WEAVE is a multi-object spectrograph for the 4-meter William Herschel telescope on the island of La Palma in the Canary Islands. There are many scientific interests for WEAVE, from galactic archaeology, for which WEAVE will complement the data collected by the Gaia mission, to stellar, circumstellar and interstellar astrophysics, the study of galaxy clusters and galaxy evolution, a joint survey with LOFAR and quasars. The instrument is currently under construction and will be on the sky in 2019.
WEAVE is a fibered instrument that collects light at the telescope’s primary focus and transports it to a Nasmyth platform, some 33 meters away, where the spectrograph is installed. GEPI is responsible for the work package associated with the fibers. The three observation modes, for individual point objects (MOS) and for extended objects with small field integrals (mIFU) and large field integrals (LIFU), lead to three geometries for the fiber optic cables. The first has a total of 1920 fibers, as one half observes while the other half is positioned, the second 740 fibers organized in 20 IFUs, the third 609 fibers organized on a single IFU.
Light is injected into the fibers via microlenses (MOS, mIFU) or a lens (LIFU) that adapts the focal ratio provided by the field corrector to minimize FRD. In the case of MOS and mIFU, a micro-prism is used to fold the beam into the fiber, as the fibers are deployed on the focal surface in a direction orthogonal to that of the incident beam. In all cases, the fibers are grouped in sheaths of variable multiplex to facilitate fiber integration and routing on the telescope. Finally, the fibers are inserted into micrometric Vs to align them and form the pseudo-slot. The four groups of fibers (2 MOS, 1 mIFU, 1 LIFU) thus form 4 interchangeable pseudo-slots, enabling the spectrograph to meet all scientific requirements.
The high number of components and the tight specifications on each of them are a challenge from the point of view of procurement, recipe, traceability requirements, stock management and, last but not least, cable assembly. The cables are assembled by the industry, with three different partners for scheduling reasons. Once delivered, they are checked using an elaborate test bench developed for WEAVE, which simulates the beam produced by the telescope and field corrector to mimic injection conditions, and enables relative photometry measurements and near and far field observations to be made for each of the fibers on the spectrograph side. It also provides the means to measure performance in different colors thanks to a motorized filter wheel. The test bench is fully programmable so that data can be acquired 24 hours a day, and is coupled with data interpretation software to produce the metrics needed to quantify cable performance.
