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Tilted illumination exit-wave restoration is compared for two aberration-corrected instruments at different accelerating voltages. The experimental progress of this technique is also reviewed and the significance of off-axial aberrations examined. Finally, the importance of higher order aberration compensation combined with careful correction of the lower order aberrations is highlighted.
Aberration correction leads to a substantial improvement in the directly interpretable resolution of transmission electron microscopes. Correction of the aberrations has been achieved electron-optically through a hexapole-based corrector and also indirectly by computational analysis of a focal or tilt series of images. These direct and indirect methods are complementary, and a combination of the two offers further advantages. Materials characterization has benefitted from the reduced delocalization and higher resolution in the corrected images. It is now possible, for example, to locate atomic columns at surfaces to higher accuracy and reliability. This article describes the JEM-2200FS in Oxford, which is equipped with correctors for both the image-forming and probe-forming lenses. Examples of the use of this instrument in the characterization of nanocrystalline catalysts are given together with initial results combining direct and indirect methods. The double corrector configuration enables direct imaging of the corrected probe, and a potential confocal imaging mode is described. Finally, modifications to a second generation instrument are outlined.
Methods for accurate and automated determination of the coefficients
of the wave aberration function are compared with particular emphasis on
measurements of higher order coefficients in corrected instruments.
Experimental applications of aberration measurement to the determination
of illumination isoplanicity and high precision local refinement of
restored exit waves are also described.
Exit wave restoration using focus series of images has become a widely
used technique to improve image resolution and interpretation. To
understand the effects of the imaging approximations used, we have
critically compared the specimen exit wave functions restored using the
efficient linear Wiener filter, with a general nonlinear maximum