Fig. 1: Schematic experimental setup for AXL characterisation with the focusing mirror (VKB).

For zero translation between the two structures of AXL (a), the outgoing wavefront of VKB remains unaffected. For opposing translations, a positive or negative parabolic perturbation of the wavefront results (b, c). Inset (d) Profile of one of the two fabricated AXL structures viewed from the side. Each of the three AXL refractors (AXL1, AXL2, AXL3) can be moved into the X-ray beam by a vertical translation.