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 sourc e located at a different position with respect to the observed scientific t arget. The differential piston error as a function of the angular distance between the reference and astronomical target can be calculated using exist ing formulae for Zernike polynomial correlation coefficients. This allows t he definition of an isopistonic angle that resembles the isoplanatic angle for Adaptive Optics System wavefront correction. Values obtained for the is opistonic angle are comparable to the isoplanatic angle showing that sky-co verage could be a potential problem for an astronomical interferometer aimi ng to correct for atmospheric fringe jitter.