The general physics goal of the HEGRA experiment is the study of the non-thermal universe. In particular, one wants to determine the origin and the acceleration mechanisms of cosmic rays, which were first detected in 1912 by Victor Hess.
The arrival direction of gamma rays can serve to trace the sources of charged cosmic rays, as they are not deflected by interstellar magnetic fields and should have the same origin. Spectroscopic analysis and measurements of the spatial distribution of sources of TeV gamma rays are decisive for providing constraints on current theories of acceleration mechanisms and source models of cosmic rays. Recently, measurements of flux variability of active galactic nuclei (AGNs) have become important, as they constrain the spatial extent of the source region.

Imaging Atmospheric Cherenkov Telescopes (IACTs) are today the most sensitive technique for the observation of TeV gamma rays. This is due to the large effective collection area, which is about 10000 times that of current satellite experiments. Given that this technique is still quite new, observation times are limited to clear moonless nights, and the field of view is limited to a few square degrees, the small number of some ten TeV gamma-ray sources detected so far is no surprise. Compared to astronomy at other wavelengths, TeV gamma-ray astronomy is just at the starting point, and is believed to open a door to new fields, probably full of new physics.

The HEGRA IACT system was unique in terms of the stereoscopic technique used. The technique of observing atmospheric showers (initiated by cosmic rays or gamma rays traversing the atmosphere) simultanously with several telescopes yields an improved resolution of the angular and spectral reconstruction of the primary particle. Based on the extraordinary background suppression available due to the stereoscopic technique, a sensitivity to very weak gamma-ray sources (at the level of few percent of the Crab Nebula flux) as well as the ability of limited sky surveys is obtained. Thus the HEGRA IACT system is able to probe the existance of gamma-ray sources in a so far nearly unknown regime.

An introduction in German language is also provided.

The Experimental Setup

The IACT system was embedded in the HEGRA experiment. The site is located on the Canary island of La Palma (28.75 N, 17.89 W, 2200 m a.s.l.).

In 1992 the first prototpye telescope CT1 (with a smaller mirror) came into operation. After a learning phase, the second prototype telescope, CT2, with a mirror area of 8.5 square meters was installed in 1993. Both prototype telescopes were already able to detect TeV gamma-ray sources and proved the abilities of the technique.

Since 1993 R&D work was done for improved cameras, readout electronics and the DAQ system. In 1995 (CT3) and 1996 (CT4,5,6), four telescopes equipped with the new camera, the electronics and the DAQ system were built up, forming an IACT system. The envisaged five-telescope system was completed in September 1998 after refurbishment of CT2 with new hardware and software.

The full CT system consisted of 5 identical telescopes, each having 8.5 square meters mirror reflector area and a camera of 271 pixels, made of photomulipliers. The telescopes were arranged on a square of 100 m side length with an additional telescope centered in the square. Stereoscopic observations based on a telescope coincidence trigger provided multiple information for the optimum discrimination between the Cherenkov light flashes of gamma-ray and cosmic-ray induced showers. Based on the stereo views, an unambiguous reconstruction of the air showers in space is possible, leading to the angular reconstruction of primary particles on an event-by-event basis. Thus a map of TeV gamma ray sources can be recorded, for example the active galactic nuclei Mkn-501. The spatial extension shown in the picture corresponds to the point spread function of the CT system.

After years of faithful operation, the HEGRA telescope system was shut down in September 2002, since the manpower was needed for the new H.E.S.S. and MAGIC telescope experiments.

More details about the experimental setup are given in the overview papers paper1, paper2 or paper3.

Some pictures are provided here :

Data analysis techniques

By using the second moments of the camera images and the known geometry of the telescopes, one can reconstruct the shower direction and the shower core position. The reconstructed shower direction yields a good discrimination of gamma/hadron induced air showers for pointlike gamma-ray sources. The 3-dimensional analytical reconstruction of air showers is unique for a system of telescopes, and permits the search for extended gamma-ray sources, such as supernova remnants or diffuse Galactic emission.
Here is a picture showing the online reconstruction of an event seen by a system of 4 telescopes . The upper left corner shows the image seen by CT6, the upper right corner contains an overlay of all images seen by the telescopes in the frame of the CT6 camera plane. The green point is an estimate of the intersection point of the 4 main axes of the elliptical images and thus an estimate of the direction of the primary particle. The lower part shows the positions of the 4 telescopes at the site. In this frame, the intersection point of the image axes determines the so-called shower impact point. The knowledge of the distance of each telescope to the shower impact point is important for the reconstruction of the shower energy, as the light yield in a telescope not only depends on energy but also on the distance to the shower.
Based on experimental data, the angular resolution of the system was determined. As an example, here is a plot of the "standard candle" Crab Nebula using some standard selection cuts. The plot contains the number of events depending on the angular difference Theta**2[deg**2] relative to the direction of the Crab Nebula. The isotropic background of charged cosmic rays shows up as a flat background.

The HEGRA CT system has the unique possibility of observing gamma-ray sources under nearly background-free conditions. This is obtained by the multiple, partly uncorrelated views of the shower, which results in the possibility to overcome the problems generated by the high intrinsic fluctuations in air showers. Here is the same Crab data using tight cuts. This ability of background suppression leads to a sensitivity of 3% of the Crab flux for a 5 sigma detection in 100 hours observation.

Publications and Talks

PhD theses of MPI Heidelberg Cherenkov telescope group members.
Conference talks, posters and proceeding papers presented by Heidelberg Cherenkov telescope group members.

Publications (CT system related and others) of the HEGRA collaboration.

See also the following annual reports of the MPI:

Getting in contact

To get in contact with the MPIK CT group write or phone to:

Mail address:

Max-Planck-Institut für Kernphysik
Postfach 103980
D-69029 Heidelberg

The CT group has several interesting topics for Diploma and PhD students.

Some interesting links:

The HESS project, a future system of Cherenkov telescopes.
Links to other atmospheric Cherenkov experiments .
ADS Astronomy Abstract Service .

HEGRA collaboration home page (now offline)
MPIK Home page .

Detailed information on CT system for HEGRA only. No public access available.
MPIK internal information on CT system . No public access available.

The CT system WWW pages were set up by Antje Kohnle and are now maintained by Konrad Bernlöhr.
Comments and additions are welcome, please send me mail.