Abstract
(Abr.) Laser guide stars employed at astronomical observatories provide artificial wavefront reference sources to help correct (in part) the impact of atmospheric turbulence on astrophysical observations. Following the recent commissioning of the 4 Laser Guide Star Facility (4LGSF) on UT4 at the VLT, we characterize the spectral signature of the uplink beams from the 22W lasers to assess the impact of laser scattering from the 4LGSF on science observations. We use the MUSE optical integral field spectrograph to acquire spectra at a resolution of R~3000 of the uplink laser beams over the wavelength range of 4750\AA\ to 9350\AA. We report the first detection of laser-induced Raman scattering by N2, O2, CO2, H2O and (tentatively) CH4 molecules in the atmosphere above the astronomical observatory of Cerro Paranal. In particular, our observations reveal the characteristic spectral signature of laser photons -- but 480\AA\ to 2210\AA\ redder than the original laser wavelength of 5889.959\AA\ -- landing on the 8.2m primary mirror of UT4 after being Raman-scattered on their way up to the sodium layer. Laser-induced Raman scattering is not unique to the observatory of Cerro Paranal, but common to any astronomical telescope employing a laser-guide-star (LGS) system. It is thus essential for any optical spectrograph coupled to a LGS system to handle thoroughly the possibility of a Raman spectral contamination via a proper baffling of the instrument and suitable calibrations procedures. These considerations are particularly applicable for the HARMONI optical spectrograph on the upcoming Extremely Large Telescope. At sites hosting multiple telescopes, laser collision prediction tools also ought to account for the presence of Raman emission from the uplink laser beam(s) to avoid the unintentional contamination of observations acquired with telescopes in the vicinity of a LGS system.
Highlights
The past few years have seen the emergence of numerous laser guide star (LGS) systems at astronomical observatories [1,2]
Following the recent commissioning of the 4 Laser Guide Star Facility (4LGSF) on Unit Telescope 4 (UT4) of the Very Large Telescope (VLT), we characterize the spectral signature of the uplink beams from the 22-W lasers to assess the impact of laser scattering from the 4LGSF on science observations
We use the Multi-Unit Spectroscopic Explorer (MUSE) optical integral field spectrograph mounted on the Nasmyth B focus of UT4 to acquire spectra at a resolution of R ≅ 3000 of the uplink laser beams over the wavelength range of 4750 Å–9350 Å
Summary
The past few years have seen the emergence of numerous laser guide star (LGS) systems at astronomical observatories [1,2]. These features consist of (1) a sharp and unresolved emission line with a peak flux density similar to that of the brightest sky emission lines in the 7000 Å–8000 Å spectral range, and (2) broad wings at a level of ≲5 × 10−20 erg s−1 cm−2 Å−1 spanning about 130 Å and 190 Å, respectively These spectral features result from the inelastic Raman scattering by the O2 and N2 molecules of the laser photons while on their way up to the sodium layer: a process seen at high peak powers with, e.g., pulsed light detection and ranging (LIDAR) lasers, and exploited for many years in atmospheric physics experiments The signature of Raman scattering from CH4 molecules [Δν0;CH4 1⁄4 2914.2 cm−1] at 7110.43 Å [60] is only tentatively detected with a signal-to-noise ratio of S=N < 3
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