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Figure 2
Anisotropic magnification of the worm-haemoglobin data set became evident from an eigenvector analysis of all amplitude spectra (see §[link]2.5). The second eigenimage revealed a 2.6% ellipticity of the ∼3.6 Å water ring (marked with large arrow in the left spectrum) and the 2.2 Å water ring (medium-sized arrow). A further water/ice ring can be seen at 1.8 Å (small arrow), which is only visible in the corners of the image (beyond the Nyquist frequency) but reflects back into the image everywhere else owing to aliasing (grey arrow). The right spectrum is the same as the left spectrum but is mirrored horizontally, thus changing the ellipticity direction. The ellipticity is in a close-to-diagonal (∼36°) direction such that the vertical and horizontal magnifications in the data set are almost identical. The Thon rings visible in the centre are elliptical owing to an astigmatism of ∼1000 Å. This astigmatism ellipticity is unrelated to the ellipticity of the water rings. The two effects may become entangled if the anisotropic magnification is not corrected prior to CTF determination.

Volume 4| Part 5| September 2017| Pages 678-694
ISSN: 2052-2525