1 . 5 Lidar System to Support Ground - Based Astronomy

All ground-based astronomical observational data include effects of the Earth’s atmosphere, which is a wavelength dependent, spatially and temporally non-stationary optical medium. The atmosphere creates two different physical types of observational effects: it absorbs and scatters light in a wavelength dependent manner, and it blurs images by turbulence-induced refractive wave front perturbations. Adaptive optics significantly reduces the effect of turbulence, but extinction from scattering and absorption is not currently being measured to high precision. Astronomical sites often incorporate ingenious techniques for estimating atmospheric extinction, such as dedicated photometric telescopes and radiometers, but these techniques are based on stellar measurements that average over long time periods during the night as the light from the celestial object being monitored passes through the atmosphere over a range of angles. Atmospheric variability during the averaging period will reduce measurement accuracy, however. The lack of precision, high-speed atmospheric characterization often leads to less than optimum utilization of expensive telescope resources. Ground-based stellar photometry measurements are made in the visible and nearinfrared wavelength region. Atmospheric extinction at these wavelengths is due to both gases and particulate matter. The gases that make up 99% of the atmosphere, N2 and O2, have wellestablished extinction spectra and are not highly variable in time. The stratospheric ozone layer causes a significant mid-visible absorption that is somewhat variable, but the main source of variable extinction is caused by airborne particulates: boundary layer aerosols; dust or smoke in the free troposphere; cloud particles, especially ice crystals in sub-visual cirrus clouds; and stratospheric aerosols, which are currently at