Focal ratio degradation in optical fibres for the Hector integral field units

Focal Ratio Degradation (FRD) in optical fibres can result in light loss. In astronomical instrumentation, FRD must be controlled because light levels are low. A new Integral Field Spectrograph (IFS) instrument called Hector, was recently completed and was installed on the Anglo-Australian Telescope (AAT) in Australia in 2021 December. The integral field units (IFUs) for Hector made with hexagonally packed multi-mode fibres are called hexabundles. In this paper, we introduce a cone-beam measurement method to measure FRD at optical wavelengths. We present the FRD and throughput performance of the hexabundles for Hector and show that the process for making hexabundles introduced no additional significant FRD beyond that of the bare fibre.