B. Huber

Design and Operation of FACT -- The First G-APD Cherenkov Telescope

The First G-APD Cherenkov Telescope (FACT) is designed to detect cosmic gamma-rays with energies from several hundred GeV up to about 10 TeV using the Imaging Atmospheric Cherenkov Technique. In contrast to former or existing telescopes, the camera of the FACT telescope is comprised of solid-state Geiger-mode Avalanche Photodiodes (G-APD) instead of photomultiplier tubes for photo detection. It is the first full-scale device of its kind employing this new technology. The telescope is operated at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands, Spain) since fall 2011. This paper describes in detail the design, construction and operation of the system, including hardware and software aspects. Technical experiences gained after one year of operation are discussed and conclusions with regard to future projects are drawn.

Long-term monitoring of bright blazars with a dedicated Cherenkov telescope

Flaring activity of Active Galactic Nuclei (AGN) in VHE gamma-ray astronomy is observed on timescales from minutes to years and can be explained either by the interaction of relativistic jets with the surrounding material or by imprints of the central engine, like temporal modulation caused by binary systems of supermassive black holes. The key to answer those questions lies in combining 24/7 monitoring with short high sensitivity exposures as provided by the third generation gamma-ray astronomy instruments like MAGIC, VERITAS and H.E.S.S. The long-term observations can be provided by a global network of small robotic Cherenkov telescopes.(1) As a first step, we are currently setting up a dedicated Cherenkov telescope, which will carry out joint observations with the Whipple 10 m telescope for AGN monitoring. The new telescope will be designed for low costs but high performance by upgrading one of the former HEGRA telescopes, still located at the MAGIC site on the Canary Island of La Palma (Spain). The main novelties will be its robotic operation and a novel camera type, resulting in a greatly improved sensitivity and a lower energy threshold.

FACT - The G-APD revolution in Cherenkov astronomy

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.

Electronics for the camera of the First G-APD Cherenkov Telescope (FACT) for ground based gamma-ray astronomy

Within the FACT project, we construct a new type of camera based on Geiger-mode avalanche photodiodes (G-APDs). Compared to photomultipliers, G-APDs are more robust, need a lower operation voltage and have the potential of higher photon-detection efficiency and lower cost, but were never fully tested in the harsh environments of Cherenkov telescopes. The FACT camera consists of 1440 G-APD pixels and readout channels, based on the DRS4 (Domino Ring Sampler) analog pipeline chip and commercial Ethernet components. Preamplifiers, trigger system, digitization, slow control and power converters are integrated into the camera.

FACT: A novel camera for Cherenkov telescopes for ground-based gamma-ray astronomy

The field of gamma-ray astronomy has expanded rapidly during the last decade. In the energy regime from 100 GeV up to several TeV, Imaging Atmospheric Cherenkov Telescopes (IACT) are the most sensitive detectors. Presently all IACTs use photomultiplier tubes for light detection, but for future projects Geiger-mode Avalanche Photodiodes (G-APD) are very promising alternatives. In order to demonstrate their potential, the First G-APD Cherenkov Telescope (FACT) collaboration has constructed an IACT camera comprising 1440 G-APDs. By means of specially designed light concentrators the collection area of each sensor is enlarged. The entire electronics for analog signal processing, digitization and triggering is fully integrated into the camera body. Event data are sent via Ethernet to the counting house. The FACT camera was installed during fall 2011 at the Observatorio del Roque de los Muchachos on La Palma, Canary Islands (Spain), on a refurbished telescope mount.