Impact of E-ELT laser light on Cherenkov Telescope Array cameras

TL;DR

This paper analyzes how the proximity of E-ELT laser guide stars could affect Cherenkov Telescope Array cameras by causing spurious light tracks, providing formulas to estimate photon scattering impacts.

Contribution

It introduces an approximate formula to estimate laser light scattering into CTA camera pixels and evaluates specific laser influence scenarios.

Findings

Estimated photon counts from scattering into camera pixels.

Laser beams can produce detectable spurious signals in CTA cameras.

Impact varies with laser direction and camera sensitivity.

Abstract

As one of the options, the Cherenkov Telescope Array (CTA) Consortium is considering the possibility to install its Southern array in Chile, in the Atacama Desert. The envisaged site is situated about 5 km from the future European Extremely Large Telescope (E-ELT), which will operate 8 parallel DC lasers emitting at 589.2 nm, to create an artificial 6.8 magnitude star at an altitude of 90 km. The guide stars are used for the adaptive optics of the telescope. Although having the artificial stars in the field-of-view of a CTA telescope would happen rather seldom, and can be avoided by coordinated scheduling, the laser beams may cross the field-of-view of a telescope more frequently and leave spurious light tracks, hence complicating the analysis of the shower images. We derive an approximate formula to estimate the expected number of photons from molecular and aerosol scattering of the laser light beam into the field-of-view of a camera pixel. We then present several specific cases of laser influence on the CTA camera pixels, based on the selected direction of the laser beam, using the expected quantum efficiency of the camera photomultipliers at the given wavelength.