Dynamically refocused Rayleigh beacons for adaptive optics

Abstract

Dynamic refocus, DR, for adaptive optics has been proposed as a process that, over a given range, will maintain a sharp image of the Rayleigh backscatter due to a laser pulse rising in the atmosphere. Sharp focus is important when the backscatter from an artificial star is used for wavefront sensing. The wavefront variance associated with a Shack Hartmann wavefront sensor is inversely related to the photon return (in the shot noise limited case) and is dependant on the square of the beacon image's angular size. For the same spot size the photon return of a dynamically refocused Rayleigh beacon is increased by a factor of twelve over a system without DR. Here we present the implementation and results of the first operational Rayleigh beacon DR system. The DR system is designed for use on the MMT with five Rayleigh laser beacons projected on a 120arcsec regular pentagon. It was tested on the Mt. Bigelow 61" telescope, configured to act as an off-axis MMT subaperture. The results show the simultaneous DR of five Rayleigh beacons where a beacon image is dynamically refocused from 7arcsec FWHM to 2.7arcsecFWHM.