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

TL;DR

This paper analyzes how Laser Guide Star systems at major telescopes impact neighboring observatories, especially the Cherenkov Telescope Array, by deriving impact formulae and assessing potential observational conflicts.

Contribution

It introduces formulae to evaluate LGS impact on nearby telescopes and applies them to specific observatories, highlighting potential observational conflicts.

Findings

Minimal disturbance from artificial star in CTA field-of-view.

Potential laser crossing issues may affect 1-5% of observations.

ELT lasers pose no conflict, but GTC and TMT lasers could impact some observations.

Abstract

Powerful Laser Guide Star (LGS) systems are standard for the next-generation of extremely large telescopes. However, modern earth-based astronomy has gone through a process of concentration on few sites with exceptional sky quality, resulting in those becoming more and more crowded. The future LGS systems encounter hence an environment of surrounding astronomical installations, some of which observing with large fields-of-view. We derive formulae to calculate the impact of LGS light on the camera of a neighbouring telescope and the probabilities for a laser crossing the camera field-of-view to occur, and apply these to the specific case of the next very-high-energy gamma-ray observatory "Cherenkov Telescope Array" (CTA). Its southern part shall be constructed in a valley of the Cerro Armazones, Chile, close to the "Very Large Telescope" (VLT) and the "European Extremely Large Telescope" (ELT), while its northern part will be located at the "Observatorio del Roque de los Muchachos", on the Canary Island of La Palma, which also hosts the "Gran Telescopio de Canarias" (GTC) and serves as an optional site for the "Thirty Meter Telescope" (TMT), both employing LGS systems. Although finding the artificial star in the field-of-view of a CTA telescope will not disturb observations considerably, the laser beam crossing the field-of-view of a CTA telescope may be critical. We find no conflict expected for the ELT lasers, however, 1% (3%) of extra-galactic and 1% (5%) of galactic observations with the CTA may be affected by the GTC (TMT) LGS lasers, unless an enhanced version of a laser tracking control system gets implemented.