplanator. It compensates the positive chromatic aberration coefficient of 1.5 mm of the objective lens.The specimen is situated near the middle plane of the condenser-objective lens. This lens produces a 60-fold magnified virtual image in a plane 122 mm above the specimen. The aplanator transfers this image 1:1 into the object plane of the intermediate lens and compensates the aberrations (6).Figure 1 shows a micrograph of a resolution test specimen.