High-energy cosmic rays produce extensive showers in the atmosphere,
which can either be detected on the ground or through fluorescence
light produced by charged particles traversing the atmosphere.
The latter detection technique used in Pierre Auger Observatory
requires continuous monitoring of light absorption between
the fluorescence source and the detector. The LIDAR system uses UV
laser beam of similar wavelength to probe a specific region of the atmosphere.
The beam back-scatters on haze and aerosols, while the
reflected light is collected with a mirror onto a photomultiplier read
by a computer. The pulse shape analysis gives absorption-coefficient
map of the sky. A simple system to measure relative absorption coefficient
is presented in the paper.