FACT - Long-term stability and observations during strong Moon light

TL;DR

The FACT telescope, equipped with G-APD detectors, demonstrates long-term stability and effective operation during bright Moon light, enabling extended observations of bright TeV blazars and improving scheduling and atmospheric monitoring.

Contribution

This study provides the first long-term stability analysis of G-APD based Cherenkov telescopes during bright Moon conditions, expanding observational capabilities.

Findings

G-APDs are robust and stable detectors for Cherenkov telescopes.

The telescope maintains performance during bright Moon light, comparable to PMT-based systems.

A new Moon light background parametrisation improves observation scheduling and atmospheric monitoring.

Abstract

The First G-APD Cherenkov Telescope (FACT) is the first Cherenkov telescope equipped with a camera made of silicon photon detectors (G-APD aka. SiPM). Since October 2011, it is regularly taking data on the Canary Island of La Palma. G-APDs are ideal detectors for Cherenkov telescopes as they are robust and stable. Furthermore, the insensitivity of G-APDs towards strong ambient light allows to conduct observations during bright Moon and twilight. This gain in observation time is essential for the long-term monitoring of bright TeV blazars. During the commissioning phase, hundreds of hours of data (including data from the the Crab Nebula) were taken in order to understand the performance and sensitivity of the instrument. The data cover a wide range of observation conditions including different weather conditions, different zenith angles and different light conditions (ranging from dark night to direct full Moon). We use a new parmetrisation of the Moon light background to enhance our scheduling and to monitor the atmosphere. With the data from 1.5 years, the long-term stability and the performance of the camera during Moon light is studied and compared to that achieved with photomultiplier tubes so far.