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An electron monochromator design is presented as an instrumental development for electron energy loss spectroscopy (EELS) and imaging in (scanning) transmission electron microscopy ((S)TEM). The main purpose of this development is enhancing the energy resolving power in spectroscopy and filtering. In addition, it helps reducing the effect of the objective lens' chromatic aberration C c in imaging and therefore, enhancing the spatial resolving power of electron microscopes. General estimates for the performance of a monochromator in energy distribution and the resulting usable beam currents are given. The special monochromator design presented is a ground-potential monochromator based on magnetic sector fields. The monochromator generates a spatially and angular un-dispersed spot and has no mechanically actuated parts in the filter sections. The optics can be operated at electron acceleration voltages from 30kV to 300kV and shows an energy resolving power of better than 2⋅10 -7 relative to the primary electron energy. The actual device is designed to be retro-fittable to microscopes from various manufacturers., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Maximilian Haider reports financial support was provided by European Research Council. All Authors report a relationship with CEOS GmbH that includes: employment and equity or stocks., (Copyright © 2023 CEOS GmbH. Published by Elsevier B.V. All rights reserved.)

After the introduction of the hexapole C s -correctors for scanning transmission electron microscopes (STEM), the next big step forward was the strong reduction of the six-fold astigmatism A 5 by means of an advanced hexapole design (DCOR/ASCOR). As a result all axial aberrations up to fifth order are sufficiently small to allow for large semi-aperture angles beyond 40 mrad for electron energies in the range of 30 to 300 kV without deterioration of the STEM resolution. In this paper we derive simple expressions for the optimum hexapole strength for minimum A 5 and the size of the residual A 5 . Both quantities are intrinsic properties of the hexapoles and the transfer lens doublet in between. The optimum hexapole strength scales with the inverse of the electron wavelength, while the residual A 5 does not depend on the electron energy directly, but on the spherical aberration C s of the pole piece. With the given properties of the DCOR/ASCOR and typical values of C s in the range of 0.5 to 2.7 mm, at all acceleration voltages A 5 remains in the range from 0.03 to 0.4 mm, the latter even for a large-gap pole piece., (Copyright © 2019 The Authors. Published by Elsevier B.V. All rights reserved.)