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.
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
MAGIC telescope experiments.
More details about the experimental setup are given in the overview papers
Some pictures are provided here :
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 (CT system related and others) of the HEGRA collaboration.
See also the following annual reports of the MPI:
The CT group has several interesting topics for Diploma and PhD students.
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
Comments and additions are welcome, please send me mail.