This is a proof of concept beamline to test an idea of how to build a low-cost
monochromtic beamline. Idea is to let a polychromatic beam impinge on a focusing
Be-lens and put a slit in the focal pt. of the lens for the desired wavelength. Any
other wavelengths present in the beam will not be focussed prefectly and may be
by the slit.
Parameters in boldface are required;
the others are optional.