FACT -- The G-APD revolution in Cherenkov astronomy

TL;DR

The FACT telescope demonstrates the successful deployment of G-APD photon counters in Cherenkov astronomy, overcoming stability challenges and establishing a new detector technology for the field.

Contribution

This work introduces the first G-APD based camera for Cherenkov astronomy and details methods to maintain stability under varying environmental conditions.

Findings

Stable operation of G-APD camera over two years

Effective methods for voltage and temperature stabilization

Successful detection of Cherenkov signals with solid state sensors

Abstract

Since two years, the FACT telescope is operating on the Canary Island of La Palma. Apart from its purpose to serve as a monitoring facility for the brightest TeV blazars, it was built as a major step to establish solid state photon counters as detectors in Cherenkov astronomy. The camera of the First G-APD Cherenkov Telesope comprises 1440 Geiger-mode avalanche photo diodes (G-APD), equipped with solid light guides to increase the effective light collection area of each sensor. Since no sense-line is available, a special challenge is to keep the applied voltage stable although the current drawn by the G-APD depends on the flux of night-sky background photons significantly varying with ambient light conditions. Methods have been developed to keep the temperature and voltage dependent response of the G-APDs stable during operation. As a cross-check, dark count spectra with high statistics have been taken under different environmental conditions. In this presentation, the project, the developed methods and the experience from two years of operation of the first G-APD based camera in Cherenkov astronomy under changing environmental conditions will be presented.