A. Overkemping

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.

Calibration and performance of the photon sensor response of FACT — the first G-APD Cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of the performance of silicon photo detectors in Cherenkov Astronomy. For more than two years it is operated on La Palma, Canary Islands (Spain), for the purpose of long-term monitoring of astrophysical sources. For this, the performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken for their temperature and voltage dependence implementing a correction method to keep their properties stable. Several measurements have been carried out to monitor the performance. The measurements and their results are shown, demonstrating the stability of the gain below the percent level. The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose.

Bokeh mirror alignment for Cherenkov telescopes

Abstract Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with large apertures and high image intensities to map the faint Cherenkov light emitted from cosmic ray air showers onto their image sensors. Segmented reflectors fulfill these needs, and composed from mass production mirror facets they are inexpensive and lightweight. However, as the overall image is a superposition of the individual facet images, alignment remains a challenge. Here we present a simple, yet extendable method, to align a segmented reflector using its Bokeh. Bokeh alig nment does not need a star or good weather nights but can be done even during daytime. Bokeh alignment optimizes the facet orientations by comparing the segmented reflectors Bokeh to a predefined template. The optimal Bokeh template is highly constricted by the reflector’s aperture and is easy accessible. The Bokeh is observed using the out of focus image of a near by point like light source in a distance of about 10 focal lengths. We introduce Bokeh alignment on segmented reflectors and demonstrate it on the First Geiger-mode Avalanche Cherenkov Telescope (FACT) on La Palma, Spain.

FACT - The first G-APD Cherenkov telescope (first results)

In October 2011, the first air-Cherenkov telescope utilizing Geiger-mode avalanche photodiodes commenced operations. The silicon-based devices display several advantages compared to classical photomultiplier tubes allowing for a more compact camera design of higher reliability, lower power consumption and bias voltage, and better prospects for improving the photon detection efficiency. Here, the first physics results are presented from a few months of data taking. Although still preliminary, the results already show a superb fidelity of the data, demonstrating the potential of avalanche photodiodes for ground-based gamma ray astronomy. The stability and high sensitivity are ideal for remote monitoring observations of variable gamma-ray sources.

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.

FACT - Calibration of Imaging Atmospheric Cerenkov Telescopes with Muon Rings

M. Nothe∗, a M. L. Ahnen b, M. Balbo c, M. Bergmann d , C. Bockermann e, A. Biland b, T. Bretz b, K. A. Brugge a, J. Buss a, D. Dorner d , S. Einecke a, J. Freiwald a, C. Hempfling d , D. Hildebrand b, G. Hughes b, W. Lustermann b, K. Mannheim d , K. Meier d , K. Morik e, S. Muller b, D. Neise b, A. Neronov c, A.-K. Overkemping a, A. Paravac d , F. Pauss b, W. Rhode a, F. Temme a, J. Thaele a, S. Toscano c, P. Vogler b, R. Walter c, and A. Wilbert d Email: maximilian.noethe@tu-dortmund.de

FACT-Tools: Streamed Real-Time Data Analysis

K. A. Brügge b∗, M. L. Ahnena, M. Balboc, M. Bergmannd , A. Bilanda, C. Bockermanne, T. Bretza, J. Bussb, D. Dornerd , S. Eineckeb, J. Freiwaldb, C. Hempflingd , D. Hildebranda, G. Hughesa, W. Lustermanna, K. Mannheimd , K. Meierd , K. Morike, S. Müllera, D. Neisea, A. Neronovc, M. Nötheb, A.-K. Overkempingb, A. Paravacd , F. Paussa, W. Rhodeb, F. Temmeb, J. Thaeleb, S. Toscanoc, P. Voglera, R. Walterc, and A. Wilbertd Email: kai.bruegge@tu-dortmund.de

FACT - Status and Experience from Three Years Operation of the First SiPM Camera

A. Biland∗a, M. L. Ahnena, M. Balbob, M. Bergmannc, T. Bretza,1, K. A. Brugged , J. Bussd , D. Dornerc, S. Einecked , J. Freiwaldd , C. Hempflingc, D. Hildebranda, G. Hughesa, W. Lustermanna, K. Mannheimc, K. Meierc, S. Mullera, D. Neisea, A. Neronovb, M. Nothed , A.-K. Overkempingd , A. Paravacc, F. Paussa, W. Rhoded , F. Temmed , J. Thaeled , S. Toscanob, P. Voglera, R. Walterb, and A. Wilbertc aETH Zurich, Institute for Particle Physics Otto-Stern-Weg 5, 8093 Zurich, Switzerland bUniversity of Geneva, ISDC Data Center for Astrophysics Chemin d’Ecogia 16, 1290 Versoix, Switzerland cUniversitat Wurzburg, Institute for Theoretical Physics and Astrophysics Emil-Fischer-Str. 31, 97074 Wurzburg, Germany dTU Dortmund, Experimental Physics 5 Otto-Hahn-Str. 4, 44221 Dortmund, Germany 1also at RWTH Aachen E-mail: biland@phys.ethz.ch

FACT — LONGTERM MONITORING OF BRIGHT TeV BLAZARS

Since October 2011, the First G-APD Cherenkov Telescope (FACT) is operated successfully on the Canary Island of La Palma. Apart from the proof of principle for the use of G-APDs in Cherenkov telescopes, the major goal of the project is the dedicated long-term monitoring of a small sample of bright TeV blazars. The unique properties of G-APDs permit stable observations also during strong moon light. Thus a superior sampling density is provided on time scales at which the blazar variability amplitudes are expected to be largest, as exemplified by the spectacular variations of Mrk 501 observed in June 2012. While still in commissioning, FACT monitored bright blazars like Mrk 421 and Mrk 501 during the past 1.5 years so far. Preliminary results including the Mrk 501 flare from June 2012 will be presented.

FACT - TeV Flare Alerts Triggering Multi-Wavelength Observations

D. Dorner∗a, M. L. Ahnenb, M. Balbod , M. Bergmanna, A. Bilandb, T. Bretzb1, K. A. Bruggec, J. Bussc, S. Eineckec, J. Freiwaldc, C. Hempflinga, D. Hildebrandb, G. Hughesb, W. Lustermannb, K. Mannheima, K. Meiera, S. Mullerb, D. Neiseb, A. Neronovd , M. Nothec, A.-K. Overkempingc, A. Paravaca, F. Paussb, W. Rhodec, F. Temmec, J. Thaelec, S. Toscanod , P. Voglerb, R. Walterd , and A. Wilberta aUniversitat Wurzburg, Institute for Theoretical Physics and Astrophysics Emil-Fischer-Str. 31, 97074 Wurzburg, Germany bETH Zurich, Institute for Particle Physics Otto-Stern-Weg 5, 8093 Zurich, Switzerland cTU Dortmund, Experimental Physics 5 Otto-Hahn-Str. 4, 44221 Dortmund, Germany dUniversity of Geneva, ISDC Data Center for Astrophysics Chemin d’Ecogia 16, 1290 Versoix, Switzerland 1also at RWTH Aachen E-mail: dorner@astro.uni-wuerzburg.de