Observational Impact of Scattered Light from the Laser Beam of a Laser Guide Star Adaptive Optics System

The scattered light from the sodium laser beam projected from the Keck II telescope during their laser guide star experiment campaign conducted at the end of 2001 was measured using the Subaru Telescope, 221 m away from the Keck II telescope. The purpose of our measurement was to evaluate the observational impact of the scattered light from the laser beam on other telescopes whose field of view happens to cross the laser beam. The actual flux level at the wavelength of 589 nm (sodium D2 line) measured when the Subaru Telescope was pointing toward the Keck II laser beam at elevation angles of 45 and 60 was roughly equivalent to 19.5 mag arcsec 2 calibrated by a standard-star magnitude in the R band. The present measurements provide the first quantitative assessment of the level of scattered light from the laser beam observed from other nearby telescopes. The results of measurements were shown to be consistent with theoretical estimation based on the laser beam flux and the efficiency of the scattering due to molecules and aerosols in the atmosphere. Although the impact of this scattered light depends on the type of observation, we show that the surface brightness of the sodium laser beam at the focal plane of the telescope is no brighter than the sky background 45 away from the full moon, when the collision of the laser beam and telescope pointing takes place at relatively low altitude, for instance several hundred meters above the ground. In this case, many observations could be performed without significant deterioration. Disturbance to astronomical observations could be significant, however, when a nearby telescope is pointed in the vicinity of the laser guide star, because of the presence of the defocused laser guide star itself at around 10 mag in the observing field of view, or because of the increase of total flux of scattered light from the laser beam that occurs when the optical axis of the telescope and the center of the laser beam collide at an altitude of around 8 km.