The Georgia Tech Research Institute and the University of New Mexico have developed an eye-safe micropulse lidar known as the Astronomical Lidar for Extinction (ALE), for measuring atmospheric extinction in support of the ground based astronomy. The atmosphere must be fairly clear when astronomical measurements are made, but some extinction is still present in the visible light region, and it has a highly variable component due to boundary layer aerosols, dust and smoke in the free troposphere, and subvisual cirrus. The lidar transmits 79 microJoule pulses through a 32-cm aperture at 1500 Hz (119 mW) at 527 nm wavelength, and a 67-cm astronomical telescope is used as the long-range receiver. The lidar meets the ANSI eye safety criteria. ALE was designed to monitor atmospheric extinction on a one-minute time scale, and the lidar was developed specifically for the astronomical observatory environment. In order to monitor minute-to-minute changes in extinction due to aerosols, ALE measures backscatter from the relatively stable stratosphere. One million photoelectrons per minute are generated from backscatter above altitudes of 20 km, in order to provide sub-one percent statistical errors. We describe the design tradeoffs leading to this “clear atmosphere” lidar, including the transmitter and the short- and long-range receivers and the elevation-over-azimuth mount on which the receiver telescope is mounted. The data acquisition system is designed to accommodate the high dynamic range required for ALE, which acquires data from ranges as short as 200m and as distant as 60km. The detection and acquisition system is described, and data acquired by ALE is compared to profiles that were modeled to evaluate the science drivers leading to the design of ALE, illustrating the excellent performance of the instrument for its task of measuring time-resolved atmospheric extinction. The lidar was designed for four different modes of operation: pointing in a fixed direction, co-aligned with a research telescope, zenith-angle scanning, and sky dome mapping. The lidar is capable of mapping the sky dome in 17 directions at a rate of three complete scans per hour in order to provide real-time measurements of the amount of atmospheric extinction as well as its cause, i.e. low-lying aerosols, dust or smoke in the free troposphere, or high cirrus. This last capability is expected to help make informed decisions about what type of observations are best made at the current time, in order to optimize the efficiency of an observatory. We describe the design process and specifications for ALE, and present results from its first year of operation. Langley plots and time-height diagrams confirm that ALE operates as designed. The goal of this project was to develop reliable, cost-effective lidar technology for observatories in order to provide greater integrity for ground based astronomical data, and the authors are hopeful that similar lidars will become commonplace at other observatories worldwide.

The ALE project was funded by NSF Grant 0421087.