(a) 3D drawing of the four-bounce crystal collimator 4BCC (situated at 34.88 m position), comprising two sets of double crystals of Si(111) or Si(311) situated on two large rotating stages for a dispersive configuration. On each rotation stage, the two crystals (C1–C2 or C3–C4) are rotated/titled (cf. large arrows) collectively; C2 and C3 crystals further can be rotated/titled (cf. small arrows) with respect to the C1 and C4 crystals, respectively. A motorized YAG crystal is installed between C3 and C4 for alignment convenience. The configuration of these two rotation stages can be improved further as detailed in SI. (b) An action mode of the 4BCC with the photon beam passing through the four crystals of the 4BCC. With 4BCC rotations (indicated by the arrows), X-rays could bypass 4BCC, through the two 80 mm-long channels in crystals C1 and C4. (c) Horizontal and vertical beam profiles of a DCM 8 keV beam measured at 49.5 m near the 52 m focus, without (2D image to the left, inset) and with (2D image to the right, inset) the 4BCC with Si(111). Note that the background scattering intensity near the primary beam is decreased by about two orders of magnitude (long arrow) with the 4BCC, at a cost of a tenfold decay in beam central intensity.