Spatial phase-referencing in dual-field optical interferometry isreconsidered. Our analysis is based on the 2-sample variance of thedifferential phase between target and reference star. We show that averagingover time of the atmospheric effects depends on this 2-sample phase variance(Allan variance) rather than on the true variance. The proper expression forfringe smearing beyond the isoplanatic angle is derived. With simulations ofatmospheric effects, based on a Paranal turbulence model, we show how theperformances of a dual-field optical interferometer can be evaluated in adiagram 'separation angle' versus 'magnitude of faint object'. In this diagram,a domain with short exposure is found to be most useful for interferometry,with about the same magnitude limits in the H and K bands. With star countsfrom a Galaxy model, we evaluate the sky coverage for differential astrometryand detection of exoplanets, i.e. likelihood of faint reference stars in thevicinity of a bright target. With the 2mass survey, we evaluate sky coveragefor phase-referencing, i.e. avaibility of a bright enough star for main delaytracking in the vicinity of any target direction.
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