Diffraction astigmatism

So far, only defocus and spherical aberration have been considered as aberrations affecting the image contrast. Both depend only on the magnitude of the spatial frequency g[ but not on the diffraction direction, thus resulting in rotationally symmetric phase shifts (Equation 11). However, the objective lens may exhibit further aberrations resulting in additional phase shifts, which are not necessarily rotational symmetric. The most important of these additional aberrations are astigmatism and coma. 

As with optical lenses, electromagnetic lenses have aberrations (chromatic aberration, spherical aberration, electron diffraction limit, astigmatism, etc.) each entailing an enlargement of the electron probe expressed by the diameter of a circle of least confusion. Under standard operating conditions, when the astigmatism has been adjusted, only spherical aberration plays a significant role. The expression of the probe diameter becomes ... 

Beam shaping of the excitation beam. The beam from a laser diode has a no-tieeable astigmatism and is not focused into a diffraction limited spot. Improv-... 

The optical diffraction pattern of an image is frequently used in high resolution TEM to determine the defocus, astigmatism and true resolution from the MTF of the image. Polymer... 

Diffraction astigmatism It is also called intermediate lens astigmatism. Use the largest condenser aperture and spread the beam with brightness fully clockwise. In the diffraction mode, a caustic image can be obtained. Adjust the intermediate lens stigmator to obtain a caustic triangle (similar to the one as shown in Fig. 5.2). Finally adjust the diffraction focus to obtain a very small and distinctly visible spot. 

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