Measurement of miscut angles in the determination of Si lattice parameters

The measurement of the angle between interferometer front mirror and diffracting planes is a critical aspect Si lattice-parameter by combined x-ray optical interferometry. In addition to being measured off-line diffraction, it was checked on-line transversely moving analyser crystal observing phase shift interference fringe. We describe procedure give miscut $^{28}$Si whose lattice parameter an essential input-datum for, yesterday, determination Avogadro constant and, today, kilogram realisation counting atoms. These data are kindness others that might wish repeat this unique crystal.