Phase-stabilised interferometry

The NPOI and PTI interferometers actively track and compensate for atmospheric phase fluctuations. A bright reference star is required in order to do this typically providing a S/N of $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}10$ for measurements of the correlated flux $C_{PS}$ on the PS. Once the fringes are stabilised, faint signals which are phase-correlated with the fringes can be coherently integrated.