Supplementary Figure 4: Technical drawing of the postsample beam pipe unit of the Roadrunner goniometer.

The beam pipe consists of a molybdenum tube with an inner diameter (ID) of 0.35 mm and an outer diameter (OD) of 0.5 mm. The upstream end of the pipe is placed about 15 mm behind the sample. The far end of the smaller diameter tube is inserted into a second larger tube with an ID of 0.55 mm and an OD of 1 mm, which extends several tens of millimeters downstream into the central hole of the CSPAD detector. The larger tube is manually pre-aligned in the direction of the direct beam path. It is mounted on a manually operated translation stage for adjustment along the beam direction. Two stepper motor operated translation stages allow fine adjustment of the beam pipe perpendicular to the X-ray beam and to guarantee that the primary beam is well enclosed by the beam pipe. By enclosing the direct beam shortly behind the sample in the molybdenum tube all X-rays scattered by air on the way down to the detector are absorbed in the beam pipe walls and therefore do not contribute to background scattering onto the detector. This results in a significant reduction of the background level from air scattering.