Abstract text (incl. figure legends and references)

In cryo electron microscopy (cryo EM) the resolution of the TEM hasn"t been the limiting factor for a long time. Recently, however, cryo EM has reached the single Ångstrom level [1] where the aberrations of the TEM start to play a significant role in the process of image formation. There are multiple aspects where aberration correctors might be very useful for cryo EM techniques:

1. Coma-free alignment: The present spherical aberration requires extremely high accuracy and stability of beam tilt alignment to prevent beam tilt induced axial coma. This condition has to be stable for a very long time to allow for long-term automated data acquisition. In a Cs-corrected TEM a beam tilt of several mrad is acceptable without noticeably introducing axial coma.

2. Beam-image-shifts: To avoid mechanical specimen shifts and improve throughput, it is common practice to use combined beam-image-shifts during image recording rather than physical specimen shifts. These virtual object shifts are affected by the unavoidable off-axial coma of the objective lens (Fig.1). Precise simultaneous adjustment of beam tilt during beam-image-shift allows to compensate for this effect [2]. Alternatively, a Cs-corrector can actively compensate the shift-induced coma below the object during image shift and in that case the beam tilt above the specimen does not need to change hence remains flexible, e.g. for dispersion correction. Cs-correctors allow for aberration-free beam-image-shifts of up to 10µm, which is extremely useful for efficient work-flows for single particle reconstructions.

3. Large cameras with a high number of pixels are important to increase throughput: The large field of view which is captured at once, however, is affected by the off-axial coma in that the coma increases linearly from the center of the image towards the outer areas. While this effect is negligible for 2k cameras, it can have a distinct impact on 4k detectors with a similar pixel size. Fortunately, this limitation can be overcome by the off-axial coma corrector (BCOR) which enables aplanatic imaging, i.e. freedom from spherical aberration and off-axial coma [3].

4. Inelastically scattered electrons usually do not contribute to the high-resolution image contrast due to chromatic aberration. A Cc/Cs-corrector is expected to significantly improve contrast for larger sample thicknesses [4].

References:
[1] K.M. Yip et al., Nature 587, 157–161 (2020).
[2] Konings et al., Microsc. Microanal. 25 S2 (2019) 1012-1013.
[3] H. Müller et al., Nucl. Instrum. Methods Phys. Res. A 645 (2011) 20-27.
[4] J.L. Dickerson et al, Ultramicroscopy 237 (2022), 113510.
[€] CEOS GmbH has received funding from the European Union"s Horizon 2020 research and innovation program under grant agreement No. 823717 – ESTEEM3.

Fig. 1 – (a) Simultaneous use of beam and image deflectors for virtual object shifts is always affected by off-axial coma (b, red). Uncorrected TEMs require an additional simultaneous beam tilt to counterbalance off-axial coma effects (magenta). In Cs-corrected TEMs (blue), the corrector elements compensate the off-axial coma contribution and the beam tilt remains flexible. For large cameras the off-axial coma even changes the image quality within the camera's field of view. To remove this effect as well, a corrector for off-axial coma (e.g. BCOR) has to be used (cyan, dashed).