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Figure 5
Our interpretation of the aberration plots in Fig. 4[link]. The presence of all four neighbouring spots in the symmetrical plot, together with the absence of the vertical neighbours from the antisymmetrical plot, indicates that the VPP spots were scanned in a vertically alternating and horizontally unidirectional sense, as shown in the first image. This partitions a majority of the spots into two classes, a and b, in which the direct vertical neighbour is located on opposite sides. The total phase shift induced by the neighboring spots is decomposed into an antisymmetrical and a symmetrical part. Both of them are averaged over particles from both classes during estimation, so the vertical neighbor partially cancels out in the antisymmetrical plot but not in the symmetrical plot.

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