DERIVING OBJECT VISIBILITIES FROM INTERFEROGRAMS OBTAINED WITH A FIBER STELLAR INTERFEROMETER

A method is given for extracting object visibilities from data provide d by a long baseline interferometer, where the beams are spatially fil tered by single-mode fibers and interferograms are obtained as scans a round the zero optical pathlength difference. It is shown how the sign als can be corrected from the wavefront perturbations caused by atmosp heric turbulence. If the piston perturbations are also removed, then t he corrected data contain both spatial and spectral information on the source (double Fourier interferometry). When the piston cannot be rem oved, object phase and spectral information are lost, and the observab le (free of detector noise bias) is the squared modulus of the coheren ce factor, integrated over the optical bandpass. In a fiber interferom eter this quantity leads to very accurate object visibility measuremen ts because the transfer function does not involve an atmospheric term. The analysis also holds for a more classical pupil plane interferomet er which does not take advantage of the spatial filtering capability o f single-mode fibers. In that case however, the transfer function incl udes a turbulence term that needs to be calibrated by statistical meth ods.