Abstract
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 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 of the uplink laser beams over the wavelength range of 4750 Å–9350 Å. We report the first detection of laser-induced Raman scattering by , , , , and (tentatively) 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 Å to 2210 Å redder than the original laser wavelength of 5889.959 Å—landing on the 8.2-m primary mirror of UT4 after being Raman-scattered on their way up to the sodium layer. Laser-induced Raman scattering, a phenomenon not usually discussed in the astronomical context, is not unique to the observatory of Cerro Paranal, but it is 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 thoroughly handle 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 (ELT). At sites hosting multiple telescopes, laser-collision-prediction tools should also 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.
- Received 28 March 2017
DOI:https://doi.org/10.1103/PhysRevX.7.021044
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Synopsis
SynopsisLaser Stars Under the Lens
Published 22 June 2017
Raman scattering could contaminate astronomical observations that use artificial, laser-generated “stars” to correct for the effect of atmospheric turbulence.
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Popular Summary
Atmospheric turbulence strongly affects the sharpness of astronomical observations from the ground. Specially equipped telescopes (and their associated instruments) can reduce this effect by directing lasers up into the sky, causing sodium atoms in the upper atmosphere to glow. Deformable mirrors can then use these “artificial guide stars” to help correct for the impact of turbulence on observations. Four such lasers were recently installed at the Very Large Telescope (VLT) at Cerro Paranal in Chile. For the first time, we characterized the astronomical consequences of laser-induced inelastic Raman scattering, a process through which the laser photons lose energy by exciting air molecules. This is a possible source of contamination for astrophysical observations, appearing in the data as complex groups of emission lines.
We used the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph—an instrument that obtains a spectrum for each pixel of an image in a given area of the sky—on the Unit 4 Telescope of the VLT to record the spectral signature of the lasers over the entire optical range. These 22-W lasers, each tuned to a wavelength of 5889.959 Å, excite sodium atoms at 90 km above the ground. We identified contaminating Raman spectral lines from molecular nitrogen, molecular oxygen, carbon dioxide, water, and, tentatively, methane. This detailed characterization of the spectral signature of laser-induced Raman scattering, and the identification of the molecules involved, is crucial to anticipating, reducing, and correcting the possible contamination of scientific observations obtained at any observatory using a laser guide star.
