Details on how located layer lines are used to choose a selection rule
which best fits the data. Based on this selection rule the filament is
then reboxed and restraightened using the original digitized image so
that the final image contains an integral number of helical repeats.
The selection rule define helical lattices which relate the layer-
line number l to the order of the Bessel function, n, contributing
to the layer-line. (n is the start number, ie number around the
circumference, of the contributing helix).
The diffraction pattern from a helix consists not of discrete spots
but of difraction spots which have been broadened into layer-lines.
The order of the Bessel functions allowed to contribute to the
diffraction pattern of a helix on a given layer line versus the
layer line along the ordinate gives a function which is described
by a lattice. Such a plot is analogous to the diffraction pattern
from a planar array corresponding to a flattened helix and is called
an (n,l) plot.
e.g. the n,l plot corresponding to the selection rule l = 5n + 12m
where m is an integer and indicates e.g. 12 (ribosomes) per repeat
five turns long, whilst the n,l plot corresponding to the selection
rule l = 5n + 17m indicates 17(ribosomes) per repeat five turns along.