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 TEM a stigmator is used to compensate for the fixed threefold astigmatism of the objective lens [1].

Note that a single aberration violating the π/4 criterion may also cancel out the other one with the same symmetry if the have different sign, such as a small positive third-order spherical aberration and a small underfocus, or a small positive twofold astigmatism and a small negative third-order star aberration with the same azimuth.

[1] Wang, Y.C., Fitzgerald, A., Nelson, E.C., Song, C., O’Keefe, M.A.& Kisielowski, C. (1999). Effect of correction of the 3-fold astigmatism on HREM lattice imaging with information below 100 pm. In Proceedings of the 57th Annual Microscopy Society of America Meeting, pp. 822–823. New York: Springer-Verlag.