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 μm. 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.