APPLICATIONS TO THE ADAPTIVE OPTICS

The characterization of the optical turbulence is fundamental to quantify the performances of adaptive optics systems (AO systems), i.e. those systems conceived to measure and correct the perturbations induced by the atmospheric turbulence on the wavefront coming from the space and arriving at the pupil of ground-based telescopes. The adaptive optics systems can not work under whatever seeing conditions and it is, therefore, important to know which is the typical temporal and spatial distribution of the turbulence developed above an astronomical site.

If extended site testing surveys of the integrated optical turbulence are fundamental to quantify the AO systems performances, for the most advanced systems, such as the Multi-Conjugated Adaptive Optics (MCAO) and Ground Layer Adaptive Optics (GLAO), the integrated turbulence budget is not sufficient to quantify their performances and a detailed turbulence vertical distribution measurement is mandatory.

For the Ground Layer Adaptive Optics (GLAO), a concept/system proposed in 2000 (Rigaut, 2000), that aims to correct the turbulence developed near the ground to obtain a uniform and homogeneous wavefront correction over a large field of view (of the order of, at least, a few arcminutes), it is extremely important to know the typical distribution of the optical turbulence developed in the first kilometer with high vertical resolution (up to a few tens of meters). The required vertical resolution depends on many parameters. Among these: the field of view of the optical instrument, the pitch size of the actuators of the deformable mirror used to correct the perturbed wavefront and the wavelength. A GLAO system can correct the turbulence below a minimum threshold (H

In a recent paper (Masciadri et al., 2010) we summarized the results of the richest statistical analysis ever done so far above Mt. Graham for many among the most important parameters useful to charcaterize the AO systems: the seeing, the wavefront coherence time, the isoplanatic angle, ect....

Fig. 1: (from Masciadri et al. 2010): Extent of the 'gray zone' i.e. H

Table1: (from Masciadri et al. 2010): Values of H

It is worth noting that the Adaptive Optics systems evolved and experience showed that more and more sophisticated new methods/technics appeared in the last decades. As a consequence, new parameters and function of merit have been introduced to characterize the performances of the new AO systems. An atmospherical model is an extremely useful tool of investigation because it can easily be adapted to quantify and characterize a new parameter without the necessity to conceive a dedicated instrument with precise technical specifications to measure it. Most of the parameters used at present in astronomy to characterize the optical turbulence and its effects on instrumentation have been implemented in the Meso-Nh Astro-Code.

GLAO performances: Dome C versus Mt. Graham (mid-latitude site)

The real challenge of Dome C for astronomical applications in the near-infrared range is the wide field because the optical turbulence is concentrated in a thin surface layer in this region. The wider the angle, the shorter is the depth of field on which the GLAO system can correct the turbulence efficiently. We can envisage two solutions to by pass the turbulence at Dome C:

(1) a telescope whose primary mirror is located above the surface layer (i.e. 50-60 m including the statistical dispersion).

(2) a telescope embedded in the strong surface turbulent layer equipped with a GLAO system conceived to correct the surface layer and for which the final performances should be better than what achievable at mid-latitudesites. But...Is there any turbulence or technical constraints in this second hypothesis ? The wavefront correction stops typically on a spatial scale roughly equivalent to the r

For this set of simulations, Dome C provides better performances than Mt. Graham for Δ X ≤ 0.38 m.

Conclusions: From a general point of view for small telescopes (2 m class telescopes) it is relatively easy to figure out a standard GLAO system with a reasonable number of actuators that fit the constraint of a small pitch size. For a larger telescope (8 m class telescopes or even more), on the contrary, we are forced to envisage a higher order AO system to be compettive with an equivalent facilities at a mid-latitude site.

Def: EE50 = angular size (arcsec) in which 50% of the PSF is included.

Fig. 2: (from Stoesz 2008): EE50 i.e. the angular size ("") in which 50% of the PSF is incldued versus the Field of View (FOV). Red lines: Mt. Graham. Blue lines: Dome C. Thick lines: median value, thin lines: first and third quartiles. Left: the pitch size Δ X = 0.1 m. Right: the pitch size Δ X = 0.5 m.

Acknowledgments: This work is funded by the Marie Curie Excellence Grant ForOT - MEXT-CT-2005-023878

E.Masciadri, 11/2009