GEPI

WEAVE is a multi-object spectroscopic instrument for the 4-meter William Herschel Telescope on the island of La Palma part of the Canary Islands. Science cases for WEAVE are numerous and include galactic archeology for which WEAVE will complement data from the Gaia mission, stellar, circumstellar and interstellar astrophysics, the study of galaxy clusters and galaxy evolution, a survey to be conducted jointly with LOFAR and quasars. The instrument is currently under construction and will be installed in 2019.
WEAVE is a fibre-fed instrument where light is collected at the telescope’s prime focus and transported to the Nasmyth platform, some thirty three meters away, where the spectrograph resides. GEPI is in charge of the fibre work-package. The three observation modes, for individual point-like objects (MOS) and for extended objects with small integral field units (mIFU) and a large integral field unit (LIFU), lead to three kinds of fibre cables. The first features a total of 1920 fibres, as half of the fibres observe while the other half are deployed, the second 740 fibres arranged as 20 IFUs, the third 609 fibre arranged as a single IFU.
Light is injected in the fibres thanks to micro-lenses (MOS, mIFU) or a lens (LIFU) which adapt the focal ratio delivered by the wide-field corrector to minimise FRD. In the cases of the MOS and mIFU fibres an additional micro-prism folds the beam into the fibre as the fibres are to be deployed on the focal surface and come in a direction orthogonal to the incident beam. In all cases, the fibres are grouped in conduits of variable multiplex to ease integration and the routing of the fibres on the telescope. Finally, the fibres are inserted in tiny v-grooves to generate an array of fibres which composes the pseudo-slit. The four groups of fibres (2 MOS, 1 mIFU and 1 LIFU) hence form 4 pseudo-slits which are interchangeable on the spectrograph so that it can serve all science cases.
The large number of parts and the demanding requirements on them are a challenge from the point of view of the procurement, their verification, the need for traceability, the management of the stock and finally, the assembly of the fibre cables. The cables are assembled by industry, with three different partners involved to meet the project’s schedule. Once delivered they are verified thanks to an elaborate test bench developed for WEAVE which simulates the beam produced by the telescope and wide-field corrector to mimic the conditions of injection and allows for performing relative photometry and for observing the near and far-field for each and every fibre on the spectrograph’s side. It also provides means to inspect the achieved performances in different colour bands thanks to a motorised filter wheel. The test bench is fully programmable so data can be collected 24 hours a day and it is coupled to data reduction software which produces the quantifiers used for assessing the performance of the cable.