4-fold Parasitic Aberrations shown in
Ronchigram after Aberration Correction
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After the corrections of lower order aberrations, the corrector normally introduces or leaves high order aberrations. For instance, the Ronchigram with a 54 mR aperture indicated in Figure 3649 shows the left 4th order coefficient of order 0.5 - 2.0 mm limiting the probe shape and size after 3rd order aberration correction by Nion quadrupole-octupole corrector. The pattern is strongly 4-fold symmetry with weak 2-fold structure in the opposing corners. Note that this uniform 4-fold symmetry was actually imposed by the octupoles in the corrector, so called parasitic aberrations.

Ronchigram obtained with 4th order coefficients

Figure 3649. Ronchigram obtained with 4th order coefficients corrected to < 60 µm [1].

 

 

 

[1] P. E. Batson, Control of Parasitic Aberrations in Multipole Corrector Optics, Microsc Microanal 14(Suppl 2), 2008.

 

 

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