Uncertainty considerations on the new Avogadro sphere interferometer

For the aim of redefining the SI unit kilogram basing on the Avogadro constant N
_A
the International Avogadro Coordination has to achieve an overall relative measurement uncertainty less than 2×10
^-8
. The most contributing part with 1×10
^-8
to this originates from the interferometric volume determination of the Si
²⁸
spheres. And here the dominant uncertainty results from the deviations of the optical system from perfection. At the present state the influence of these wavefront aberrations is only based on a careful estimation. For a distinct knowledge of the influence of surface deviations and adjustment errors a ray tracing routine and evaluation procedures were implemented.