Comparing High-Precision Stellar Diameters From the Navy Prototype Optical Interferometer With Stellar Atmosphere Models as a Function of Wavelength

In this paper we use the coherent integration technique to obtain extremely high precision stellar diameters as a function of wavelength in the visible band using observations from the Navy Prototype Optical Interferometer (NPOI). We then compare these diameters with stellar atmosphere models as a function of wavelength. Coherent integration is a post-processing technique which corrects for atmospheric optical-path-difference shifts in interferometric data and allows, effectively, to increase the integration time of optical interferometric data indefinitely by summing many short exposures suitably phase-shifted. This is significant because coherent integration, which allows the complex visibilities to be summed in phase, greatly improves the signal-to-noise ratio over traditional techniques which average squared visibilities. The improvement is particularly dramatic for faint targets, and/or when the visibility amplitude is very small. This is important when measuring stellar diameters on resolving baselines that include visibility nulls. Those baselines best constrain the diameters. However in order to take full advantage of the null as a diameter measure it is necessary to obtain high-SNR measurements around the null, which is possible with coherent integration. We present wavelength-dependent uniform disk diameter measurements with precision up to 1:500 to 1:1000. We then compare these with the corresponding uniform-disk diameters obtained from stellar atmosphere models