Experimental Demonstration of Real Time Gradient Gain Correction for Sodium Beacon Laser Guide Star: Postprint

摘要

One of the problems associated with the use of the sodium laser guide star as a beacon for an adaptive optics system is the changes in the sensitivity of the wavefront sensor caused by the extended nature of the sodium layer. Shack-Hartmann wavefront sensors use quad cell detectors to estimate the amount of x- and y- gradients in each sub-aperture. The gradients are multiplied by a constant gain factor that is equal in the x and y directions. Due to the non-zero thickness of the sodium layer, the wavefront sensor sees elongated, rather than round, spot, which causes different gains in the x- and y- directions. This directly impacts the reconstructed phase and thus degrades the closed loop AO performance. This paper presents an experimental demonstration of the null space technique that allows for real time extended spot gain correction in a closed loop AO operation. The implementation of the technique in the ASALT laboratory testbed is described, system performance degradation for incorrect gains is measured, and null space corrected performance for different atmospheres is presented.