Characterization of hexabundles: Initial results

TL;DR

Hexabundles, multi-core imaging fibre bundles, enable simultaneous integral field spectroscopy over wide fields, improving astronomical observations by reducing biases and enhancing resolution, with initial performance comparisons favoring lightly-fused bundles.

Contribution

This study compares the optical performance of fully-fused and lightly-fused hexabundles, highlighting the trade-offs between fill fraction and optical quality for astronomical applications.

Findings

Lightly-fused bundles have better optical performance than fully-fused ones.

Higher fill fraction increases modal coupling and cross-talk.

Lightly-fused bundles with 100 cores are feasible for future instruments.

Abstract

New multi-core imaging fibre bundles -- hexabundles -- being developed at the University of Sydney will provide simultaneous integral field spectroscopy for hundreds of celestial sources across a wide angular field. These are a natural progression from the use of single fibres in existing galaxy surveys. Hexabundles will allow us to address fundamental questions in astronomy without the biases introduced by a fixed entrance aperture. We have begun to consider instrument concepts that exploit hundreds of hexabundles over the widest possible field of view. To this end, we have compared the performance of a 61-core fully-fused hexabundle and 5 lightly-fused bundles with 7 cores each. All fibres in the bundles have 100 micron cores. In the fully-fused bundle, the cores are distorted from a circular shape in order to achieve a higher fill fraction. The lightly-fused bundles have circular cores and five different cladding thicknesses which affect the fill fraction. We compare the optical performance of all 6 bundles and find that the advantage of smaller interstitial holes (higher fill fraction) is outweighed by the increase in modal coupling, cross-talk and the poor optical performance caused by the deformation of the fibre cores. Uniformly high throughput and low cross-talk are essential for imaging faint astronomical targets with sufficient resolution to disentangle the dynamical structure. Devices already under development will have between 100 and 200 lightly-fused cores, although larger formats are feasible. The light-weight packaging of hexabundles is sufficiently flexible to allow existing robotic positioners to make use of them.