Optical scheme of the LUPI stage, coupled to the XUV Raman monochromator chamber. The beamspliter (BS) splits the beam from the single-mode He–Ne laser into reference and test arms and recombines them again on the CCD. The beam in the test arm is focused by the FP to create a source point for the M1 mirror of the monochromator. During different stages of the alignment process, the optical flat mirror (OF) has to be placed at points B, C and D. An imaging lens can be placed at position A for pre-alignment purposes. A retro-reflector was used at position C as explained in Section 3.2.