Research

.... Here the inspiring principles of my work ....

SCIENCE DRIVERS

My research develops in the field of the interaction between the turbulence and the electromagnetic fields, in particular those from the visible and the infrared ranges. When the electromagnetic field such as a wavefront propagate through the atmospheric turbulence, the latter destroys the original characteristics of the wavefront. Wavefront perturbations are generated by simultaneous fluctuation of wind speed and temperature that are main causes of the atmospheric turbulence. Physically speaking, fluctuations of temperatures generate, in cascade, fluctuations of density and of the refractive index. The latter generate fluctuations of the amplitude and the phase that univocally determine the wavefront conditions. Such turbulence, that is able to impact on the electromagnetic fields through the refractive index, is called optical turbulence. Such a destructive phenomenon plays a role in many contexts: ground-based astronomy, free space optical communication between satellites, remote sensing, etc. In the ground-based astronomy, perturbations of the wavefront generated by the optical turbulence drastically decrease the angular resolution of telescopes and make telescopes of 8-10 m equivalent to telescopes of 10 cm. The angular resolution is the parameter that measures how detailed is an image. Adaptive Optics (AO) technics have been conceived to correct these wavefront perturbations with the final goal to reconstruct the original wavefront characteristic and, as a consequence, the original potentialities of the telescopes.

Fig.1 show examples of the effects of the AO techniques on images obtained at the focus of telescopes. Performances of the these techniques are, however, strongly dependent on the turbulence conditions. In other words, the efficiency of the correction depends on the turbulence itself (Fig.3). For this reason, it is extremely important to know in advance turbulence conditions (i.e. to forecast the optical turbulence) to optimize the use of the adaptive optics and, finally, to optimize the science operation of present top class telescopes and of new generation telescopes. More precisely it is extremely critical how much turbulence is developed, how it is distributed in the space and at different heights in the atmosphere, how fast is the turbulence, etc.

EFFECT OF THE ADAPTIVE OPTICS ON OBSERVATIONS (Fig.1)

In the above picture are shown astronomical observations of the globular cluster Omega Centauri at 2.2 microns obtained with a multi-conjugated adaptive optics instrument (MCAO) called MAD (Marchetti et al. 2008). In the panel on the left side is shown the image as obtained through the atmospheric turbulence. In the panel on the right side the image as obtained using MAD. The smallest observable detail improves of a factor 6. The angular resolution is 0.6 arcsec in the image on the left and 0.1 arcsec in the image on the right side. The correction introduced by the MCAO system allows to detect much more fainter stars that are not visible at all on the image on the left side.

AO PERFORMANCES AS A FUNCTION OF THE OPTICAL TURBULENCE (Fig.3).

The AO performances depend strongly on optical turbulence conditions. In general, the stronger is the turbulence developed in atmosphere, the lower are the AO performances. Fig. 3 shows the Strehl Ratio (SR) as a function of the magnitude of the observed object for different values of the 'seeing' obtained for the AO system FLAO that has been implemented at the Large Binocular Telescope (LBT) some years ago (Esposito et al. 2011). The SR is a number between 0 and 1 that measures the quality of the AO correction. The larger is SR, the better is the correction. The 'seeing' is the parameter used in astronomy to measure the amount of the turbulence present in atmosphere and it is proportional to the integral of the turbulence between the ground and the top of the atmosphere. It is expressed in arcsecond ("). The larger is the seeing, the stronger is the turbulence developed in atmosphere, From Fig.3 we observe that the SR decreases with the magnitude of the observed objects. However, for a fixed magnitude, SR strongly decreases if the seeing increases. When the turbulence is above a threshold, the AO can even not work at all. This shows clearly why it is extremely crucial to quantify and even better, to forecast (therefore to know in advance) the conditions of the turbulence. The highest is the challenge of a scientific program, the better are the turbulence conditions. To optimize the use of top-class telescopes of present time and the new generation facilities, the prevision of the turbulence is extremely critical.

CHALLENGES

Optical turbulence forecast is much more challenging than the forecast of classical meteorological parameters because turbulence fluctuates on spatial and temporal scales that are much more smaller than the typical scales on which evolve the classical atmospheric parameters such as wind speed, temperature, relative humidity, etc. Optical turbulence is indeed a stochastic phenomenon. This parameter is, therefore, hardly resolved explicitly in realistic applications and it is preferably parameterized. It is moreover worth to remember that the forecast performances depends on the time scale at which forecasts refer and in general they decrease with the time scale.

OPTICAL TURBULENCE MODELLING/FORECAST (Fig.4)

The animation on the right side is a 2D map of the seeing extended on a 32.5 km x 20 km having a horizontal resolution ΔX = 500 m around the Cerro Paranal and Cerro Armazones in Chile respectively sites of the Very Large Telescope (VLT) and the Extremely Large Telescope (ELT). The two peaks are indicated with the letters 'P' and 'A'. The seeing is obtained by integrating, in each grip point, the optical turbulence (C_N² profiles) from 5 m above the ground up to the top of the atmosphere. The color table indicates a weak turbulence level (small seeing values) in yellow and a strong turbulence level (large seeing values) in black. The simulation has been obtained with the Meso-Nh model joint to the Astro-Meso-Nh code, a dedicated package for the optical turbulence originally created by Masciadri et al. 1999 and, since there continuously supported by the OT group of INAF-OAA. The animation covers the whole night (9 hours) with a sampling of 5 minutes. Overlapped to the seeing map is visible the 2D array map of the wind speed at 10 m above the ground level (a.g.l.). In this paper, actually, for the first time, a non hydrostatical mesoscale model has been used to reconstruct the vertical structure of the turbulence along the typical 20 km.

FORECASTS AT SHORT TIME SCALE (Fig. 5)

Many progresses in the field have been achieved in the last two decades and we are now in front to new challenges as the new generation telescopes aim to attain very challenging goals and this implies a very detailed knowledge of the conditions of the turbulence to use in efficient way the AO techniques. More recently, with my collaborators of the Optical Turbulence group of INAF-OAA, we dedicated a great effort in improving forecast performances at short time scale i.e. of the order of 1h and 2h. These are the most critical time scales for the science operation therefore for the optimisation of the observations of top-class telescopes. Fig. 5 shows the forecast performances in predicting the seeing (i.e. the total quantity of turbulence developed above Mt.Graham (Arizona), site of the Large Binocular Telescope (LBT). Fig.5-right shows the bias, RMSE and SD expressed in arcsecond of the seeing observed and forecasts over a sample of one full solar year (Masciadri et al. 2020). This has been obtained thank to a method of autoregression (AR). I am the PI of the project ALTA (http://alta.arcetri.inaf.it) that provides nightly forecasts of the astroclimatic parameters and atmospheric parameters relevant for the ground-based astronomy for the LBT. ALTA is a project carried out by the Optical Turbulence group of INAF-OAA. The astroclimatic parameters are those paramaters that characterise the optical turbulence conditions in the atmosphere.