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Figure 2
Antisymmetrical and symmetrical aberration experiments on aldolase and the 20S proteasome at 200 keV. The upper image of each pair shows the independent phase-angle estimates for each pixel, while the lower image shows the parametric fit using Zernike polynomials. These types of aberrations are referred to as trefoil or threefold astigmatism (left) and fourfold astigmatism (right). The proteasome trefoil exceeds 180° at the very high frequencies, so the sign in the per-pixel plot wraps around. This has no impact on the parametric fit. The dashed circles indicate resolutions of 1.94 and 1.98 Å, respectively.

IUCrJ
Volume 7| Part 2| March 2020| Pages 253-267
ISSN: 2052-2525
https://doi.org/10.1107/S2052252520000081