Astronomers have been plagued for centuries by “atmospheric extinction” – scattering and absorption of light as it travels through Earth's night-time atmosphere for the last millisecond of its often billion-year journey. This effect is the principal limiting factor to the precision and accuracy of observations made with terrestrial telescopes. In an effort to significantly enhance the precision of ground-based astronomical observations we have initiated a program to quantitatively measure the atmosphere in the direction a nearby astronomical telescope is pointed, and to do so contemporaneously with its scientific observations.

The first instrument we have fielded to address this problem is the Astronomical Lidar for Extinction (ALE), an eye-safe 527nm lidar that provides high precision extinction measurements when the atmosphere is clear and the majority of astronomical observations are being made. ALE is thus a “clear air lidar” primarily designed to make sub-1% measurements of atmospheric extinction every minute of time, and to make these measurements through an optical path sensibly the same as that through which astronomical observations are being made.

ALE is also used to provide quantitative assessment of the transmission over the visible hemisphere of the atmosphere to help make informed decisions about whether to observe, and what type of astronomical observations are best made through the atmosphere at the current time. It is anticipated that this support function can increase the observing efficiency of existing observatories by up to 30%, simply by precluding unsuitable observations that, because of atmospheric extinction, would not yield valid or useful scientific data.

The bulk of temporal and spatial variability that affects ground-based astronomical observations originates in the troposphere where aerosols and water vapor exhibit large variation. Therefore, ALE principally monitors the Rayleigh backscatter from the relatively stable stratosphere, and from the stratospheric measurements we deduce the transmission of lower layers. ALE was designed to gather approximately one million photons from above 20km per one minute scan under clear conditions, enabling the high precision we require.

Because of its observatory-class utility, it is anticipated that operation of ALE-like lidars will become ubiquitous at observatories worldwide. This raises the interesting issue of the contributions to understanding Earth's atmosphere, especially over timescales of years and decades, made possible by a database of synoptic, high-precision lidar observations.

We describe ALE and present results from the first year of operations. We further discuss its astronomical applications and investigate the contributions to understanding Earth's atmosphere, weather and long-term atmospheric changes from analyzing globally-obtained precise, synoptic night-time lidar observations of Earth's atmosphere.

Development of ALE was funded by NSF Grant 0421087.