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Laser Guide Star Adaptive Optics (LGS-AO) is becoming routine in severalastronomical observatories. The use of powerful lasers generates sensible Ramanemissions on the uplink laser beam path, plus secondary Rayleigh scatteringfrom atmospheric molecules and Mie scattering from aerosols. This paper reportsthe results of a campaign done with the 10.4m Gran Telescopio CANARIAS (GTC);this campaign was undertaken to assess the spectral and photometriccontamination coming from a 589 nm laser uplink beam scattering and Ramanemission induced on the GTC spectro-imager OSIRIS by laser launched about 1 kmoff-axis. The photometric contamination is due to primary and secondaryscattering of the uplink photons, as well by the Raman inelastic scattering. Wehave propagated the laser beam creating a mesospheric LGS, then pointed andfocused the GTC telescope toward the uplink laser beam, at different heightsand up to the LGS, taking into account the observing geometry. In ourobservations, the Raman emissions for O2 and N2 vibrational lines are visibleat 20 km, weakening with altitude and becoming undetectable above 30 km. Thescattering of the focused uplink beam is detectable at less than +/-0.2 arcminfrom the center of the beam, while for the focused LGS the scattering isnarrower, being detectable at less than +/-0.1 arcmin around the plume.Recommendations for Laser Traffic Control Systems (LTCS) are given accordingly.

Laser Guide Star uplink beam: scattering and Raman emission measurements with the 10.4m Gran Telescopio CANARIAS