About | Working Groups

In September 2019, the ELT Programme Scientist at ESO initiated the formation of a set of Working Groups (WGs) that have as main goal to improve several critical aspects that are needed for the ELT to do transformative science and for the telescope and instruments to be operated smoothly. These WGs bring together expertise from within ESO, the instrument Consortia, and the wider Community.  

Currently there are 10 active WGs, each with its own coordinator(s) and with more than 100 contributing members. The overall coordination and the inter-WG deliverables is led by Remco van der Burg, Myriam Rodrigues, Ruben Sanchez-Janssen and Michele Cirasuolo. The ELT WGs are open to the community and volunteers are very welcome. If you are interested in contributing to any of these WGs please contact Remco van der Burg and Michele Cirasuolo.  

The diagram below shows how the different ELT WGs are closely connected, where the output from a given WG can feed directly into (an)other WG(s).

Guide/AO stars

The ELT will be able to steer to any new target with a pointing uncertainty of about 10 seconds of arc. While impressive for such a big telescope, we will rely on reference stars that fall within the field-of-view to finetune the pointing to the required, much higher, precision. Another purpose of such reference stars is to help correct the astronomical images for the blurring effects caused by atmospheric turbulence. By measuring the “twinkling” of these bright reference stars within short time intervals, computers can determine how to counteract these effects by adapting the mirror shapes of the telescope and instruments. This application is called adaptive optics (AO) and it helps to substantially improve the image quality obtained with an Earth-based telescope.

The purpose of this WG is to construct reliable star catalogues that can serve both these purposes. While star catalogues exist and are being used by other telescopes and observatories, the requirements for the ELT are far more stringent. Besides the considerable astrometric requirements, we need a catalogue that is as free as possible from binary stars with a certain angular separation and flux ratio, since those would be problematic for the AO correction to work properly. The goal of this WG is to set up a learning algorithm that can help understand how reliable different stars are to be used by the ELT. 

Coordinator: Remco van der Burg (ESO) 

Simulated PSF and AO performance

How well the ELT can correct for the blurring effect by the Earth’s atmosphere depends on the atmospheric/weather conditions at a given time (which can be wildly varying during an observing night), but also on the particular stars that are available to help map out the turbulence, and inform the AO system of the required mirror adjustments. In particular their photon flux and their location with respect to the science target and to other stars (if multiple are being used in a given setup), are critical parameters.

This WG defines an algorithm to inform the user which stars are optimal to use for the AO correction, for a typical, or a specific, atmospheric condition. That is, which setup would lead to the sharpest Point Spread Function (PSF) at a defined target position. A secondary goal of this WG is to simulate the PSF, and its variation over the science detector, for a given observing condition, AO mode, and set of chosen reference stars. A particular goal is to be able to simulate the PSFs within a few seconds, via an analytic approximation that is accurate enough so that it can serve the Exposure Time Calculator, and the night-time scheduling of the ELT. 

Coordinator: Benoit Neichel (Laboratoire d’Astrophysique de Marseille) 
Key contributors: Guido Agapito, Olivier Beltramo-Martin, Cédric Plantet, Fabio Rossi

Data simulators and Exposure Time Calculator

Since exposure time at the ELT will be so valuable to the community, it is essential for a prospective user to be able to simulate the performance of the ELT and its instruments in any given observing condition, and for any given target, as realistically as possible. The instrument consortia are developing instrument simulators that are specifically tuned for their instruments. This WG aims to identify common layers between all those individual simulators, and to develop a single simulator that can serve all the ELT instruments. 

The data image simulator will be able to run with different levels of “abstraction”. On the one hand, one may require a highly sophisticated full end-to-end model that includes the entire optical path, realistic sky conditions, and different detector effects to be included. On the other hand, to be able to make a quick assessment of the observability of a given group of targets, a simulator mode should be available that can run in a few seconds. This WG helps to incorporate the latter simulator in a new Exposure Time Calculator that is being developed by ESO. Following the standards, the simulator will be maintainable in the long term, and its robustness will be ensured by comparison with “real” data once available. 

Coordinator: Kieran Leschinski (Universität Wien)
Key contributors: Myriam Rodrigues, Miguel Pereira Santaella, Olivier Hainaut, Henri Boffin, Jakob Vinther, Lars Lundin

Detector Characterisation

To be able to optimally observe with the ELT, several dozen highly sensitive detectors will be integrated in the instruments. These have a range of different properties to be able to cover the different science cases, and a total wavelength range from 0.4–14 μm. To be able to use the detectors for science, they have to be carefully tested and characterised in the lab. Particular aspects are individual pixel responses to incoming photons, charge storage and transfer efficiency, cross-talk between pixels, noise of the readout amplifiers. All these properties may depend on the photon wavelength, temperature, read-out speed and exposure time, making this a substantial task.

The main goal of this WG is to understand to what level of accuracy these effects can, and have to be, characterised for the different relevant science cases. And secondly, to provide recipes that can be integrated within the data reduction pipelines of the different ELT instruments. There is a sub-WG that deals specifically with the detector persistence (“memory effect”) that is most prominent after being exposed to relatively bright sources; namely how to effectively prevent this to occur, and to best correct for it. Finally, another sub-WG integrates all experience gained from these detector tests in a full detector model (called PyXel, a collaborative project with ESA) that can be used as input for the end-to-end simulations. 

Coordinator Overall: Elizabeth George (ESO)
Coordinator Persistence mitigation and correction: Mark Neeser (ESO)
Coordinator PyXel detector simulations: Benoit Serra (ESO)

Line calibrations

Several extensions and alternatives to the Standard Model of particle physics have prompted interest to investigate whether some of the fundamental physical constants may in fact vary by location or over time. Of particular interest is a potential variation of the fine-structure constant, which characterises the strength of the electromagnetic interaction between elementary particles. The ELT will play a big role in such experiments, given its large light-gathering power and new calibration techniques that bring the wavelength calibration residuals down to the photon limit.

However, accurate knowledge on the rest-frame wavelengths of certain atomic spectral transitions are essential for the ELT to perform such tests of fundamental physics. Even with the current generation of telescopes and instruments, knowledge of the laboratory reference wavelengths are often already the limiting factor. For the ELT, there will be an even stronger call for an accurate database of rest-frame transition wavelengths.

This WG will review and assess the current status quo of this field, and identify which transitions are most urgent to be revised, and which laboratory experiments would be needed for those experiments. Since these improvements will be difficult to make with current techniques, this WG seeks and engages in close collaborations with spectroscopy laboratories.

Coordinator: Carlos Martins (CAUP, Porto)
Key contributors: Teruca Belmonte, Joe Liske, Dinko Milaković, Michael Murphy, Gillian Nave, Tobias Schmidt, John Webb


An accurate measurement of the current and upcoming weather and atmospheric conditions is essential for the ELT to work efficiently. This way programs can be chosen and executed that best match each observing condition. There are a multitude of different meteorological tools that could help to assess these conditions, parameterised in different ways, ranging from local temperature and wind speed to cloud cover, turbulence and humidity profiles. The ELT aims to have an Astronomical Site Monitor (ASM) that will combine several of these tools.

The goal of this WG is to gather the requirements from the telescope and the different instruments to help make an informed decision on the to-be-assembled components of the ASM. The choices will be optimised in terms of scientific value versus financial means for the different parts. The experience of the nearby VLT at ESO’s Paranal Observatory will also be paramount to help make the right decisions. Furthermore, this WG will be in close contact with the Cherenkov Telescope Array, yet another nearby observatory, to share information that will hopefully improve the forecasting of the conditions on the ELT site.

Coordinators: Julien Milli (Institut de Planétologie et d'Astrophysique de Grenoble), Angel Otarola (ESO)

Key contributors: Miska Le Louarn, Elena Masciadri, James Osborn

Telluric correction

Several molecules, such as water, oxygen and ozone, are abundant in the Earth atmosphere and will leave an imprint in astronomical observations by absorbing light at specific wavelengths. This “telluric” imprint will be substantial in the wavelength range the ELT will operate in and will need to be taken out of the observations to correctly interpret them. Furthermore, following changes in temperature and pressure profiles, and different molecular volume mixing ratios, the individual absorption lines will be highly variable over time. While standard practice used to be to correct for this by observing a “telluric” standard star close in time and sky coordinate to the science target, this would be an expense of valuable ELT observing time.

This WG will study how an accurate telluric correction can be accomplished by combining data from the instruments, the ASM and satellites in space, without having to rely on specific calibration data taken with the ELT. Such a synthetic alternative, Molecfit, has already been successfully employed to process data from different observatories. This WG will define the exact requirements for the ELT, and how these can be met with those auxiliary data and by expanding current molecular data bases. Furthermore, this WG will study how this algorithm can be interfaced with the data processing pipelines. Tests with high-resolution spectroscopic data taken with the VLT, in a relevant wavelength range for ELT, are also planned.

Coordinator: Alain Smette (ESO)

PSF reconstruction

Continuous and accurate knowledge of the varying PSF, that is, the smearing by the telescope and atmosphere of an ideal point source, is critical to be able to do science with the ELT. This is especially true since the ELT will rely heavily on its AO correction, resulting in a highly complex PSF. Since the field-of-view of the telescope will be limited, there is little chance that a truly isolated point source is available to help define the PSF. The necessary blind PSF reconstruction may thus rely on the AO telemetry data, read-outs from the wave-front sensors and the commands to the deformable mirrors.

Since several tools already exist that may help reach the requirements of the PSF reconstruction for the different science cases, the goal of this WG is to make a detailed investigation of those tools, and to confront these with the different requirements for the ELT instruments and science cases. Furthermore, this WG aims to understand which telemetry data need to be stored, within the limit of what is feasible given the high data rates involved, and which site monitoring data can be explored to further improve the PSF reconstruction.

Coordinator: Joel Vernet (ESO)
Key contributor: Olivier Beltramo-Martin

Sky subtraction

Ground-based astronomical observations suffer from different contributions of sky, or background, radiation. These are largely due to atmospheric photon diffusion and radiative processes, but also due to thermal emission from the telescope itself. Depending on the wavelength of observation, different sources dominate. This WG aims to characterise both the thermal and non-thermal components of the sky, so that their contributions can be removed in the ELT data processing.

At wavelengths from the near-ultraviolet to the near-infrared, the high atmosphere emits strong airglow lines, which, especially at longer wavelengths, dominate over the continuum sky radiation at their spectral positions. Being mostly caused by different OH molecular transitions, they are extremely variable, both temporally and spatially. There have been model-based algorithms that can replicate these line variations by cataloguing and grouping them by type. However, with high-resolution (R~5,000-10,000) spectroscopy such as taken with the CRIRES instrument on the VLT, a multitude of new lines can be unravelled.

This WG aims at cataloguing these faint new sky lines and to characterise their variability. Ultimately, this will lead to an improved algorithm and a better way to subtract the airglow emission from the ELT data. Furthermore, this WG will characterise the different contributors to the continuum sky background, and to study their temporal and spatial variation. Finally, besides these physically motivated approaches, the WG will explore a completely data-driven (empirical) approach, motivated by the substantial recent progress of different signal processing techniques, to help characterise and subtract the different background components.

Coordinators: Ruben Sanchez-Janssen (STFC UK Astronomy Technology Centre), Elena Valenti (ESO)
Key contributor: Myriam Rodrigues 

Spectro-photometric standards

The ELT and its instruments will rely on a set of extremely well-characterised standard stars to quantify the wavelength-dependent sensitivity of the instruments (including the telescope and science detectors). These stars will provide the absolute zero point of the flux scale, an essential step to make the data coming from the ELT scientifically useful. The goal for the ELT is to use these standard stars only to track the relatively slowly varying instrumental response. In contrast, the rapidly changing atmospheric throughput will be determined via other means (see WG on telluric correction), saving as much of the valuable ELT time as possible to perform science observations.

In close contact with the instrument consortia, this WG aims to first make an inventory of which types of stars are needed for the different instruments. Requirements are set based on the stellar brightness, and to what extent they have been characterised in terms of wavelength range and spectral resolution. If an expansion of the existing catalogues of standard stars turns out to be necessary (which appears to be the case for MICADO), this WG will seek suitable candidates, follow them in time to ensure they are photometrically stable, and acquire the observations required to be able to model them to the required wavelength coverage and resolution.

Coordinator: Sabine Möhler (ESO)
Key contributors: Leo Burtscher, Wolfgang Kausch, Miguel Pereira Santaella