Obtaining increasingly accurate astronomical images requires more than just increasing the size of new telescopes or equipping them with state-of-the-art instrumentation. The performances of most instruments mounted on top-class ground-based facilities, especially in visible and infrared light, is highly dependent on the weather conditions prevailing during operations, and in particular on the optical turbulence above them.  Knowing these conditions sufficiently in advance is therefore becoming increasingly important and decisive for optimising the use of the world's best telescopes, such as the current Very Large Telescope (VLT) and the future Extremely Large Telescope (ELT), in the Chilean Andes, both from the European Southern Observatory (ESO). It is crucial to be able to exploit in the best way the potentialities of these jewels of technology in a weather-compatible manner while maximising the scientific return produced.  In fact, the typical cost of a night of observations for just one of the four UTs of VLT is of the order of several tens of thousands of euros: a figure that explains in itself how critical it is to make the most of ideal atmospheric conditions.

With these objectives in mind, the National Institute for Astrophysics has won an ESO international call for proposals aimed at producing forecasts of optical turbulence (OT) and the main atmospheric parameters to optimise astronomical observations of the VLT and all the instruments with which it is equipped. The selected project, called FATE (Forecasting Atmosphere and Turbulence for ESO sites) sees the collaboration of the CNR/Regione Toscana consortium LaMMA (Laboratorio di Monitoraggio e Modellistica Ambientale per lo sviluppo sostenibile), which also provides meteo services for the Italian Civil Protection.

The FATE project began in November 2022 and entered the commissioning phase in September - December 2023, with tests to verify the technical and operational specifications. Once completed, it will enter in the operational phase in which ESO will be able to optimise observing strategies for the VLT and start planning those for ELT, which is currently scheduled to come into operation in 2028.

"The commissioning lasted four months and was aimed at verifying the robustness of the prediction system and compliance with the technical specifications required by ESO, i.e. the accuracy of the predictions of the different parameters at different time scales," says Elena Masciadri, INAF researcher and principal investigator of the FATE project. "The spatio-temporal fluctuations of optical turbulence have much smaller typical scales than those of classical atmospheric parameters and therefore the prediction of optical turbulence is a much more difficult objective to achieve. ESO's technical specifications are also quite stringent, as one would naturally expect, considering that the VLT is undoubtedly one of the most prestigious telescopes in the world, but also one of the most complex, consisting of four 8.2-metre diameter telescopes plus four 1.8-metre auxiliary telescopes, with a great variety of instrumentation and therefore observing possibilities. We can say that we are satisfied with the commissioning,' continues Masciadri, 'as it has allowed us to demonstrate the robustness and reliability of the system, and at the same time to better define the margins for improving the accuracy of the predictions where we will concentrate in the second phase of the project.

The magnificent Milky Way stretching over the Very Large Telescope (VLT) at ESO’s Paranal Observatory, demonstrating the astounding level of detail visible in the night sky from this remote site in the Chilean Atacama Desert. Credit: P. Horálek/ESO

Modern telescopes are now equipped with interchangeable instruments that have specific conditions of use, which also depend on the atmospheric conditions prevailing during observations. Some of these instruments are not very sensitive to, for example, a high concentration of humidity in the air, while others are almost completely 'blinded' by it. For certain types of scientific programmes, it is very important to collect data in the presence of a weak optical turbulence, for example in all observations requiring a high level of detail in small portions of the sky that exploit the benefits of adaptive optics, such as in the search for exoplanets. In general, knowledge of optical turbulence is crucial in all observations supported by adaptive optics.  The ELT will be a facility supported 100 per cent by adaptive optics, so the prediction of optical turbulence is certainly crucial for next generation astronomy.

Beside to the prediction above the VLT of a number of atmospheric parameters such as temperature, wind strength and direction, relative humidity, water vapour and cloud cover, the FATE project will also be concerned with the prediction of so-called astroclimatic parameters at night, including the so-called seeing, a parameter indicating the level of the atmospheric perturbation in the quality of astronomical images. But what is the optical turbulence? Temperature fluctuations in the air generate fluctuations in the refractive index, which in turn perturbs the wavefront of light from the celestial objects observed. This wavefront is thus 'imperfect' and the image collected by the telescope looses details accuracy, thus limiting the potential of the instrumentation used. Adaptive optics techniques aim to correct for these perturbations, but their performance depends on the state of the turbulence, which is why an accurate prediction of the optical turbulence is essential.

A forecasting system such as the one envisaged in the FATE project is based on hydrodynamic models that are defined as 'mesoscale': the model is applied to a limited region of the Earth, achieving a higher resolution than a forecast on a global scale could provide. This forecast is made using, as initialisation data, those produced by general circulation models, i.e. applied to the entire globe by the European Centre for Medium Range Weather Forecast (ECMWF), the centre acting on behalf of the entire European community.

INAF's experience in the field of optical turbulence forecasting for astronomy acquired over the years was fundamental in arriving at the FATE project: "We developed a model for forecasting optical turbulence, called Astro-Meso-NH in the 1990s, and since then the system has evolved, it has been applied to several among the best observatories in the world and more recently has been automated, making the model usable in operational mode and not just for research purposes", recalls Elena Masciadri "the development of modern 'assimilation data' techniques and more generally the statistical techniques of spatial filtering have guaranteed us levels of accuracy that were inconceivable only a decade ago. INAF,' concludes Masciadri, 'has the scientific responsibility for the FATE project, taking care of the development of the automatic operational forecasting system, the study and development of the algorithms required to obtain the technical specifications of the forecasting system, and all the activities necessary to improve performance that will be implemented during the first years of the operational phase. The LaMMA has the operational responsibility to manage and monitor the forecast system, both on a daily basis and over longer time intervals, and thus to ensure optimal coverage of the system”. “Software for the OT forecasts is operational in LaMMA and use High Performance Computing ressources (HPC) that are expressely dedicated to FATE and have been purchased thanks also to a contribution from the Tuscany Region. The role of LaMMA in the project is based mainly on trustworthiness of its data center that, since  more than 20 years has shown reliability in terms of strenghtness and resilience in the field of the meteo service done for the Tuscany Region – says Alberto Ortolani, LaMMA researcher and responsible of the LaMMA unit – the deep scientific comptences of INAF in the field of the OT forecast and the long-time experience del LaMMA in the management of operational meteo services lead to the win of the international call opened by ESO. The fact that a tuscany proposal won this international competition make us very proud”.