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 in TEM, as shown in Figure 1923 the presence of astigmatism of the condenser lens (CL) result in an elliptical beam (the bright strip) on the specimen, causing an elliptical shape of the caustic curve even though there is no astigmatism of the objective lens (OL).

Figure 1923. Caustic image taken from an amorphous carbon (C) film under
an illumination condition with condenser lens astigmatism. Adapted from [1]

[1] Q. Xing, T. A. Lograsso, A rapid method to correct objective lens astigmatism in a TEM, Ultramicroscopy 109 (2009) 287–290.