Interferometer for the IMGC rise-and-fall absolute gravimeter

Absolute gravity measurements are performed by absolute gravimeters that sometimes show significant differences between their averaged values, as borne out by the international comparisons organized periodically by the Bureau International des Poids et Mesures. Many sources of error have been taken into consideration to evaluate each uncertainty budget; nevertheless in some cases the observed differences from a general averaged value are statistically larger than expected on the basis of these evaluations. The main reason for this is that some causes of uncertainty cannot be well quantified as they are based only on theoretical hypotheses. Having investigated some sources of error in the optical interferometer at the Istituto di Metrologia "G. Colonnetti" (IMGC) we propose: (a) an optical design that minimizes errors arising from relative movements between the interferometer parts; (b) a simple apparatus that is easy to use to equilibrate the corner-cube prism that constitutes the moving mirror of the interferometer. These two modifications to the IMGC absolute gravimeter have reduced the uncertainty arising from the interferometer by around a factor of ten.