CTF fitted Fourier transform of RSV CA 2D crystal. FFT of RSV
CA1-226 2D crystal clearly indicates Thorn rings that
represent CTF plotted as a function of spatial frequency (red graph).
Positions of zero CTF are indicated by green circles in the image.
Diffraction spots that belong in every second Thorn rings in the FFT (i.e.
positive CTF in the plot) must be phase-flipped in order to correctly
extract information to a highest possible resolution.