(a) Experimental setup for double-crystal X-ray topography of the α-SiO2 (X-cut) single crystals. In the inset (i) is shown the measured incoming rocking curve (in) and the estimated outgoing rocking curve (out) based on the asymmetry b factor. (b) and (c) show the X-cut double crystal topographies taken on the two different samples. The images shown are homogeneous except for the cutting blade scratches. Dashed areas a1 and a2 are the regions where the lattice space maps were acquired.