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Schematic sketch of the undulator source model with its parameters. 
is the r.m.s. lateral size of the electron beam, σ the r.m.s. diffraction-limited source size, and 
the overall r.m.s. lateral size of the source. 
is the natural divergence of the undulator radiation and 
the r.m.s. divergence of the electron beam. 
is the lateral coherence length of the source.
