The H.E.S.S. Telescopes

For an overview of the H.E.S.S. I telescopes, see, e.g., the ICRC 2001 proceedings. See also the chronology of the construction of H.E.S.S. I (1994-2004) and images from the construction of the H.E.S.S. I telescopes.

The Cherenkov technique

event sequence
Short (300 kB) and long (4 MB) movies (in animated GIF format) showing Cherenkov images recorded with the first H.E.S.S. telescope in 2002. One finds the typical elongated shower images as well as muon "rings" generated when a air-shower particle reaches the ground and hits the telescope

The detection of high energy gamma rays with the H.E.S.S. telescopes is based on the imaging air Cherenkov technique. 

Arrangement of the telescopes

H.E.S.S. is a stereoscopic telescope system, where multiple telescopes view the same air shower.
array

Mount and dish

mirror

In the design of the H.E.S.S. telescopes, emphasis was placed on the mechanical stability and rigidity of the mount and dish.

The 12 m H.E.S.S. I telescopes

Detailed information about the telescope construction, the mirrors and their optical characteristics can be found in the publications:

The 28 m H.E.S.S. II telescope

The design of the the H.E.S.S. II telescope structure follows the same guiding principles as for the 12 m telescopes: an alt-az mounted dish with high intrinsic rigidity. The basic parameters of the telescope are

The camera is supported by a quadrupod. The quadrupod connections to the dish and the support hubs of the dish with the elevation bearings illustrate the size of the structure.

The weight of the complete H.E.S.S. II telescope amounts to 580 tons.

Mirror

mirror

The mirror focuses the Cherenkov light of an air shower onto the camera. Relevant for the performance of a telescope are the net mirror area and the quality of the image, i.e. the point spread function (size of the image of a point source).

The 12 m H.E.S.S. I mirrors

Detailed information the telescope mirror, its alignment and optical characteristics can be found in the publications The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes, Part I: layout and components of the system (1.8 MB) and Part II: mirror alignment and point spread function (2.0 MB).

The 28 m H.E.S.S. II mirror

The H.E.S.S. II telescope uses a parabolic mirror shape to minimize time dispersion. The parabolic shape is approximated by a grid of 5 x 5 planar mirror support segments, aligned to approach a parabola. Hexagonal rather than round facets optimize coverage. Facets are also larger than for the H.E.S.S. I telescopes, with about 2.5 times larger area per facet. Mirror parameters are The same alignment technique as for H.E.S.S. I is used. Shown here is a section of the mirror before alignment of the facets.

Camera

The cameras of the H.E.S.S. telescopes serve to capture and record the Cherenkov images of air showers. Design criteria included a small pixel size to resolve image details, a large field of view to allow observations of extended sources and surveys, and a triggering scheme which allows to identify the brief and compact Cherenkov images and to reject backgrounds, such as the light of the night sky. The complete electronics for image digitization, readout and triggering is integrated into the camera body.

Cameras of the 12 m telescopes

mirror
Key features of the cameras include:

More details about the camera are given, e.g., in the ICRC 2001 proceedings. The processing and calibration of the camera data is described here.

The camera of the 28 m telescope

HESS II Camera
In its concept, the H.E.S.S. II camera follows the design of the H.E.S.S. I cameras: Photomultiplier tubes are group into 16-PMT drawers which also contain the electronics for signal storage, signal digitization, triggering and readout. However, the H.E.S.S. II camera is much larger - it contains 2048 PMTs in 128 drawers - and virtually every detail is improved and the H.E.S.S. II drawers were largely redesigned. The photomultiplier pixels of the camera have the same physical size, but due to the larger focal length shower images are much better resolved. A next-generation analog ring sampling ASIC is employed, and a new digitization scheme and faster readout bus allow a ten-fold increase in image recording rate. Camera parameters are: A special mechanism allows semi-automatic unloading on the sensitive camera in periods of bad weather, or for mounting a second camera module for special measurements.

Central trigger system

H.E.S.S. employs the stereoscopic reconstruction of air showers to determine their direction in space, the type and the energy of the primary particle. Therefore, only air showers which generate images in at least two telescopes are recorded. This requirement reduces the load on the DAQ system, reduces the read-out dead time and allows the trigger thresholds and energy thresholds to be lowered. The central trigger system receives trigger signals from the individual telescopes and searches for coincidences between telescopes, properly accounting for the delays of the signals from the different telescopes, and their dependence on telescope pointing. Coincident triggers result in the read-out of telescope data; for non-coincident triggers, the telescope readout electronics is cleared after a few microseconds and is ready for the next event.

The central trigger system is described in the publication The trigger system of the H.E.S.S. telescope array.

To include the 28 m H.E.S.S. II telescope, the central trigger system was upgraded, it now allows triggering on arbitrary combinations of the five telescopes. The usual operating mode is that either a concidence of any two of the five telescopes will trigger image readout, but that also air showers seen only in the 28 m telescope will be recorded, to provided minimal energy threshold.

Data acquisition

The data acquisition system (DAQ) serves to collect data from the telescopes and monitoring instruments on site. Once collected, it is processed by the DAQ and a real time analysis is performed. Data is stored locally on RAID servers and tapes are used for distribution to Europe; however, a small fraction of monitoring data is transmitted using the Internet.

Telescope monitoring

Permanent monitoring of the performance of the telescopes is crucial to achieve optimum data quality. Currents and counting rates of the camera photon detectors are continuously recorded, as are the temperatures in all parts of the camera. Additional monitoring instruments include

Atmospheric monitoring

Atmospheric parameters and optical transmission of the atmosphere need to be known in order to relate the measured Cherenkov light yield and the energy of the incident particle. Instruments used in H.E.S.S. to probe the atmosphere will include

W. Hofmann, July 2012