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Technical details

Technical solutions chosen for MAGIC

Many technologies employed in MAGIC were simply not available a generation ago, and some are clear innovations in astroparticle physics: detectors use the know-how of techniques taken from accelerator experiments; fast electronics and automatic control allow to economically build devices of astounding performance and complexity; computers and networks provide sufficient capacity to record and reconstruct large volumes of data and find their interrelations.
The most critical parameters of the MAGIC telescope (more details are given in the Technical fact sheet) are the following:

  • Active mirror surface 236 sq.m., made of square elements 49.5cm x 49.5cm; f/D = 1.03;
  • Support frame of carbon fibre made for minimum weight and maximum stiffness;
  • Hexagonal camera of 1.05 m diameter, with an inner area of 396 PMTs of 1" diameter (ET 9116A) each, surrounded by 180 PMTs of 1.5" diameter (ET 9117A), arranged in four concentric rings. All tubes have an effective quantum efficiency (QE) of 25 to 30 %;
  • The camera is kept as light as possible, held by an aluminium support stiffened by a web of thin cables;
  • Analogue signals are transmitted from the camera to the control house via optical fibres; only the amplifiers and laser diode modulators for transmission are inside the camera housing. Digitization was achieved initially by 300 MHz FADCs, new FADCs with a sampling frequency of 2 GHz have been in use since February 2007;
  • The threshold for gamma detection is around 60-70 GeV with classical PMTs; future high-QE red-extended PMTs are expected to achieve a lower threshold.
  • The average time to reposition the MAGIC telescope anywhere on the observable sky is 40 seconds (despite a moving weight of ~60 tons);

Technological innovation in MAGIC

On the technology side, MAGIC innovates in several key aspects, most likely preparing the ground for future experiments. The following is a list of innovative features:

  • MAGIC is characterized by the largest collection surface of any existing or projected gamma-ray telescope world-wide, an assembly of nearly 1000 individual mirrors, together resulting in a parabolic dish with 17 m diameter; the diamond-grinding and polishing of the individual aluminium mirrors and their mounting (in altitude/azimuth controlled position) on a light-weight carbon fiber structure are technological challenges not solved at this level before.

  • Elaborate computer controlled control mechanisms are needed to maintain the individual mirror elements in their optimal place and collect all possible photon quanta, counteracting effects of mechanical distortion by gravity, atmosphere, weather, and cleaning observations of any background light. All these effects are detrimental to high-resolution measurements. The individual mirrors also carry a heating loop to avoid inefficiencies occurring due to rain, snow or simply dew.

  • A very fast (average time 40 seconds) repositioning of the telescope axis is an important design parameter; this is achieved by minimizing the device weight and automating axis control. Repositioning in a matter of seconds is important when short-lived phenomena are signaled by other active devices, e.g. by satellite-based wide-angle detectors in the X-ray band, in particular the enigmatic gamma ray bursts, whose understanding is hoped to contribute to understanding current cosmological models.

  • The high-resolution 'camera' of MAGIC is composed of 576 ultra-sensitive photomultipliers; their development, jointly with industry, was crucial to the success of the experiment. Both wide-band response and quantum efficiency have been pushed to or beyond existing limits, and an improvement program for a phase-2 camera is already under way. The present photomultipliers have been developed by a company in the UK. Developments resulting in higher light yield are under way.

  • The detailed time analysis of the camera output is another key element. This is achieved by permanent digital sampling of the photomultiplier signal, presently at a rate of 300 MHz, in a FADC developed by one of the collaborating partners (Univ. of Siegen), Also in this domain, future improvements, needed for optimal suppression of the trivial cosmic ray background, are foreseen, e.g. by faster sampling and intelligent signal processing.

  • MAGIC is also innovating in the area of data transmission: the analogue signals pass through optical fibers, developed by industry; the readout chain uses, for economical reasons, standard parallel high-performance computers, with interfaces and driver software developed (by a company in Germany) for applications in medical imaging and high-energy physics.

Why MAGIC has an edge over other Cherenkov telescopes

A list of existing and planned Cherenkov telescopes for VHE gamma observations, with pointers to their home pages, is given on a links section of these pages.

Here are some points given much weight in designing the MAGIC telescope project, and which make it a superior instrument for VHE gamma ray physics, particularly at lower energies:

  • MAGIC has the best light collection that has been attempted so far: the largest mirror with an active surface of 234 sq.meters, combined with the best available photomultiplier tubes that can be obtained, of a quantum efficiency around 30%. As a result, MAGIC is more sensitive to electromagnetic showers of lower energy, and does much to close the gap existing between satellite gamma ray detectors (that can go up to some 10 GeV energy) and Cherenkov telescopes (that presently start at >100 GeV). MAGIC-I has a threshold trigger energy of ~50 GeV, and an analysis threshold of ~70 GeV at small zenith angle, which also permits to observe sources with higher redshift than in the past.
  • For the first time, the MAGIC telescope is constructed with a quick reaction to Gamma Ray Burst alarms in mind. Such alarms are broadcast by satellite experiments seconds after observing a signal, and MAGIC is be able to react to them within a short delay. This includes redirecting the telescope axis and reloading software and trigger tables.
  • MAGIC initially is a single telescope. Several experiments in operation (e.g. HESS, VERITAS) put much emphasis on the 'stereo' effect of multiple telescopes operating synchronously on the same source. Although this issue is far from settled, particularly at lower energies, there are clear benefits from stereo observation beyond the added sensitivity due to the added surface. The stereo effect clearly improves the determination of the shower impact point, but this is less obvious for the more critical shower energy and direction. Improved position of impact helps mostly at the largest observed energies, where the energy changes rapidly with the distance from the shower axis (see graph: lateral energy density for gammas, at different energies).

    The MAGIC collaboration has opted, therefore, to put its main effort and resources into improving light collection and hence obtain a lower energy threshold; however, the collaboration has also decided to install a second MAGIC type telescope, albeit without the constraint of running both telescopes on the same source.

  • With this priority given to light collection, MAGIC will also be able to probe earlier parts of the universe than other experiments, existing or planned. This can be shown graphically (click on the thumbnail image):

Like most astrophysics experiments, MAGIC has been constructed and is being operated by an international collaboration. The leading institutes for the existing telescope, MAGIC I, were (in alphabetic order) Barcelona, München and Padova; the full list of the present collaboration can be found in the list of collaborators; more bread and butter information on the collaboration through the navigation panel on the left: mail addresses, publications, and also the MAGIC picture gallery for more visual information.

This page was created by Rudolf Bock. Last modification 20.07.2008 by Robert Wagner.
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