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Figure 3666a shows the schematic illustration of axial coma aberration C_{2,1} (or B_{2}).
Figure 3666a. Schematic illustration of aberration coefficient C_{2,1} (B_{2}): axial coma aberration.
The aberration coefficient C_{2,1} will displace radially the centers of concentric circles, for instance, the three circles in different sizes has different displacements, therefore, the coma aberration distorts a round beam into a characteristic comet shape, namely forming axial coma aberration in the imaging as shown in Figure 3666b.
Figure 3666b. Schematic illustration of the coma effects on TEM imaging.
Figure 3666c shows the simulated intensity distribution patterns of 200 keVelectron probes at 100 nm of B_{2} (secondorder axial coma), 100 nm of A_{2} (threefold axial astigmatism), 1 µm of S_{3} (axial star aberration of the 3rd order), and 2 µm of A_{3} (fourfold axial astigmatism).
Figure 3666c. Simulated intensity distribution patterns of 200 keVelectron probes: (a) B_{2} = 100 nm, (b) A_{2} = 100 nm, (c) S_{3} = 1 µm, and (d) A_{3} = 2 µm. In this simulation, the defocus C_{1} was set to 3 nm, the imaginary parts of the aberrations to zero, and the illumination semiangle to 30 mrad. [1]
Note that the C_{2,1} (B_{2}) can be corrected with Ronchigram when the microscope is operated in STEM mode.
[1] Simulation of Rolf Erni.
