Electron-beam-tilt-induced Coma
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As the resolution of TEM imaging increases, a small amount of beam tilt can result in a significant amount of coma.

The electron-beam-tilt-induced coma can be given by,

           electron-beam-tilt-induced image displacement in TEM ---------------- [1913a]
where,
        w = α exp(iϕ) ------------------ [1913b],
        Cs -- The spherical aberration coefficient,
        b = B exp(iϕB),
        α -- The scattering angle,
        B -- The coefficients corresponding to coma displacement,
        ϕB -- The initial phase of corresponding aberration.

Note that in Cs-corrected EMs, due to the absence of spherical aberration it is not possible anymore to correct the residual axial coma by tilting the illumination beam. In this case an appropriate coma compensator is needed to eliminate the coma.

 

 

 

[1] Koji Kimoto, Kazuo Ishizuka, Nobuo Tanaka, Yoshio Matsui, Practical procedure for coma-free alignment using caustic figure, Ultramicroscopy 96 (2003) 219–227.

 

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