Photometry of a Sodium Laser Guide Star at the Starfire Optical Range

Measurements of a laser‐pumped sodium guide star produced over the Starfire Optical Range in 2002 November show that the brightness of the spot produced by 11.5 W of linearly polarized power on the sky was equivalent to a V = 8.0 mag star. However, taking into account that the transmission through a V filter is only 55% at the wavelength of sodium, its corrected magnitude, V 1 , was 7.4, or 800 photons s −1 cm −2 at the top of the telescope. In 2003 March, tests with linearly and then circularly polarized beams out of the telescope showed that a circularly polarized beam from 12 W of power out of the telescope produced a spot with V 1 = 7.1 (1015 photons s −1 cm −2 at the top of the telescope), 0.7 mag brighter than a linearly polarized beam from 11.1 W of power out of the telescope. Over the 4 nights of experiments over two seasons, the apparent 2σ width of the spot varied between 36 and 46, or 1.6 and 2.0 m at 92 km altitude, and its length through the sodium layer was 4.6–8.5 km, but no variation of spot size with power on the sky was seen.