Influence of the optical quality of the interferometer optics on the diameter measurement of the Avogadro silicon sphere

Within the International Avogadro Project measurements of molar mass and lattice parameter of a silicon single crystal and mass and volume of a high precision silicon sphere made of the same crystal are combined to calculate Avogadro's constant. As an overall uncertainty of 2ldr10
^-8
is aimed at, the determination of the volume of the Avogadro sphere requires unequalled precision in absolute interferometry. Especially systematical influences on an experimental setup can only be detected, if relevant influencing parameters can be varied. The spheres interferometer of PTB offers a unique way to investigate the influence of its optical components.