The H.E.S.S. Telescopes

The Cherenkov technique
Arrangement of the telescopes
Mount and dish
Mirror
Camera
Central trigger system
Data acquisition
Telescope monitoring
Atmospheric monitoring

For an overview of the H.E.S.S. telescopes, see, e.g., the ICRC 2001 proceedings. See also the chronology of the construction of H.E.S.S. including pictures, and the a collection of high-resolution images of the experiment.

The Cherenkov technique

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.

An incident high-energy gamma ray interacts high up in the atmosphere and generates an air shower of secondary particles. The number of shower particles reaches a maximum at about 10 km height, and the shower dies out deeper in the atmosphere. Since the shower particles move at essentially the speed of light, they emit Cherenkov light, a faint blue light.
The Cherenkov light is beamed around the direction of the incident primary particle and illuminates on the ground an area of about 250 m diameter, often referred to as the Cherenkov light pool. For a primary photon at TeV energy (10¹² eV), only about 100 photons per m² are seen on the ground. They arrive within a very short time interval, a few nanoseconds.
A telescope located somewhere within the light pool will "see" the air shower, provided that its mirror area is large enough to collect enough photons. The "effective detection area" of a Cherenkov telescope is therefore given roughly by the area of the Cherenkov light pool, about 50000 m², to be compared with the sub-m² detection area of satellite instruments aiming to detect gamma rays before they interact with the atmosphere.
The image obtained with the telescope shows the track of the air shower, which points back to the celestial object where there incident gamma ray originated. The intensity of the image is related to the energy of the gamma ray. The shape of the image can be used to reject unwanted "background", such as showers induced by cosmic ray particles.
With a single telescope providing a single view of a shower, it is difficult to reconstruct the exact geometry of the air shower in space. The achieve this, multiple telescopes are used which view the shower from different points and allow a stereoscopic reconstruction of the shower geometry.

Arrangement of the telescopes

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

The initial four H.E.S.S. telescopes (Phase I) are arranged in form of a square with 120 m side length, to provide multiple stereoscopic views of air showers. The telescope spacing represents a compromise between the large base length required for good stereoscopic viewing of the showers, and the requirement that two or more telescopes are hit by light generated by a shower. Showers emit their Cherenkov light at a height of about 10 km, and at a corresponding distance from the telescopes; hence even the 120 m spacing results in rather small angles between different views. On the other hand, given the 250 m diameter of the Cherenkov light pool, a larger spacing would make it increasingly unlikely that multiple telescopes are illuminated simultaneously.
The diagonal of the square is oriented north-south.
In Phase II of the project, an single huge dish with about 600 m² mirror area will be added at the center of the array, increasing the energy coverage, sensitivity and angular resolution of the instrument.

Mount and dish

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 telescopes (here before mirror installation, side view, front view) use an alt-azimuth mount, to point the telescope at any point in the sky. A "base frame" rotates around a vertical axis and carries the dish, which rotates around the elevation axis. Both the base frame and the dish are realized as steel space frames. Technical drawings show a telescope from the front and back. Both axes are driven under computer control to track a celestial object across the sky.
The drive system combines for each axis a servo-controlled AC motor and a backup battery-driven DC motor. In order to reduce the drive forces, the drives act on circular rails of about 7 m radius. The maximum speed of the drive systems is about 100^o/min, in order to allow rapid slewing from one object to another. The servo controllers and batteries for the DC drive are located in a small hut on the base frame.
The pointing is controled by angular position encoders connected to both axes, which provide a resolution of a few arc-seconds (17 bit digitial readout plus an additional analog track). Telescope pointing is in addition monitored by an optical guide telescope (f=800 mm) equipped with a CCD camera, which serves to correct deviations from perfect pointing.
Weights: complete telescope about 60 t incl. camera, drive systems, mirrors.

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

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).

For cost reasons, the mirror is segmented into 382 round mirror facets of 60 cm diameter, made of aluminized glass with a quartz coating.
The mirror has a focal length of 15 m and a d/f ratio of 0.8; the mirror facets are arranged in a Davies-Cotton design (on a sphere of radius f), which provides good imaging also for off-axs rays.
The total mirror area is 108 m² per telescope.
Mirror reflectivity is >80% (300 to 600 nm). Each mirror tile is individually tested for reflectivity and image quality before it is mounted.
The orientation of each facet is adjustable by two remote-controlled motors. To align the mirror facets, the image of a star in the focal plane is viewed by a CCD camera in the center of the dish. Before alignment of the facet, each facet generates a light spot. One mirror at a time, the individual mirrors are then moved until all spots converge in the center. The procedure is fully automatic; the initial alignment requires a few nights. The effect of the alignment is visualized by comparing the image of a star before and after alignment. Alignment of mirror will be check regularly; when required, a re-alignment can proceed in a few hours.
The mirrors of H.E.S.S. telescopes are focused for an object distance of about 10 km, corresponding to the typical distance of an air shower from the telescopes.
The specified point spread function including alignment errors is 0.03^o (rms) on axis (0.5 mrad), 0.06^o (1 mrad) for rays 2^o off axis. The measured point spread functions exceed. Over most of the field of view, the spot is well contained within a single pixel (indicated in the figure by its hexagonal outline). Consistent with raytracing simulation, the spot width increases with increasing angle to the optical axis. Caused by gravity-induced deformations of the dish, the point spread function varies slightly with elevation; the degree of variation is as predicted by FEM simulations and is uncritical.

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).

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. Key features of the camera include:

A 5^o field of view.
960 photon detector elements ("pixels", see front face of the camera), each subtending 0.16^o angle, using 29 mm, 8-stage photomultiplier tubes (PMTs) with borosilicate windows, equipped with Winston cones to improve light collection, operated at a gain of 2x10⁵. Operating voltages for the PMTs are supplied by DC-DC converters integrated into each PMT base, with active stabilization for the last four dynodes for best linearity. A PMT with its base.
Modular construction using 60 "drawers" which slide into the camera body (Exploded view of the camera body, cut-away few of the camera body, front view of the actual camera body, view into the rear section). Each drawer contains 16 PMTs and the associated electronics. The rear part of the camera houses power supplies and crates with interfaces to the digital readout bus, with a CPU and with the trigger processors.
PMT signals are captured using dual-range 1 GHz Analog Ring Sampling ASICs (ARS) which sample the signal every nanosecond and which record the last 128 ns of signal history. The signal processing provides a dynamic range of 0.1 to more than 2000 photoelectrons. Here a trace of a signal sampled by the ARS, and the pulse-height distribution measured for single photoelectrons, illustrating the resolution and noise performance.
The camera is triggered by a coincidence of signals detected in 3 to 5 pixels in (overlapping) 8x8 pixel sectors; typical signals required in a pixel ("pixel trigger thresholds") are around 5 photoelectrons. The fast coincidence circuitry provides an effective coincidence window of about 1.5 ns, and allows for the efficient rejection of uncorrelated PMT signals caused by photons of the night sky background light.
Once a camera triggers on a shower image, it alerts a central trigger station. If two or more telescope trigger simultaneously, providing stereo images of an air shower, a trigger confirmation is sent back to the telescope, and the analog signals stored in the ARS are digitized, processed, and read out via a readout bus into the local processor of the camera.
Monitoring circuitry inside each drawer provides information of PMT currents, PMT trigger rates, supply voltages and temperatures.
Air cooling is used to remove close to 5 kW of heat dissipated in the circuitry of the camera. Air flow is provided by about 80 computer-controlled fans.
Camera dimensions: about 1.6 m diameter, 1.5 m length; weight about 800 kg.

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.

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.

Data acquisition

The data acquisition system (DAQ) serves to collect and combine data from the telescopes and the monitoring instruments, and to perform a first analysis. Data will be stored to tape and distributed; a small fraction of monitoring data will be transmitted using the internet.

Data from the different telescopes are sent via 100 Mb Ethernet and a commercial switch to a farm of processors.
The processor farm currently encompassed 16 dual-Pentium 800 MHz processors with local disks. On these processors, data from the different telescopes are combined to complete events, and event data and monitoring information are analyzed and stored on the local disks.
Special tape server nodes collect data from the disks of the farm nodes and assemble a single data stream.
The DAQ software is written in C++, uses CORBA for interprocess communication and ROOT as an analysis framework.

An overview of the DAQ system can be found in the ICRC 2001 proceedings.

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

An "active camera lid" which contains an LED light pulser for each phototube, and allows to simulate arbitrary image patterns.
A laser light pulser in the center of the dish, which illuminates the camera uniformly and is used to flat-field the camera.
A "sky CCD" with an f=800 mm telescope, mounted parallel to the optical axis on the telescope and employed to correct telescope pointing using stars.
A "lid CCD" mounted in the center of the dish and viewing the camera, used to align mirrors and to monitor the point spread function by viewing the images of stars on the white camera lid, and to monitor deformations of the camera masts under gravity by viewing reference LEDs on the camera body.
A reflectometer continuously measuring the reflectivity of one of the telescope mirrors.

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

Infrared radiometers on each telescope to measure the effective sky temperature in the field of view of the telescope. Clouds in the field of view manifest themselves through an increased sky temperature.
A scanning infrared radiometer to survey the entire sky every few minutes.
A "ceilometer", an active cloud sensor scanning the sky with a laser beam and detecting light backscattered by clouds and aerosols.
An optical telescope measuring atmospheric transmission using stars.
A weather station.

W. Hofmann, August 2004

H.E.S.S.

High Energy Stereoscopic System