Experimental estimation of the spatial-coherence outer scale from a wavefront statistical analysis

Abstract

Estimations of the spatial-coherence outer scale L0 relevantfor high angular resolution imaging are deduced from measurements ofboth-angle-of-arrival normalized covariance and difference of thevariances measured with two different size circular apertures. Thesemeasurements have been obtained by means of an experimental deviceidentical to a differential image motion monitor. Simultaneousestimations of Fried's parameter r0 are obtained from thisdifference and compared to those deduced from the structure functionEuropean Southern Observatory's ((ESO's) Differential Image MotionMonitor (DIMM) method). The sensitivity of these techniques to thedifferent noises as well as to the model describing atmosphericturbulence, is deeply analyzed and discussed. The effect of a finiteouter scale on the temporal power spectrum is also studied in the caseof three different models. The values obtained for L0 appearto be in good agreement those presented in literature. The mainconclusion of this work is that this parameter varies widely in time(and probably in space) and that a monitor dedicated to its measurementbecomes a necessity.