Detection and Implications of Laser-Induced Raman Scattering at Astronomical Observatories

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 the 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 4750Å → 9350Å. 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 480Å to 2210Å redder than the original laser wavelength of 5889.959Å -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, a phenomenon not usually discussed in the astronomical context, 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 (ELT). 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.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 isa 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...

Rotational and Rotational-Vibrational Raman Spectroscopy of Air to Characterize Astronomical Spectrographs

“…The possible presence of wind along the lineof-sight (and thus the spectral shifting of the signal over time) implies that the Raman lines are not suitable to characterize the goal precision of 10 cm s −1 over 10 years for radial velocity measurements with ESPRESSO. We find, however, that the accuracy with which the Raman lines (<5 fm for the case of 16 1 3 3 5 5 7 7 9 9 11 11 12 12 13 13 14 14 16 16 18 18 20 20 22 22 24 24 26 26…”

“…We link the majority of these fainter lines to the existence of a non-zero electronic spin in the ground state 3 Σ − g of 16 O 2 . It is the interaction between this electronic spin and the molecular rotation that leads to the fine-splitting of the pure rotational Raman lines of 16 [32][33][34][35], we can successfully identify side-lines up to N = 9 (where N denotes the end-over-end rotation). We are left with 6 lines of (yet) unknown origin, all located within 13Å from the main laser line (see Fig.…”

“…with G n the vibrational component, and F n (J) the rotational component. Following [26], for diatomic molecules like 14 N 2 with a 1 Σ electronic ground state or 16 O 2 with a non-singlet electronic state but unresolved line splitting structure:…”

“…O 2 ; see the Supplemental Material for details) and the exciting laser wavelength (<11.6 fm at the 3σ level) are known is ideally matched to the target accuracy of ESPRESSO of 10 m s −1 ≡20 fm at the 4LGSF laser wavelength. The 4LGSF lasers have a spectral full-width at half maximum < ∼ 5 MHz≡6 fm, so that the full-width at half maximum of the observed Raman lines is actually dominated by thermal broadening (of the order of 2.3 GHz for 14 N 2 and 2.1 GHz for16 O 2 at…”

Impact of Laser Guide Star facilities on neighbouring telescopes: the case of GTC, TMT, VLT, and ELT lasers and the Cherenkov Telescope Array

“…The main parameters of the LGS systems of VLT, ELT, GTC and TMT used for this study are reported in Table 1 together with their closest distance to CTA telescopes and altitude differences. Based on the experience with the 4 Laser Guide Star Facility (4LGSF) on Unit Telescope 4 (UT4) of the VLT (Vogt et al 2017), an LGS system will be implemented on the ELT (Fusco et al 2010), which will rely on continuous wave lasers similar to those of the 4LGSF (according to the current design) . The TMT will operate six lasers, each with a power of 22 W (16.5 W after exiting the beam transfer optics) (Herriot et al 2014).…”