VERITAS Observations under Bright Moonlight

TL;DR

This paper discusses how VERITAS telescopes have adapted to observe under bright moonlight by implementing new modes that extend observation time and maintain sensitivity, addressing previous limitations caused by moonlight.

Contribution

It introduces and evaluates new observing modes for VERITAS under bright moonlight, enabling increased observation time without damaging the equipment.

Findings

UV bandpass filters allow observations up to 80% moon illumination

Reduced PMT voltage decreases noise and PMT aging

29% more observing time achieved annually

Abstract

The presence of moonlight is usually a limiting factor for imaging atmospheric Cherenkov telescopes due to the high sensitivity of the camera photomultiplier tubes (PMTs). In their standard configuration, the extra noise limits the sensitivity of the experiment to gamma-ray signals and the higher PMT currents also accelerates PMT aging. Since fall 2012, observations have been carried out with VERITAS under bright moonlight (Moon illumination $> 35\%$), in two observing modes, by reducing the voltage applied to the PMTs and with UV bandpass filters, which allow observations up to $\sim80\%$ Moon illumination resulting in $29\%$ more observing time over the course of the year. In this presentation, we provide details of these new observing modes and their performance relative to the standard VERITAS observations.