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Optical analysis and performance verification on ALADIN spectrometers

The Aeolus mission will take an innovative wind lidar called ALADIN (Atmospheric LAser Doppler INstrument) into space to measure wind profiles in the lowermost 30 km of the Earth's atmosphere. ALADIN is a direct detection wind lidar capable of using the backscatter signal from both molecular (Rayleigh-) and aerosol (Mie-) scattering to retrieve independent wind information. To achieve the mission goal, two separate spectrometers have been manufactured. The Rayleigh spectrometer is using a Fabry-Perot etalon with 2 paths and works like 2 narrow band filters. The detector measures the power reflected by the atmosphere for each thin band. The Mie spectrometer core is a Fizeau etalon. A CCD matrix measures directly the spectral response with a very fine resolution. For both etalons, the critical parameters are the FWHM (Full Width Half Maximum) and the Finesse. High optical quality and extremely narrow FWHM are needed to achieve mission performance but also request high quality system for the verification of those performances. The optical performance predictions, the verification philosophy and the test results are presented. The description of the different measurement setups including a system able to do spectral measurement with a resolution of some femtometers, the characteristic of the equipment and mathematical method used for calibration and to optimize the measurement accuracy are described. For the 2 spectrometers, a numerical model has been developed to analyse and predict the spectral response. The model and the results of the analysis are presented in the documents. The comparison between analysis and measurement results is discussed.