Schematic drawing of the in-vacuum double phase-retarder set-up in the optics hutch. Two individual diffraction stages with opposite θ and 2θ circles mounted on the roll (χ) stages can be seen. Three diamond phase plates are mounted on each of the crystal changers. The inclination of the diffraction plane shown is ±45° for the two circles. With both phase plates in the QWP condition this results in circular polarization after the first phase plate and 90° rotated linear polarization after the second phase plate.