..
   Athena document is copyright 2016 Bruce Ravel and released under
   The Creative Commons Attribution-ShareAlike License
   http://creativecommons.org/licenses/by-sa/3.0/

Spectral resolution and k-range
===============================

The first and second coordination shells in iron metal are at 2.49 and
2.87 |AA|. These closely spaced shells overlap considerably when the
Fourier transform of the |chi| (k) data are plotted. Consequently, the iron
foil data can be used to demonstrate the relationship between the
Fourier transform k-range and spectral resolution.

We see the iron foil data plotted in the figure below using three
different values for :procparam:`kmax`. The data are three identical copies of the
60K iron foil data and have had the background function removed in the
same way.

For the smallest value of :procparam:`kmax`, 10 |AA| :sup:`-1`, the second shell
appears only as a shoulder at about 2.5 |AA|. The shoulder becomes
more pronounced when :procparam:`kmax` is increased to 16 |AA| :sup:`-1`. Only
when :procparam:`kmax` is set to 22 |AA| :sup:`-1`, the entire measured data
range, is the second coordination visible as a resolved peak.

.. _fig-krange:
.. figure:: ../../_images/plot_fe_krange.png
   :target: ../_images/plot_fe_krange.png
   :align: center

   Iron foil data, plotted with three different k-ranges.