Over the next decade, we expect a deluge of high-quality data delivered by the upcoming large ground-based (e.g., LSST/VRO, SKAO, CTA) and space (e.g., JWST, Euclid, Nancy Grace Roman Space Telescope, Athena) telescopes. In parallel the Gaia astrometric mission will have its final release. The imaging capabilities of some of these new facilities will detect and classify a huge number of objects. However, to learn what they are, spectroscopic follow-up on the main representatives as well as rare cases at adequate spectral resolution and cadence is required. Given the expected number of sources (e.g., 20 billion galaxies and 17 billion stars down to R~27.5 for the Vera Rubin Observatory alone), only a dedicated spectroscopic facility will be able to fully realise the scientific potential of these wide-field imaging surveys. This has been ably demonstrated by the many previous major stellar and extragalactic surveys that have had a huge scientific impact and strong legacy value thanks to their spectroscopic follow-up on photometric and astrometric observations.
The demand for a 10m-class telescope dedicated to spectroscopic surveys is an interest shared worldwide and figures explicitly in many national strategic science plans (e.g., the Astronet Science Vision and Infrastructure Roadmap for European Astronomy, US 2020 decadal survey, 2016-2025 decadal plan for Australian astronomy, Canadian astronomy long range plan 2020-2030). In Europe in particular, a recent poll among the ESO users showed that 75% of that community identified such a facility as the most crucial one for the future (Merand et al. 2021).
Motivated by this widespread scientific interest, a project named SpecTel has been studied at ESO in the period 2016-2020. First, a group of ESO scientists and engineers lead by Luca Pasquini and Bernard Delabre proposed two innovative designs for a wide-field 10m spectroscopic telescope (Pasquini et al. 2016). One concept involved a five mirror telescope design optimised for gravity invariant instruments, and the other was a very wide field Cassegrain telescope optimised for fibre feeding. At the same time, ESO established a Working Group, chaired by Richard Ellis, with the mandate of developing the scientific case for a large aperture (10-12m class) optical spectroscopic survey telescope. The scientific requirements that emerged from the Working Group report (Ellis et al. 2017) defined the need of the largest possible field of view in a 10m class telescope and the use of optical fibres feeding intermediate and high resolution spectrographs. In addition, the availability of a large IFU at a separate focal station was considered a valuable additional option. A design concept for a facility full-filling such requirements was presented in Pasquini et al. (2018).
In 2021, a large European and Australian consortium was formed to propose WST for the EU Horizon 2022 call for an infrastructure concept study. The project ranked highly but was ultimately not funded. The consortium is currently self-financing an interim study in preparation of the next call in 2024. The revised project inherits from the SpecTel studies, with one major evolution, namely the ambition to have simultaneous operation of a large field-of-view, high multiplex MOS, including both medium- and high-resolution modes, and a giant panoramic central IFS. Current instrumentation at 10m class telescopes has demonstrated the great complementarity between MOS and IFS instruments, therefore their simultaneous operation at the WST will greatly increase the scientific return of this facility. Also the project is paying particular attention to the carbon impact including it as a tradeoff parameter in the concept study.
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