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20-30 nm using fast-spinning aluminium (Al) polishing tips of conformal parabolic shape and a diamond slurry. This is a chemical-mechanical polishing process.
Manufactured lenses were measured both individually and stacked as a compound refractive lens (CRL). X-ray Speckle Vector Tracking (XSVT)  is a differential measurement technique that compares the X-ray wavefield after an optical element to the wavefield without the optical element and can deliver a height map of the optical element under investigation. The height error map in Figure 145 (as compared to a perfect paraboloid of
revolution for a 2D focusing concave X-ray lens) can be used to quantify lens aberrations.
XSVT was performed at beamline BM05 on 39 individual bi-concave C* lenses of radius R=100 µm (14 unpolished and 25 polished) and compared with 24 Be lenses of equivalent focusing power (R=50 µm) and identical physical aperture. Three CRLs were also measured: a stack of 10 unpolished C* lenses, a stack of 10 polished C* lenses and a stack of 11 Be lenses of equivalent focusing power. The results show similar height error rms values in the range between 0.85 µm and 0.9 µm for the
Fig 144: a) One side of a polished bi-concave C* lens as seen from the side. b) Lens inside ø12mm frame. c) CRL lens stack of 48 C* lenses ready for beamline use.
Fig. 145: Radiographs and height error maps of a single unpolished (a) and polished (b) C* lens of radius R=100 μm. For the height error map, the best fit parabola is removed. c) Beam profile in the focal plane and 25 mm up- and downstream of that plane, for a N=10 polished C* lens stack. d) SAXS signal for unpolished and polished C* lens, as well as for an equivalent commercial Be lens.