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

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In caustic image theory, a coma-free condition means that the projection of the direct electron beam lies in the center of the caustic curve. This alignment method uses the caustic figures, such as caustic curve or aberration free point (the center point of caustic curve), for finding the optical axis. The positions of the caustic curve and aberration-free point do not depend on the direction of the incident electron beam.

Therefore, coma aberration can be corrected by centering the direct beam [1]. This beam centering can be done by bringing the direct electron beam to the center of caustic curve using the beam tilt controls. For crystalline samples, either tilting the sample from the zone axis or using an adjacent thin region is required to perform the coma-free alignment. Note that it is convenient to employ a caustic curve to accomplish the coma-free alignment and then align the voltage center or current center if necessary.

[1] K. Kimoto, K. Ishizuka, N. Tanaka, Y. Matsui, Ultramicroscopy 96 (2003) 219.