Field size refers to the width of each small image cropped.
An _em_micrographs.averaging_overlap implies that the
successive images have an overalp of
(1-_em_micrographs.averaging_overlap)*field size.
A higher value of _em_micrographs.averaging_overlap means
a greater number of smaller images are used for averaging.
If the signal to noise ratio is very low, the averaging overlap
should be changed for better estimate of power spectrum.