Coherent Integration in Optical Interferometry

Optical Interferometry has long been limited by low SNR making it nearly impossible to measure the small visibilities required to make resolved images. Although the SNR exists in the raw data, much SNR is lost in the conventional squared-visibility processing. In modern interferometers fringes are recorded simultaneously at many wavelengths and baselines. This makes phase-referencing possible, which is the key to coherent integration, which in turns can greatly improve the SNR of measurements, making small-amplitude resolving measurements possible. In this paper we will detail the theory of coherent integration. We will also explain why coherent integration should, in most cases, be carried out during post-processing in software rather than in real-time in hardware. We will then compare it to conventional processing approaches for some data from the Navy Optical Interferometer. We will demonstrate how coherent integration can improve the accuracy of observations.