Raman-scattered laser guide star photons to monitor the scatter of astronomical telescope mirrors

TL;DR

This study demonstrates that Raman-scattered laser guide star photons can be used as a non-invasive method to monitor the evolving scatter properties of telescope mirrors, revealing dust accumulation effects over time.

Contribution

It introduces a novel technique to track mirror scatter properties using Raman photon flux measurements from laser guide star systems.

Findings

Dust on mirrors causes ~8% reflectivity loss at 6000Å.

Raman photon flux correlates with mirror dust accumulation.

Monitoring Raman lines offers a new way to assess mirror health.

Abstract

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.