  We are now moving into an era where multi-object wide-field surveys, which
traditionally use single fibres to observe many targets simultaneously, can
exploit compact integral field units in place of single fibres. Current
multi-object integral field instruments such as SAMI (Croom et al. 2012; Bryant
et al. 2012a) have driven the development of new imaging fibre bundles
(hexabundles) for multi-object spectrographs. We have characterised the
performance of hexabundles with different cladding thicknesses and compared
them to that of the same type of bare fibre, across the range of fill-fractions
and input f-ratios likely in an IFU instrument. Hexabundles with 7-cores and
61-cores were tested for focal ratio degradation (FRD), throughput and
cross-talk when fed with inputs from F/3.4 to >F/8. The five 7-core bundles
have cladding thickness ranging from 1 to 8 microns, and the 61-core bundles
have 5micron cladding. As expected, the FRD improves as the input focal ratio
decreases. We find that the FRD and throughput of the cores in the hexabundles
match the performance of single fibres of the same material at low input
f-ratios. The performance results presented can be used to set a limit on the
f-ratio of a system based on the maximum loss allowable for a planned
instrument. Our results confirm that hexabundles are a successful alternative
for fibre imaging devices for multi-object spectroscopy on wide-field
telescopes and have prompted further development of hexabundle designs with
hexagonal packing and square cores.
