PHASE-REFERENCED VISIBILITY AVERAGING IN OPTICAL LONG-BASE-LINE INTERFEROMETRY

In the photon-starved regime, the signal-to-noise ratio of interferome tric data depends on the coherent integration time. To extend the inte gration time beyond the limit imposed by atmospheric fluctuations, the phases taken in a narrow ''signal'' channel can be corrected by refer encing them to those taken in a wider ''tracking'' channel. A number o f instrumental and atmospheric effects decorrelate the phases in the t wo channels and thus constrain the range of conditions under which the phase-referencing technique can be used. In the case of the MkIII ste llar interferometer, differential refraction at intermediate to large zenith angles is the most important limitation. Tests with MkIII data in the photon-rich regime demonstrate that the phase-referencing techn ique works well at moderate zenith angles. In the photon-starved regim e, the expected improvement of the signal-to-noise ratio is readily ob served. We use phase-referenced data taken on the bright star a Boo cl ose to the first null of the visibility function to show that the MkII I data are free from additive bias at the V2 less-than-or-equal-to 10( -4) level. The absence of any bias larger than this value is an import ant requirement for future imaging interferometers.