Forecasting Wavefront Isoplanaticity

 

Assuming the decision is taken to use adaptive optics, how far away from the line of sight can a natural guide star be chosen? In case no convenient natural guide star is available for a given scientific object, will the cone effect be small enough to allow laser guide star operation?

The answers depend on the relative vertical distribution of the turbulence expected above the observatory, expressed as an isoplanatic angle $\theta 0$ along:

$$\theta 0 = \big[ \sum C_{n}^{2}(h) h^{5/3} dh \big] ^{-3/5}$$ (1)

Let us assume that is it possible to model the vertical distribution of C_n² with an input vertical profile of temperature measured locally by a radiosonde as described in Section 4.2. Could the radiosonde measurement be replaced by a forecasted temperature profile to obtain from meso-nh a forecast of the vertical profile C_n²(h) above Paranal?

According to the conclusion of the evaluation of ECMWF forecast skill by [CRS4 97], summarized on Table 6, the temperature profile is accurately represented and correctly forecast at the two chilean radiosonde launching sites. Question was raised if a profile obtained under the hydrostatic assumption could be used to initialize a non-hydrostatic model such as meso-nh. It was answered that although this would normally not be possible, the particular case of the chilean coast was allowing this approximation: the general motion being eastwards, air masses are reaching the coast undisturbed by orography. The verification of this assumption is the object of the next phase of the study by [Masciadri 97], comparing model results to actual C_n² profiles measured from Paranal with balloon borne microthermal probes and by the SCIDAR technique.

 

2||c|| 3c||1993 89-93

$$\mid \mid \mid \mid$$ site & hour ECMWF corr err

Antofagasta Analysis 0.986 1.39 2.54 2.50

12GMT 24h For. 0.985 1.41 3.00 2.91

48h For. 0.984 1.32 3.24 3.11

Quintero Analysis 0.991 1.15 2.48 2.52

12GMT 24h For. 0.989 1.35 3.39 3.25

48h For. 0.988 1.37 3.96 3.71