Using UV-pass filters for bright Moon observations with MAGIC

TL;DR

This paper explores the use of UV-pass filters on MAGIC telescopes to extend their observation capabilities during bright moonlight, effectively increasing their duty cycle by about 30%.

Contribution

The study introduces a cost-effective UV-pass filter system that allows MAGIC telescopes to operate under brighter moonlight conditions, significantly expanding their observational window.

Findings

Transmission of about 20% for moonlight and 45% for Cherenkov light.

Observation capability during full Moon with sources as close as 5 degrees to the Moon.

Extended duty cycle of MAGIC by approximately 30%.

Abstract

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACT) that observe Very High Energy (VHE) gamma ray sources. The PMTs in their cameras are designed to operate under moonlight, but they are limited to Moon phases below 93% (300 Moon hours per year), as they can get damaged if the amount of light they receive is too high. As a result, they cannot be used in the three to five nights around full Moon. We have selected commercial inexpensive UV-pass filters rejecting light above a wavelength of 420 nm, where the moonlight intensity is stronger. We mounted them on light-weight frames that can be easily installed on the telescope cameras. Test observations have been performed during the last nine months, from which a moonlight transmission of about 20% and a Cherenkov light transmission of about 45% are estimated. This allows the observation of sources down to an angular distance of 5 degrees to the Moon during Full Moon: essentially in the whole sky and all possible moonlight conditions. Therefore, the duty cycle of MAGIC can be extended by about 30%, including nights when VHE observations with IACTs are currently not feasible. Here we evaluate the preliminary performance, in terms of sensitivity and energy threshold, of the MAGIC telescopes equipped with the UV-pass filters under different moonlight intensities, as inferred from Crab Nebula observations and Monte Carlo simulations.