| Summary: | Aerosols play an important role in atmospheric interactions and in radiative balance. Aerosols have a major effect on climate and other phenomena (like Arctic Haze). Monitoring of the fluctuating aerosol content is usually done with solar spectrophotometry, which is not possible during the night and in the polar winter. The system presented aims to complete our observations of aerosol optical depth by making nighttime measurements. Its innovative but simple design is based on the use of a diffraction grid mounted on a robotic telescope (Meade LX200 10 inches) combined with a CCD array. This system measures the atmospheric attenuation of the emission spectrum (spectral resolution: 2.5 nm) of a star selected from a group of bright stars. The aerosol optical depth (AOD) is obtained by comparing the spectra of a pair of these stars at different apparent angular elevations and hence different air masses. The different steps of signal processing are discussed: acquisition, measurements protocols, retrieval of the spectrum in the image, air mass calculation, spectral response and the extraction of the aerosol optical depth (AOD). The system performance, tested with selected stars: Capella, Vega, Deneb, Altaïr and Arcturus, shows the ability of this method to measure the aerosol optical depth spectrum. Results for two sets of"golden days" (4-5 April 2001 and 6-7 May 2001) indicate that the system can make measurements of aerosol optical depth that are consistent with daytime solar spectrophotometry («0.05 for AOD = 0.1) provided the differences in air mass between the two stars considered are greater than 1.
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