Theoretical and experimental study on photometry of a sodium laser guide star

Coupling efficiency between the sodium laser and sodium atoms in the mesosphere is one of the most important parameters for the sodium laser guide star system. A photometry system of the sodium beacon had been built on the 1.8-meter telescope at Lijiang Observatory, Yunnan Province, to measure the brightness of the beacon and coupling efficiency of the laser. The system consisted several parts:sodium laser, beam transfer optics and laser launch telescope, receiver telescope, atmospheric coherence length measuring instrument, and ground-based sodium lidar. Measurements of the sodium beacon on different output power, polarization status and central wavelength, excited by the 20 W class quasi-continuous wave sodium laser of Technical Institute of Physics and Chemistry, Chinese Academy of Sciences had been carried out Since 2011. The apparent full width with half maximum of the beacon spot could be as small as 3', or 1.3 meter at 90 km altitude. Detail of calculating the returned photons had been presented. Taking into account the transmission through astronomical V-band filter and quantum efficiency of the CCD camera, the corrected brightness of the sodium beacon produced by 19 Watts of circular polarized light was equivalent to a 7.4-V-magnitude natural star, or 9.55106 photonss-1m-2 on the top of the atmosphere.