DEVELOPMENT OF TECHNIQUES FOR OPTICAL TURBULENCE MEASUREMENTS

The developement of new techniques/instruments for the measurements of the optical turbulence is certainly a crucial element in this research field and certainly useful to achieve the ForOT scientific goals. Since the beginning of the ForoT project, the team has been concerned on two topics:

(1) the exploitation of a new method/technique that has been proposed very recently by Egner & Masciadri (2007) and called High Vertical Resolution Generalized Scidar (HVR-GS)

(2) a joint project carried out in collaboration with the J. Storey team (University of New South Wales, Sydney, Australia), A. Pellegrini team (ENEA/PNRA, Roma, Italy) and the Italian Air Force - Meteo Experimental Section, (Vigna di Valle, Roma, Italy) aiming at developing microthermal sensors to be mounted on balloons for CN2 measurements above Antarctica. J. Storey's team lead the development
of these sensors. This work has been done in the context of the project ForOT@DC (PI: E. Masciadri) approved by the Concordia Station Steering Committee of Dome C  and aiming at performing an intense site testing campaign with different vertical profilers running simultaneously for the validation of the numerical technique for optical turbulence prediction above Antarctica.

(1) HVR-GS (High Vertical Resolution - Generalized SCIDAR)

The High Vertical Resolution Generalized Scidar (HVR-GS) is a technique recently proposed by Egner & Masciadri (2007). The Generalized Scidar instrument is used in a particular way to provide CN2 profile in the first kilometer with a very high vertical resolution (~25-30 m). The HVR-GS technique has been exploited using data from intense site testing campaigns carried out at Mt. Graham whose results are summarized in Masciadri et al. (2010).

The HVR-GS technique is based on the simultaneous acquisition of auto and cross-correlation of scintillation maps produced on the pupil of the telescope by binary stars with a separation θ of the order of 30"-35";   m1, m2 ≤ 5.8 mag; Δ m ≤ 1.5 mag..


Fig1: (from Egner & Masciadri 2007) : The figure shows a set of triplets obtained calculting the cross-correlation of scintillation maps taken at the Vatican Advanced Technology Telescope observing β Cyg (θ = 35") with  time lag of 50 msec. Beside each triplet is indicated the height of the correspondent turbulent layer and its wind speed.




Fig2:
(from Masciadri et al. 2010): Optical turbulence vertical distribution structure obtained with the HVR-GS technique and the richest available sample of nights (43 nights) above Mt. Graham. Mean CN2 profiles calculated above Mt. Graham from the corresponding 45-55 %  of the J cumulative distribution.
J is the integral of the CN2 on a thickness equal to Δ h. The sample is composed of 43 nights. The dome contribution is excluded. Thin layers are well visible in the first kilometer  above the ground.


(2)
MICROTHERMAL SENSORS FOR CN2

On 1-4 July 2008 a site testing campaign has been carried out at Vigna del Valle, Rome, (Italy) to validate the experimental set up for measuring the CT2 and CN2 vertical profiles in the whole 20 km above the ground.



Fig. 1: Meteorological station of the
Italian Air Force - Meteo Experimental Section at Vigna del Valle, Rome, Italy where experiments have been carried out.


Fig.2: Top-left:  The electronic board for the microthermal sensors. Top-right:  The Digicora Station for the reception of the signal from the Vaisala meteo  sonde. Bottom-left:  D. Luon-Van and J. Storey with the meteo Vaisala sonde. On the top of the sonde it is placed a very light bar (1 m long) with two thermocouples at the extremities (the white circles indicate the position of the microthermal sensors). Bottom-right: The balloon in the hangar is ready for the lunch.


Fig. 3: Top-left: Preparation of the radiosonde battery. Top-right: People are ready to launch the balloon. Bottom-left: Test done during the daytime. The balloon is on sky and, some tens of meters behind is visible the radiosonde where the microthermal sensors are placed.  Bottom-right: Test done during the nighttime. J. Storey, E. Masciadri and a member of the Italian Air Force team are looking at the balloon flying away just after the launch.


Fig.4: Example of the CT2 (left panel) and CN2 (right panel)
extended on the 20 km and retrieved from the launch of a balloon. The vertical resolution of ~ 6 m permits to put in evidence several thin layers all along the 20 km. CN2 values are as small as 10-19 and this indicates that the sensitivity of the microthermal sensors is reasonably good as expected by such a kind of sensors for measurements above the Internal Antarctic Plateau (raw measurements from: D. Luon-Van, UNSW).

Acknowledgments: This work is funded by the Marie Curie Excellence Grant ForOT - MEXT-CT-2005-023878.
For the experiment of microthermal sensors for CN2 measurements, ForOT funded the thermocouples purchase.

  E.Masciadri, 11/2009