Fig. 1. Spectrum-measurement scheme for the double-spherically bent crystal, with Bragg angles ${\theta }_1$ and ${\theta }_2$. The red, green, and blue lines represent the maximum and minimum grazing incidence angles from the source position for light incident at the edges of the crystal surface, and radiation hits the center of the crystals (C1 and C2) at the Bragg angles (${\theta }_1$ and ${\theta }_2$), respectively.

Fig. 2. DSBCS-measured spectrum of cold Cu. (a) The raw spectral data recorded by using an IP. (b) The relative intensity obtained by averaging over the photon counts in the spatial direction versus the spectral resolution.