The Nyquist frequency is double the sampling step. e.g. for data
sampled at 5 angstroms the Nyquist frequency is 1/10 angstroms.
It is important to have the sampling rate of the data high enough
so that the image information stays sufficiently away from the
Nyquist frequency, otherwise there is a danger that the high-
resolution information in both of the reconstruction volumes
may be correlated.