Raman-scattered laser guide-star photons to monitor the scatter of astronomical telescope mirrors
Mieske, Steffen; Milli, Julien; Kuntschner, Harald; Hibon, Pascale; Bellhouse, Callum; Opitom, Cyrielle; Hau, George; Vogt, Frédéric P. A.; Jaffé, Yara L.; Schmidtobreick, Linda; Bourget, Pierre; Selman, Fernando; Smoker, Jonathan; Luis Álvarez, José; Bonaccini Calia, Domenico; Hackenberg, Wolfgang; Aranda, Ivan; Blanchard, Israel; Cerda, Susana; Cid, Claudia; Comin, Mauro; Espinoza Contreras, Marcela; Holzlöhner, Ronald; Kolb, Johann; Madec, Pierre-Yves; Parraguez, Diego; Romero, Cristian; Vera Urrutia, Sergio; Zins, Gerard
Chile, Germany, United Kingdom
Abstract
The first observations of laser guide-star photons that are 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 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, which are 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, which contaminates scientific observations that are acquired with optical spectrographs, thus provides a new, non-invasive means to monitor the evolving scatter properties of the mirrors of astronomical telescopes that are equipped with laser guide-star systems.