Coma-Free Point/Plane
- Practical Electron Microscopy and Database -
- An Online Book -  

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.


Because the spherical aberration is unavoidable for rotationally symmetric electron lenses in EMs (electron microscopes), it can never be aplanatic. However, each electron lens has a coma-free point that locates on the optic axis within the field of the lens. No coma is introduced into the images by any off-axial pencil of the rays whose central ray intersects the coma-free point. In aberration-corrected TEMs, in order to eliminate the off-axial coma, the coma-free point of the lens next to the corrector must match the corresponding point of the corrector field.

Later on, Haider et al. [1] corrected the aberrations in TEM imaging mode successfully. Figure 2766 shows an example of diffractogram tableaus of the aligned TEM under both uncorrected and corrected conditions. The figures taken under the uncorrected condition demonstrates that the electron beam tilt about the coma-free pivot point introduces primarily a defocus and an axial twofold astigmatism. The strength of these aberrations does not depend on the azimuthal direction of incident beam due to the rotational symmetry of the aligned TEM. After the aberration correction all diffractograms in the tableau exhibit approximately the same appearance revealing the properties of an aplanatic lens. In this case the illumination tilt does neither introduce defocus nor two-fold astigmatism confirming the correction of spherical aberration.

Diffractograms obtained in an aligned microscope without (a) and with (b) correction of the spherical aberration

Figure 2766. Diffractograms obtained in an aligned microscope without (a) and with (b) correction of the spherical aberration. τ stands for tilt angles. [1]


[1] Maximilian Haider, Harald Rose, Stephan Uhlemann, Bernd Kabius and Knut Urban, Towards 0.1 nm resolution with the first spherically corrected transmission electron microscope, Journal ofElectron Microscopy 47(5): 395-405 (1998).



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