Letter to the Editor Differential piston angular anisoplanatism for astronomical optical interferometers

This letter considers the problem of measuring the differential piston of a two-pupil astronomical optical interferometer which uses a reference source located at a different position with respect to the observed scientific target. The differential piston error as a function of the angular distance between the reference and astronomical target can be calculated using existing formulae for Zernike polynomial correlation coefficients. This allows the definition of an isopistonic angle that resembles the isoplanatic angle for Adaptive Optics System wavefront correction. 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.