Automatically guiding a telescope to a laser beam on a biaxial antarctic light detection and ranging system

Abstract

The operating principle of atmospheric Rayleigh LIDAR (light detection and ranging) systems is that the range-corrected return-backscatter signal is directly related to atmospheric density. For this to be the case full overlap is required between the backscattered laser signal and the field of view of the receive telescope. Time-dependent errors in this alignment compromise the experimental method, and confuse the interpretation of geophysical signals present in the data. We describe a means of locking the alignment of a small LIDAR telescope to the backscattered laser beam, using images obtained with a commercial charge-coupled device camera, to reduce the effects of relative movement of telescope and laser on field overlap. This “autoguiding” system is implemented on a biaxial Rayleigh LIDAR in operation in Antarctica. We achieve a positional precision near 3 camera pixels (1 pixel ~ 1 arc) across the beam, and 7 camera pixels along the beam. Positional corrections are generated once per minute. The system is capable of removing medium- and long-term drifts in the relative alignment of our telescope and laser during an observing run.