The Generalized FASS (Full Aperture Seeing Sensor): filling the lower altitudes of the Cn2 profileRevista : Proceedings of the fifth International Conference on Adaptive Optics for Extremely Large Telescopes
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Turbulence monitors based on scintillation are in common use among the astronomical community. One of their weaknesses is that they are blind to turbulences located near the ground where the wavefront traversing these layers have not propagated over sufficiently long distances to generate scintillation patterns with significant contrast. One approach to solve this problem is to allow propagation below the telescope pupil by means of additional optics. The problem with such approach is the diffraction caused by the telescope aperture on the negatively conjugated images. We evaluate the impact of this undesired effect on a recently proposed technique called FASS (Full Aperture Seeing Sensor) that aims to monitor the atmospheric turbulence with a dedicated small telescope of typically 30-centimeter diameter. The method uses a Fourier analysis of the scintillation patterns to estimate the turbulence strength distribution in altitude. So far, the concept and experimental results have been applied to pupil conjugation only, thus limiting the reconstruction of the turbulence profile to altitudes higher than 500m. In this paper, we simulate the generalized configuration for FASS (negative conjugation) and we show how the diffraction of the annular aperture affects the profile estimation as a function of the conjugation altitude. We show that one of the features of the FASS Fourier method, which is using one-dimensional transforms of the image points along concentric rings, makes the technique very robust with respect to the diffraction ring generated by the annular aperture. Finally, we define a method to find the most adequate negative altitude for conjugation, in order to design the future generalized FASS system.