Simultaneous Water Vapor and Dry Air Path Length Measurements with the Keck Interferometer Nuller

K-band phase and group delay measurements of atmospheric turbulence are used as part of the Keck Interferometer nuller cophasing system to provide high-bandwidth path length compensation at N band. Because of atmospheric dispersion, the path length fluctuations from dry air and water vapor must be estimated separately, and the computation of the N-band feedforward quantities from the K-band measurements is described. Simultaneous K and N-band sky data are presented that show good correspondence with the underlying atmospheric models. Simultaneous power spectra of dry air and water vapor path length turbulence measured on an 85 m baseline with the Keck Interferometer over 44 nights between 2007 June and 2009 July are also presented. From the median power spectra, the rms path length fluctuations at K band from water vapor are found to be 48 times smaller than those for dry air, and the absolute level of the water vapor path length fluctuations is found to be a factor of 2 smaller than predicted based on archival data from the CSO test radio interferometer. It is postulated that part of the difference is attributable to surface-layer water vapor turbulence, which would be smaller at the elevation of the Keck telescopes than for the test antennas, similar to surface-layer effects seen in dry air seeing at Mauna Kea. The midfrequency power spectral amplitude better characterizes the residuals for a feedforward compensation system, and this value is found to be a factor of 65 smaller at K band for water vapor than for dry air; the difference compared to the ratio of the rms values is attributable to the lower effective wind speed of the water vapor turbulence, and thus lower control bandwidths are required for compensation than a simple scaling based on rms fluctuations would indicate. The water vapor coherence time is also shown to have modest correlations with both precipitable water vapor and the dry air coherence time, although the variability of the water vapor turbulence strength is larger than that of the dry air turbulence.