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...
Context. High-contrast imaging is a powerful technique for detecting and characterizing planetary companions at orbital separations 100 mas from their parent stars. Aims. We aim to study the limiting magnitude of the VLT/SPHERE adaptive optics (AO) system and the corresponding instrument performance for faint targets (G ≥ 11.0 mag). Methods. We computed the coronagraphic H-band raw contrast and the full width at half maximum (FWHM) of the non-coronagraphic point spread function (PSF), for a total of 111 different stars observed between 2016 and 2022 with IRDIS. For this, we processed a large number of individual frames that were obtained under different atmospheric conditions. We then compared the resulting raw contrast and the PSF shape as a function of the visible wave front sensor (WFS) instant flux, which scales with the G-band stellar magnitude. We repeated this analysis for the top 10% (TCAT10 ) and top 30% (TCAT30 ) best turbulence conditions on Cerro Paranal. Results. We found a strong decrease in the coronagraphic contrast for stars fainter than G ∼ 12.5 mag, even under the best atmospheric conditions. In this regime, the AO correction is dominated by the read-out noise of the WFS detector. In particular we found roughly a factor of ten decrease in the raw contrast ratio between stars with G ∼ 12.5 and G ∼ 14.0 mag. Similarly, we observed a sharp increase in the FWHM of the non-coronagraphic PSF beyond G ∼ 12.5 mag, and a corresponding decrease in the strehl ratio from ∼ 0.5 to ∼ 0.2 for the faintest stars. The decrease in the contrast ratio and PSF sharpness is slightly more pronounced for TCAT30 than for TCAT10 .
The first observations of laser guide star photons Raman-scattered by air molecules above the Very Large Telescope (VLT) were reported in June 2017. The initial detection came from the Multi-Unit Spectroscopic Explorer (MUSE) optical integral field spectrograph, following the installation of the 4 Laser Guide Star Facility (4LGSF) on the Unit Telescope 4 (UT4) of the VLT. In this Letter, we delve further into the symbiotic relationship between the 4LGSF laser guide star system, the UT4 telescope, and MUSE by monitoring the spectral contamination of MUSE observations by Raman photons over a 27 month period. This dataset reveals that dust particles deposited on the primary and tertiary mirrors of UT4 -responsible for a reflectivity loss of ∼8% at 6000 Å -contribute (60 ± 5)% to the laser line fluxes detected by MUSE. The flux of Raman lines, contaminating scientific observations acquired with optical spectrographs, thus provides a new, non-invasive means to monitor the evolving scatter properties of the mirrors of astronomical telescopes equipped with laser guide star systems.A&A proofs: manuscript no. Vogt2018_Raman of the series of experiments presented by Vogt et al. (2017). In the remainder of this Letter, all wavelengths are quoted in air, unless explicitly mentioned otherwise. Whenever we refer to laser photons, we mean photons that were originally emitted by the laser guide star system, irrespective of whether they were subsequently Rayleigh-scattered, Mie-scattered, Raman-scattered, or absorbed and re-emitted by a sodium atom.
During the 2nd semester of 2016, a new Deformable Secondary Mirror (DSM), part of the Adaptive Optics Facility Project at Paranal Observatory, was installed on the Unit Telescope 4. Starting end of November 2016, we then re-commissioned the telescope and the following three instruments : HAWK-I, MUSE and SINFONI. It is important to understand how a DSM, even if used in non-adaptive optics mode (i.e., in its rest/flat position), can impact the operations and the quality of the observations. We discuss here the results of this telescope and instrument re-commissioning, the challenges and problems we met, and compare the new performance to the ones obtained previously with the traditional Dornier Secondary Mirror.
We report the serendipitous discovery of a redshift 3.68 quasar while validating the star WD 0308‐565 as a spectrophotometric standard star for the Multi‐Unit Spectroscopic Explorer (MUSE) calibration plan. Based on the MUSE observations, the luminosity of the quasar at 1350A (L1350) is 4.71 × 1045 erg s−1. The black hole mass is 2.5 × 109 Mʘ, and bolometric luminosity is 1.57 × 1047 erg s−1. Present in the field of view of a star in the calibration plan of the instrument makes this a very valuable quasar as it will receive many repeated visits in the coming years making it an ideal candidate for reverberation mapping studies.
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