Degrees of freedom for the G2 beamsplitting monochromator: top arc - rotation around
q to avoid Umweg reflections (for use with forbidden reflections), lower arc - parasitic tilt
of crystal / bending beam down for surface scattering, 410 turn table - Bragg angle,
CHESS xz stage - positioning crystal into the beam, long y-translation - optimizing
scattering angle range as well as shutter and exit window size