# Gamma-ray Observations Under Bright Moonlight with VERITAS

## TL;DR

This paper discusses how VERITAS gamma-ray telescopes have been adapted to operate under bright moonlight using reduced PMT voltage and UV filters, increasing observational time and enabling new astrophysical measurements.

## Contribution

It introduces and evaluates two new observing modes for VERITAS under bright moonlight, expanding operational capabilities and scientific opportunities.

## Key findings

- Enabled observations at up to 80% Moon illumination
- Achieved 30% more observing time annually
- Detected a gamma-ray flare from 1ES 1727+502

## Abstract

Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive photomultiplier tube (PMT) cameras. Exposure to high levels of background illumination degrades the efficiency of and potentially destroys these photo-detectors over time, so IACTs cannot be operated in the same configuration in the presence of bright moonlight as under dark skies. Since September 2012, observations have been carried out with the VERITAS IACTs under bright moonlight (defined as about three times the night-sky-background (NSB) of a dark extragalactic field, typically occurring when Moon illumination > 35%) in two observing modes, firstly by reducing the voltage applied to the PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to the cameras. This has allowed observations at up to about 30 times previous NSB levels (around 80% Moon illumination), resulting in 30% more observing time between the two modes over the course of a year. These additional observations have already allowed for the detection of a flare from the 1ES 1727+502 and for an observing program targeting a measurement of the cosmic-ray positron fraction. We provide details of these new observing modes and their performance relative to the standard VERITAS observations.