On the night of 11–12 October, just a few hours after installation of its camera, the First G-APD Cherenkov Telescope (FACT) recorded flashes of Cherenkov light from air showers induced by cosmic rays. Remarkably, the shower images were recorded during a full moon – a feat that would not have been possible with a conventional air Cherenkov telescope.

FACT, installed at an altitude of 2200 m at the Roque de los Muchachos Observatory on La Palma, in the Canary Islands, uses newly developed Geiger-mode avalanche photo-diodes (G-APDs) instead of the photomultiplier tubes (PMTs) normally used in Cherenkov telescopes. These first images, taken in ambient light 100 times brighter than PMT-based telescopes could tolerate, demonstrate for the first time the use of silicon detectors capable of recording images at a rate of 109 a second.

The pioneering camera was designed and built by a collaboration from the universities of Dortmund, Geneva and Würzburg as well as EPF Lausanne (EPFL) and led by ETH Zurich. It consists of 1440 G-APDs, each one a square with a side of only 3 mm. To increase the active area, the collaboration developed solid light concentrators together with the University of Zurich. Each concentrator has a hexagonal entrance window 9.5 mm across and a square exit window with a side of 2.8 mm to match onto a G-APD. The result is a 10-fold concentration area for light reflected from the telescope mirror, while at the same time rejecting background light from outside the area of the mirror. There is one concentrator glued to each G-APD, providing a field of view of 0.1° per pixel and 4.5° for the full camera.

The electronics to read each of the 1440 pixels individually is based on the DRS-4 analogue ring sampler chip operating at a frequency of 2 gigasamples/s. The complete electronics package is integrated into the camera body, and data are sent to the counting house via standard Ethernet. The complete camera weighs about 150 kg and has a power consumption of around 500 W.

The camera was assembled and tested at ETH Zurich, before being installed in the refurbished HEGRA CT3 telescope at the Roque de los Muchachos Observatory, next to the MAGIC telescopes (CERN Courier June 2009 p20). The telescope, which has a total mirror area of 9.5 m2 , was equipped with a new drive system and improved mirror facets.

The installation of the camera is the first step towards establishing a monitor telescope for variable gamma-ray sources. It has already begun to demonstrate that G-APDs are a viable alternative to PMTs in Cherenkov telescopes. Future developments with these devices promise even higher photon detection efficiencies and availability at lower costs than PMTs. Moreover, their bias voltages of about 70 V render their operation under the harsh conditions of Cherenkov telescope sites stable and robust.