Characterising modal noise in fibre-coupled spectrographs for astronomy

Abstract

Fibre modal noise occurs in high spectral resolution, high signal-to-noise applications. It imposes fundamental limits upon 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. Explicitly theoretical approaches are often restricted to specific cases, therefore this paper focuses on the conditions relevant to astronomy. The goal of this work is to derive a reliable model which can be used for the optimization of future spectrograph designs. We give a description of experimental investigations undertaken in Durham, displaying preliminary results. The first laboratory tests of a square fibre have shown modal noise characteristics that are interestingly similar to standard circular fibre.