Effects of atmospheric water vapor on infrared interferometry

Water vapor, while usually a small contributor to the atmospheric refractive index for astronomical observations at IR wavelengths, is highly dispersive and can introduce optical path length errors affecting high-precision interferometer observing modes. The refractive index of atmospheric water vapor can be computed from a summation over the various IR resonances, and we present values over a range of 1.2 - 13.5 mum. The dispersion of water vapor introduces phase errors across the instrument passband and produces excess noise in interferometer group delay, residual errors in cophasing using a different source wavelength, and coherence loss over broad optical bandwidths. We quantify these effects for the J through N bands, discuss means of amelioration, and consider their implications for differential phase and nulling observing modes.