Differential piston angular anisoplanatism for astronomical optical interferometers

This letter considers the problem of measuring the differential piston of a two-pupil astronomical optical interfer- ometer which uses a reference source located at a different posi- tion with respect to the observed scientific target. The differen- tial piston error as a function of the angular distance between the reference and astronomical target can be calculated using ex- isting formulae for Zernike polynomial correlation coefficients. This allows the definition of an isopistonic angle that resembles the isoplanatic angle for Adaptive Optics System wavefront cor- rection. Values obtained for the isopistonic angle are comparable to the isoplanatic angle showing that sky-coverage could be a potential problem for an astronomical interferometer aiming to correct for atmospheric fringe jitter.