Modal noise prediction in fibre spectroscopy - I. Visibility and the coherent model

Fibre modal noise occurs in high spectral resolution, high signal-to-noise ratio applications. It imposes fundamental limits on the photometric accuracy of state-of-the-art fibre-spectrograph systems. In order to maximize the performance of current and future instruments it is therefore essential to predict fibre modal noise. To attain a predictive model we are using a dual approach, bringing theoretical assumptions in line with the experimental data obtained using a test-bench spectrograph. We show that the task of noise prediction can be reduced to determining the visibility of the modal pattern which can be measured at the detector plane. Subsequently, the visibility dependence of essential parameters is presented. This work will soon provide a basis for prediction of modal noise limitations in fibre-coupled spectrograph designs.