Dimensions of (a) crystal 1 (X1), and (b) crystal 2 (X2) in the DCM, given in millimetres. (c) X1 mounted in the liquid-N2-cooling housing. The `white beam' impinges from the left, while the monochromatic beam (here, yellow) emerges on the right. (d) The crystal surface itself is obscured by a copper `cap' used to protect it from long-term deposits originating from residual-gas carbon-containing species, by limiting the solid angle to which the surface is exposed. Materials: orange = Cu; grey = Al; blue = Si.