(a) Schematic of the optics layout of BL41XU showing distances from the light source. (b) Design of the new optics layout for the upgrade project. The slit in the experimental hutch 1 (EH1) is a high-precision slit which defines the virtual source size. The dotted-line rectangle in EH1 shows where the second diffractometer will be installed for the high-energy mode. ID, insertion device; FE slit, front-end slit; DCM, double-crystal monochromator; HM, horizontal-focusing mirror; VM, vertical-focusing mirror.