Hexabundles: imaging fibre arrays for low-light astronomical applications

We demonstrate for the first time an imaging fibre bundle ("hexabundle") that is suitable for low-light applications in astronomy. The most successful survey instruments at optical-infrared wavelengths today have obtained data on up to a million celestial sources using hundreds of multimode fibres at a time fed to multiple spectrographs. But a large fraction of these sources are spatially extended on the celestial sphere such that a hexabundle would be able to provide spectroscopic information at many distinct locations across the source. Our goal is to upgrade single-fibre survey instruments with multimode hexabundles in place of the multimode fibres. We discuss two varieties of hexabundles: (i) closely packed circular cores allowing the covering fraction to approach the theoretical maximum of 91%; (ii) fused noncircular cores where the interstitial holes have been removed and the covering fraction approaches 100%. In both cases, we find that the cladding can be reduced to ∼2μm over the short fuse length, well below the conventional ∼10λ thickness employed more generally. We discuss the relative merits of fused/unfused hexabundles in terms of manufacture and deployment, and present our first on-sky observations.