FREQUENTLY ASKED QUESTIONS
The great quests of modern astrophysics and cosmology.
Table of contents
1. About the WST
1. About the WST
Traditionally, ground-based telescopes are designed independently of their instrumentation. The telescope is optimised to deliver the best possible image quality at a predefined set of focal stations and instruments are then designed to fit within the available space.
In many cases, first-generation instruments offer limited performance because most of the effort — and often the budget — goes into building the telescope. As a result, instrumentation receives fewer resources than initially planned and the original scientific ambitions must be scaled back. These shortcomings are typically addressed only with the development of a second generation of instruments.
In contrast, the WST is conceived as a fully integrated facility, in which the telescope and instruments are co-designed from the outset. Design trade-offs are made at system level to maximise overall scientific return, which may mean investing more effort and resources into the instruments than into the telescope itself.
This is exactly the case for the WST: the instrumentation accounts for a substantial portion of the total effort, and the telescope is engineered specifically to accommodate the instruments' volume and spatial requirements.
The WST will address science across all spatial and temporal scales in the Universe. It will allow the community to tackle key questions in most areas of cosmology and astrophysics, including, to cite just a few examples: the nature and expansion of the dark Universe; fundamental physics; black-hole formation; the study of dark and baryonic matter in the cosmic web, as well as the small-scale matter cycle in galaxies; galaxy assembly, including the formation history of the Milky Way and dwarf galaxies in the Local Group; star formation across space and time; the origin of the chemical elements; the characterisation of planet-host stars; the astrophysics of extreme objects, transient or time-variable events; and multi-messenger astrophysics.
For a detailed description of the science goals, see here.
Predicting the scientific landscape 15 years ahead is always challenging. Past experience has consistently shown that major, and often unexpected, discoveries emerge from the advent of new facilities. It is therefore essential to assess the discovery potential of the WST.
We identify three key areas of discovery potential for the WST:
• the statistical power of MOS-LR, enabled by its massive multiplexing capability, which will allow the collection of homogeneous samples comprising hundreds of millions of targets;
• the intrinsic discovery space of the IFS, with its “spectroscopy of anything” capabilities and the generation of billions of spectra;
• the WST’s unique spectroscopic time-domain capability, which will explore this largely uncharted domain.
For a more detailed discussion of this important topic, we refer the reader to the second issue of the Chronicle.
The concept of combining both MOS and IFS capabilities in a dedicated wide-field 10-metre-class spectroscopic telescope would fill a clear gap in the European and international research infrastructure landscape.
In the 2040s, the major ESFRI infrastructures, including the ELT, SKAO, CTAO, and possibly the Einstein Telescope, together with the WST, will represent a key set of facilities to ensure the full scientific exploitation of the other major facilities on the ground and in space.
2. Why the WST / Why not the VLT
2. Why the WST / Why not the VLT
The Integral Field Spectrograph (IFS) for the WST has a field of view nine times larger than that of MUSE on the VLT. Its spectral range, extending from 0.37 to 0.98 microns, is also significantly broader than MUSE’s 0.48–0.96 micron coverage. This broader range requires a dual-channel configuration, with separate blue and red arms, which in turn increases the volume of each spectrograph compared with those in MUSE.
On the VLT, MUSE already occupies the full volume of the Nasmyth platform. It is therefore evident that an instrument with a field of view nine times larger, multiplied by two channels, could not be hosted on a VLT platform. More generally, the total volume required for the WST IFS exceeds what could be accommodated on any conceivable Nasmyth platform of a 10-metre-class telescope.
The only viable solution, which we have adopted, is to place the 192 spectrographs at a dedicated Coudé focus, located in the basement of the telescope structure. The so-called IFS station occupies a volume of 22.5 m in diameter × 6.75 m in height, corresponding to approximately 2,700 cubic metres and representing a significant portion of the entire telescope building. Such a volume is not available at the VLT Coudé focus.
This possibility was explored by the AAO and presented by M. Casali at an ESO conference in 2019. The study showed that, by replacing the M2 mirror, the original VLT field of view could be extended to 2.6 sq. deg., bringing it close to the 3.1 sq. deg. of the WST MOS field of view.
However, the study was not developed in sufficient detail to assess the feasibility of the concept. Key questions remain unresolved, such as the volume available for positioning systems and instruments, or the potential impact on VLTI operations.
It is also important to note that, with an 8.2-m aperture (compared with 12 m for the WST) and a smaller field of view, the telescope’s étendue, which is closely tied to survey speed, would be 2.3 times lower than that of the WST. The original AAO proposal envisioned a multiplex of 7,000, far below the 30,000 planned for the WST.
In addition, the design did not include a high-resolution MOS mode (R = 40,000), which would require extremely large spectrographs that are unlikely to fit within the confined space of the azimuth platform.
With limited multiplexing, lower étendue, and no provision for MOS-HR, a VLT-based wide-field spectroscopic telescope would be far less competitive than the WST, even considering the MOS only, and would support a much narrower range of science cases.
In the end, while it might be technically feasible to adapt the VLT for a lower-performance MOS, the cost savings would be minimal, especially since a new facility would still be required for the IFS. Furthermore, decommissioning the existing Nasmyth and Cassegrain foci and their instruments would significantly impact the VLT’s capabilities, even looking ahead to the 2040s.
Using two telescopes, each dedicated to a single instrument, would simplify the optical design by eliminating the need for parallel operation of the IFS and the MOS, as implemented in the WST. However, this approach would come with a significant cost impact, essentially doubling the investment required for both the telescope and the building, and it would also double the environmental footprint.
Our studies have demonstrated that the added complexity of implementing two parallel focal planes is both manageable, in terms of design and operations, and cost-effective.
With the emergence of heavy-lift launch vehicles, one might imagine building a similar spectroscopic facility in space. The largest space telescope to date, JWST, has a 6.5-metre aperture, and the next proposed large mission, HWO, envisions a 6–8 metre telescope. Launching a significantly larger telescope, however, would be prohibitively expensive.
It could be argued that a 6–8 metre telescope in space would offer performance comparable to, or better than, a larger ground-based telescope. While this is true for angular resolution and for observations in the infrared or ultraviolet, where space offers clear advantages, it does not apply in our case.
First, the high multiplexing capability of the MOS and the wide-field coverage of the IFS rely on a large number of spectrographs and detectors. These components simply cannot be accommodated within the strict mass and volume constraints of space-based missions.
Moreover, the WST operates primarily in the visible range, where the Earth’s atmosphere is sufficiently transparent. In addition, the science goals do not require diffraction-limited resolution. The large field of view necessitates relatively large spaxels in the IFS and large fibre apertures in the MOS to collect light from resolved sources.
In short, building a comparable facility in space, even with a reduced 6–8 metre aperture, is currently not feasible, would involve prohibitive costs, and is unnecessary given the science requirements driving the WST.
One could argue that the WST MOS is simply a larger version of 4MOST on a larger telescope: a significant upgrade, but not truly transformative. However, such an argument could be applied to any MOS on any telescope. The fact that we have consistently developed new MOS instruments for increasingly large telescopes shows a strong and ongoing need for greater aperture, wider fields of view, and higher multiplex.
Looking at the numbers, with a telescope three times larger in aperture and a multiplex 12 times greater, the WST enables surveys an order of magnitude larger and allows the WST to observe much fainter sources.
The same logic applies to the WST IFS compared with MUSE on the VLT. The four billion spectra expected from the WST over five years would require 43 years of continuous MUSE observations on the VLT.
All the science cases we have developed are truly transformative, as they address fundamental questions that no existing or planned facility, not even 4MOST or MUSE, will be able to tackle.
3. Design and Technology
3. Design and Technology
We have deliberately chosen to reuse the 1.4-metre segment size and the technology developed by ESO for the ELT. This strategy minimises risk and significantly reduces cost, as it relies on an existing, proven design, differing only in the optical prescription of the mirror.
Based on this approach, the overall mirror diameter is defined by the number of concentric segment rings. Using 7, 8, or 9 rings results in mirror apertures of 11.1, 12.6, and 14 metres, respectively.
As expected, a larger aperture improves sensitivity to faint sources. However, because a wide field of view is a key requirement, increasing the aperture also introduces significant challenges: maintaining good image quality across a large field becomes increasingly difficult and, due to étendue conservation, it would also require very fast spectrograph cameras.
The selected configuration, a 12-metre aperture with 78 segments, strikes a balance between sensitivity and feasibility. It provides a twofold increase in collecting area compared with the VLT (approximately 100 m² versus 50 m²), while preserving good image quality and allowing for a manageable spectrograph design with a camera speed smaller than f/1.
Although parallel-mode operation is common on space telescopes such as Hubble or JWST, it remains rare among ground-based observatories. Allowing only one mode at a time, either the IFS or the MOS, would have slightly simplified the design. However, the impact on cost would be marginal, since the IFS must still be located at the Coudé focus and a large number of mirrors would still be required.
Operating in parallel does introduce greater complexity: two focal planes must be maintained simultaneously, and the two instruments must function independently without mutual interference. Nevertheless, we have demonstrated that this is entirely manageable, and the significant scientific gains far outweigh the operational challenges.
From the standpoint of telescope time, parallel operation effectively doubles the science return at no additional cost, a particularly important factor given the overall investment in the facility. The key question is whether this additional time can be used productively for science.
The IFS and MOS serve distinct yet complementary purposes. The MOS features a very large field of view, capable of placing up to 30,000 fibres (MOS-LR) or 2,000 fibres (MOS-HR). The IFS, by contrast, covers a smaller field but captures full spectra for all spaxels. Together, they enable a multi-scale approach: the MOS is ideal for sampling sparse objects across wide areas, while the IFS is optimised for detailed study of compact or crowded regions.
This complementarity is well illustrated in the science cases developed by the WST team.
In summary, the parallel operation of the MOS and IFS is a novel and powerful feature of the WST. It increases observing efficiency, maximises the return on investment, and enables a wide range of science cases requiring both high spatial resolution and large-scale coverage.
Yes. The WST will integrate several innovative technologies. Key developments currently under study include:
• High-density fibre positioners: Existing technologies support up to approximately 2,000 fibres, whereas the WST aims for 30,000. Four alternative concepts are being evaluated to enable such unprecedented fibre density.
• Curved detectors: With 192 spectrographs for the IFS alone, spectrographs represent a significant fraction of the overall cost. Curved detectors simplify the optical design, enhance throughput, and reduce component count. We are currently testing curved CMOS prototypes from multiple industrial partners.
• Dielectric mirror coatings: Advanced multi-layer coatings will be applied to most telescope mirrors to maximise throughput and minimise recoating frequency, thereby improving operational efficiency.
• Energy-efficient cooling and readout systems: To support the operation of approximately 500 detectors, new compact and low-power cooling and readout solutions are under development, significantly reducing energy consumption.
This initial choice is driven by two main factors: cost and risk.
Cost: hundreds of near-infrared detectors are currently prohibitively expensive and would have a major impact on the overall cost of the facility.
Risk: reliable sky subtraction for fibre-fed near-infrared spectroscopy at the faint magnitudes the WST will reach has not yet been fully demonstrated.
To support future technological advances and reflect the facility’s long-term expected lifetime, the telescope is nevertheless designed to accommodate an extended UV-to-H-band range (350–1600 nm). Future upgrades might therefore include a near-infrared extension.
As with any large project, the WST comes with its own challenges and risks. Among the various subsystems, the telescope itself is probably the least challenging, as it reuses part of the technology developed by ESO for the ELT, for example the primary mirror segments. The parallel focal planes are a new feature and will bring their own challenges to the telescope control system, but these should remain manageable given the expertise gained at ESO with the VLT, adaptive optics systems, and the ELT.
On the instrumentation side, the challenges are greater. First, the extremely high fibre density in the positioner, featuring an order of magnitude more fibres than existing systems, requires careful investigation. This is why we are currently exploring several new concepts to meet this challenge.
The MOS-LR and IFS spectrographs are not intrinsically difficult to design, but their number, several hundred units, calls for a radically different approach. The challenge will be to manufacture small series of complex opto-mechanical systems at an affordable cost. This will require close collaboration with industry from the early stages of the project.
The MOS-HR spectrographs, although far fewer in number, may involve very large optics and gratings, which will be technically challenging. That said, their size is comparable to similar spectrographs under development for the ELT, so the experience gained there should be transferable to the WST.
The large number of detectors (approximately 650) also poses significant challenges and will require close attention. Curved detectors are foreseen for the IFS, but have not yet reached the required TRL. We are therefore developing prototypes with industrial partners to assess their performance. A backup plan is in place to use traditional flat detectors, although this would increase the complexity and cost of the spectrographs.
The WST IFS slicers follow a similar design to those used in MUSE and subsequent integral-field spectrographs such as BlueMUSE and HARMONI. However, due to the large quantity required, manufacturing processes will have to be adapted in order to produce them within a reasonable timeframe and at reasonable cost.
Operations may be the most significant challenge. The survey-oriented mode, massive data volume, and time-domain science requirements will require the development of an entirely new operational model, distinct from the current one used at ESO.
While ESO currently has limited experience with such models, the upcoming operation of 4MOST will already offer valuable insights. Moreover, although ESO is not yet operating at full big-data scale, the broader European community has significant experience in this area. Missions such as Euclid and Gaia are excellent examples, and our involvement in Vera Rubin/LSST also allows us to benefit from that experience.
Therefore, while we fully acknowledge the operational challenges posed by the WST, we are confident that, through close collaboration with ESO and by leveraging the community’s existing expertise, we have the capacity to succeed.
4. Science Case
4. Science Case
Many synergies are expected. First, the WST will allow the ESO community to uniquely complement and/or extend the capabilities of instruments such as MOONS@VLT, MUSE@VLT, BlueMUSE@VLT, 4MOST, and MOSAIC@ELT.
Second, given the huge discovery space of the WST, we expect its surveys to provide sources for follow-up with ESO instrumentation at higher spatial resolution (e.g. MAVIS@VLT, MICADO@ELT, HARMONI@ELT), higher spectral resolution (e.g. ESPRESSO@VLT, ANDES@ELT), or at different wavelengths (e.g. CUBES@VLT, ALMA, METIS@ELT).
The WST’s synergy with existing or forthcoming large ground- and space-based facilities will allow the community to fully realise their scientific potential, as it will provide the essential spectroscopic characterisation of their targets.
Thanks to its unique capabilities, even in the 2040s the WST will remain an unparalleled complementary facility for Vera Rubin Observatory/LSST, JWST, and the Gaia and Euclid missions. The WST will also greatly enhance the science capabilities of upcoming facilities such as SKAO, CTAO, ARIEL, Nancy Roman, and New Athena, as well as those that are planned or proposed on longer timescales such as Haydn, Gaia-NIR, and the Habitable World Observatory.
In addition, the WST will crucially provide the possibility to identify and characterise electromagnetic counterparts of sources found by the next generation of gravitational-wave detectors, including the Einstein Telescope and LISA.
To quote the ASTRONET Science Vision and Infrastructure Roadmap 2022–2035, a WST-like facility would “help capitalise on other large investments by providing follow-up capabilities for facilities…”.
[Go to the Chronicle article]
The WST is being conceived as a time-domain facility that will allow unprecedented characterisation of the variable and dynamic Universe. The telescope and instrument design, as well as the operational model, survey plan, and data management framework, are all being devised to optimise the time-domain capabilities of the facility. The WST will allow both fiber-level and telescope-level Target of Opportunity observations.
Thanks to these efforts, the WST will represent a key facility in the multi-messenger landscape, enabling follow-up of transient events, repeated observations of variable targets discovered in photometric surveys, and spectroscopic alerts generated by the WST itself.
5. Operations and Data
5. Operations and Data
We expect the WST to adopt a fundamentally different operational model.
The current ESO model was developed for general-purpose telescopes, where users are granted observing time to pursue specific science goals. While effective for individual investigations, or even for ESO surveys, this model is not well suited to the large-scale, multi-survey operations envisioned for the WST.
Moreover, the WST will produce data volumes an order of magnitude greater than those generated by existing ESO facilities. In addition, time-domain science introduces new constraints, such as the need for rapid target assignment for transient follow-up and fast data processing to generate real-time alerts.
In response to these demands, we are developing a new operational framework tailored to the WST’s unique requirements and scientific ambitions.
Delivering 54 GB in a single exposure, 7.5 times more than Vera Rubin Observatory/LSST, the WST is unequivocally a big-data facility. It will be the first ESO instrument to bring the organisation into the big-data era, alongside other large-scale observatories such as Vera Rubin Observatory and SKAO.
While this presents significant challenges, it also opens up remarkable opportunities.
The field of AI is evolving rapidly, making it difficult to anticipate what its capabilities will be in 15 years. However, AI is intrinsically linked to large data samples, precisely what the WST will provide. The homogeneous quality of the data will also be essential for applying AI methods effectively.
The development of AI-based processing will be explored and prototyped in the upcoming phases of the project. Even so, it is already clear that AI will play a significant role in the processing and analysis of the WST data.
A duration of five years appears to provide the optimal balance between different requirements. It allows the survey to cover substantial areas of sky and achieve the required depth without locking the facility into a decade-long plan.
It also provides enough observing time to meet ambitious science goals, since many of the WST key science drivers require millions to hundreds of millions of spectra. Five years is sufficient to assemble these large, uniform samples at high completeness.
At the same time, five years is short enough to adapt to new science priorities, respond to major discoveries, and/or incorporate new instrumentation. It also aligns well with possible technology and instrument upgrades.
Finally, a five-year survey allows for early legacy value. Shorter surveys may lack depth or completeness, while longer surveys delay broad scientific return.
If the WST is selected as the next ESO programme, this will ultimately be for ESO to decide. However, as with all previous, current, and upcoming ESO public surveys, we expect the WST spectroscopic surveys to follow the same procedure and that both raw data and advanced data products, such as source catalogues with derived parameters, will be released publicly on a regular basis.
As is already the case for all ESO data, the WST data will be stored in a publicly accessible archive following the FAIR (Findable, Accessible, Interoperable, and Reusable) Guiding Principles for scientific data management and stewardship.
The long-term legacy of the WST surveys will be represented by the long-lived archive of a large, high-quality, homogeneous dataset of calibrated spectra for hundreds of millions of sources, together with advanced product catalogues and tools.
These resources will remain foundational for future astronomical research, calibration, artificial intelligence, and even educational use for decades.
For example, the calibrated, homogeneous database of stellar, galaxy, and transient spectra will keep the WST discovery space open for decades. Future facilities producing, for example, X-ray, radio, or infrared maps, will be able to cross-match the WST catalogue and/or retrieve spectra.
The WST dataset will also represent a consistent reference for other spectroscopic surveys. To maximise the long-term legacy value of the WST, a suitable calibration strategy is being devised.
6. Community and Access
6. Community and Access
As discussed in the FAQ on challenges and risks, we anticipate that the community will play a key role in the operation of the WST. This involvement will be essential, as ESO does not yet possess significant in-house big-data expertise, whereas the community has substantial experience in operating large-scale data facilities, for example Gaia and Euclid.
The precise form of this collaboration between ESO and the community will need to be defined and agreed during the next phases of the project.
If selected by ESO as its next major programme, the WST will become an ESO facility, fully owned by the organisation. It will therefore be accessible to the entire ESO community without preferential access.
In the next phases, ESO will develop a collaborative plan to further study and construct the facility. While the details remain to be defined, we anticipate that all interested institutes from the Member States will be eligible to respond to the calls that ESO will issue. Institutes from the current consortium will of course be candidates, but these will be open calls and therefore also accessible to institutes not yet part of the collaboration.
Scientists from virtually all research areas in astrophysics are welcome to join the WST Science Team and contribute ideas, expertise, and scientific input. You can join by filling in this form.
7. Timeline and Landscape
7. Timeline and Landscape
The WST will be proposed as the next programme of the European Southern Observatory after completion of the Extremely Large Telescope. A proposal to ESO will be submitted within the Expanding Horizon initiative.
The current timeline can be found on the ESO webpage.
As indicated on the ESO webpage, “the approval of a concept for the next programme does not indicate the programme’s beginning”, as it would need to be followed by an early design study phase.
If selected by ESO as its next major programme, the WST will become an ESO facility, fully owned by the organisation. It will therefore be accessible to the entire ESO community without preferential access.
In the next phases, ESO will develop a collaborative plan to further study and construct the facility. While the details remain to be defined, we anticipate that all interested institutes from the Member States will be eligible to respond to the calls that ESO will issue. Institutes from the current consortium will of course be candidates, but these will be open calls and therefore also accessible to institutes not yet part of the collaboration.
Scientists from virtually all research areas in astrophysics are welcome to join the WST Science Team and contribute ideas, expertise, and scientific input. You can join by filling in this form.
8. Sustainability
8. Sustainability
Today, it is imperative for the astronomical community to review and minimise its environmental impact. Traditionally, telescopes located in remote areas have had a significant ecological footprint. It is therefore essential to consider sustainability from the very beginning of the design process, with the aim of reducing the WST’s impact as much as possible.
In our concept study, we have begun evaluating the life-cycle assessment of various components of the facility, both during construction and operation. This approach will help us make informed decisions when facing different trade-offs.
Choosing one of the available peaks in the Paranal–Armazones area is also motivated by our desire to avoid the environmental cost of developing an entirely new site and its associated infrastructure. At Paranal, we can take advantage of the large solar plants already in place to partially power the facility with green energy.
We have already identified several key areas in which new technologies could help reduce the facility’s carbon footprint. One example is the cooling and readout electronics for the 650 detectors, which could have a positive impact if we succeed in lowering power consumption.
We are also examining data-processing solutions in order to identify the most sustainable options available.
Synergies
The WST will fill a critical gap in the global astronomical infrastructure of the 2040s.
Next section
The wst main challenges
The great quests of modern astrophysics and cosmology
Our use of Cookies
Technical and Strictly Necessary Cookies
Always active
These tools are essential for the proper functioning of the website and the correct display of content. Since they are strictly necessary, they do not require prior user consent.
Preferenze
L'archiviazione tecnica o l'accesso sono necessari per lo scopo legittimo di memorizzare le preferenze che non sono richieste dall'abbonato o dall'utente.
Analytical cookies
These cookies are used to perform statistical analysis on how users navigate the site. We use Matomo Analytics configured to collect data in aggregate form with anonymized IP addresses. For this reason, these cookies are treated as technical cookies and do not require prior consent.
These cookies are used to perform statistical analysis on how users navigate the site. We use Matomo Analytics configured to collect data in aggregate form with anonymized IP addresses. For this reason, these cookies are treated as technical cookies and do not require prior consent.
Marketing
L'archiviazione tecnica o l'accesso sono necessari per creare profili di utenti per inviare pubblicità, o per tracciare l'utente su un sito web o su diversi siti web per scopi di marketing simili.
Addressing oustanding science
Issue #1 The WST Chronicle