This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

=================================================================================
In coma case, most of its intensity distributes on one side and a “comet tail” on the other.
Figure 4239a shows typical Ronchigrams taken at the edge of an amorphous carbon film. At defoci (defined by zheight), there is a distance between the electron crossover and the point on the specimen along the optic axis. At large underfocus, electron rays at all angles cross the optic axis after the specimen and it shows a shadow image of the specimen edge. At small underfocus, lowangle rays cross the optic axis after the specimen, while highangle rays cross before the specimen due to spherical aberration. Therefore, the shadow image changes in magnification as a function of the angle. The lowangle asymmetry indicates the presence of astigmatism. At Gaussian focus, the lowestangle rays cross the axis at the specimen, while higherangle rays cross before the specimen due to the spherical aberration. The coma free axis is defined at this focus and all alignment and positioning of detectors and apertures can be performed with respect to the lowangle “disk”. Defocus and spherical aberration can effectively cancel each other at those lowest angles. Axial astigmatism can be accurately corrected by using the stigmator coils, resulting in circularly symmetric Ronchigram features. At overfocus, rays at all angles cross the axis before the specimen.
Figure 4239a. Ronchigrams of a thin amorphous carbon (C) film at: (a) Large underfocus, (b) Small underfocus, (c) Gaussian focus, and (d) Overfocus.
[1]
In TEM observations, axial coma can easily be distinguished from twofold astigmatism, because the axial coma pattern does not change its intensity orientation from overfocus to underfocus.
Figure 4239b shows the schematic comparison of Zemlin (diffractogram)tableau characteristics for the axial aberrations up to third orders. Firstorder aberration (e.g. defocus and twofold astigmatism, A_{1}) shows the elliptical distortion even without electron beam tilting because the impact of firstorder aberrations does not depend on the tilt angle. For the aberrations of higher orders (n≥2), such as secondorder axial coma B_{2}, threefold astigmatism A_{2}, thirdorder spherical aberration C_{3} (>0), thirdorder star aberration S_{3} and fourfold astigmatism A_{3}, there is no elliptical distortion observable for the untilted case. In these cases, only at illumination tilts the characteristic distortion due to the aberrations becomes discernible. Note that the higherorder aberrations have equal symmetries to the ones in Figure 4239b as discussed in page3740.
Figure 4239b. Schematic representation of Zemlin (diffractogram)tableau characteristics for the axial aberrations up to third order.
[1] E.M. James, N.D. Browning, Practical aspects of atomic resolution imaging and analysis in STEM, Ultramicroscopy 78 (1999) 125139.
