  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{\AA} --
contribute (60$\pm5)% 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.
