The GALAH Survey: Relative throughputs of the 2dF fibre positioner and the HERMES spectrograph from stellar targets

TL;DR

This paper analyzes the stability and variation in the throughput of the 2dF/HERMES system over two years, finding overall stability but some unexplained variations that do not affect scientific outcomes.

Contribution

It provides the first detailed assessment of the throughput stability and variations of the 2dF/HERMES system during the GALAH survey's initial years.

Findings

Overall throughput stability over two years

Approximately 2.5% of fibres have significantly lower throughput

Unexplained variations between HERMES bandpasses and fibre groups

Abstract

We present an analysis of the relative throughputs of the 3.9-metre Anglo-Australian Telescope's 2dF/HERMES system, based upon spectra acquired during the first two years of the GALAH survey. Averaged spectral fluxes of stars were compared to their photometry to determine the relative throughputs of fibres for a range of fibre position and atmospheric conditions. We find that overall the throughputs of the 771 usable fibres have been stable over the first two years of its operation. About 2.5 per cent of fibres have throughputs much lower than the average. There are also a number of yet unexplained variations between the HERMES bandpasses, and mechanically & optically linked fibre groups known as retractors or slitlets related to regions of the focal plane. These findings do not impact the science that HERMES will produce.