Scientific and technical results from VINCI using coherent estimation of fringe visibility

Although primarily intended as a test and alignment instrument in order to commission the VLTI, VINCI has taken useful scientific data in its first year and a half of operation. Our results employ coherent integration of fringe visibility in which the actual amplitudes of the raw scans are combined linearly after correcting for the position of the fringe within each scan. In addition to reducing the effect of noise compared to incoherent integration, the result contains a broader range of information, including an estimate of the complex visibility spectrum. Such an estimator is thus sensitive to instrumental phase and spectral characteristics, including the variable component of dispersion introduced by the excess air paths in the delay lines. Calibration of such instrumental effects demonstrates the ability to detect source phase at a fine level as will be required for direct interferometric detection of extrasolar planets. We present diameters for five stars obtained by observing the visibility null in their correlated spectra. Using coherent integration we have also observed the peculiar correlated spectra seen in many Mira variables, possibly due to changes in the apparent diameter with wavelength. Calibration of the zero-baseline power from o Ceti is used with other interferometric observations of this star over a period of 90 days to plot diameter variations associated with its pulsation cycle.