Figure 3666a shows the schematic illustration of axial coma aberration C2,1 (or B2).
Figure 3666a. Schematic illustration of aberration coefficient C2,1 (B2): axial coma aberration.
The aberration coefficient C2,1 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 keV-electron probes at 100 nm of B2 (second-order axial coma), 100 nm of A2 (three-fold axial astigmatism), 1 µm of S3 (axial star aberration of the 3rd order), and 2 µm of A3 (four-fold axial astigmatism).
Figure 3666c. Simulated intensity distribution patterns of 200 keV-electron probes: (a) B2 = 100 nm, (b) A2 = 100 nm, (c) S3 = 1 µm, and (d) A3 = 2 µm. In this simulation, the defocus C1 was set to -3 nm, the imaginary parts of the aberrations to zero, and the illumination semi-angle to 30 mrad. 
Note that the C2,1 (B2) can be corrected with Ronchigram when the microscope is operated in STEM mode.
 Simulation of Rolf Erni.