Multi-Object Fibre Spectroscopy at the Wht: Performance Enhancements of Af2+Wyffos

AF2+WYFFOS is the multi-object one degree field-of-view fibre-fed spectrograph at the prime focus of the 4.2 m William Herschel Telescope (WHT) at the Observatorio del Roque de Los Muchachos (ORM). Demand for this has been decreasing over the years due to its low overall throughput. Given the strategic importance of multi-fibre spectroscopy for the WHT's future, with the coming of WEAVE (the powerful next-generation multi-object spectrograph for the WHT to be commissioned in 2017), during 2013 and 2014 ING staff carried out an end-to-end analysis of the reasons for low throughput. The investigations addressed target-acquisition/positioning/guiding, focal-plane geometry, optical transmission and overall system throughput measurements.AF2+WYFFOS performance has improved significantly as a result of these enhancements, and it is ready now to perform as a useful precursor and science test-bed for surveys with WEAVE, the new wide-field multi-object spectroscopy facility at the WHT.