Extremely Accurate Stellar Diameter Measurements With the Help of Coherent Integration: ν Ophiuchus

Astronomy and Astrophysics – Astronomy

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Scientific paper

Coherent integration can be used to improve the signal-to-noise ratio (SNR) of optical interferometric measurements. Coherent integration refers to the fact that the fringe signal is integrated for a period much longer than the atmospheric coherence time. By coherently integrating fringes we can in many cases convert what would be low-SNR measurements using traditional power-spectral techniques into high-SNR measurements. To coherently integrate for long periods of time requires that the total optical path difference be held constant (ideally zero) during the entire integration. This can for example be achieved by real-time fringe tracking and stabilization through a rapid control loop. However, fringe tracking and stabilization can also be performed after the fact in software if the instrument has multiple spectral channels and records short exposures. In fact, under some circumstances, coherent integration in post-processing will outperform real-time coherent integration, because it is not subject to the time-lags and processing limitations of real-time systems. The Navy Prototype Optical Interferometer measures fringes in up to 32 spectral channels, and at a 2 ms cadence, making those data ideally suited for post-processing coherent integration. In this presentation we will give an overview of the theory of coherent integration and its limitation. We will then illustrate the application of this theory with an example in which we use a modest amount of NPOI data (270 seconds) to measure a stellar diameter with the unprecedented accuracy of better than 0.2% (1:500) using coherent integration techniques. This is a substantial improvement beyond the current state of the art, and opens up the possibility of measuring small-amplitude pulsations. Despite the small amount of data used, in this case the accuracy of the diameter determinations is limited by the spectral resolution of the instrument, and not by the amount of data recorded.

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