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Technical implementation

Technical solutions chosen for MAGIC

Many brand new technologies were employed for the MAGIC telescopes, including some major innovations in astroparticle physics: detectors that use techniques taken from accelerator experiments; fast electronics and automatic controls to economically operate devices of astounding performance and complexity; computers and networks with sufficient capacity to record and reconstruct large volumes of data and find their interrelations.

The most critical parameters of the MAGIC telescopes (more details are given on the subpages about the mirrors, mounts and cameras) are the following:

  • Active mirror surface of 236 sq.m., made of square elements 49.5x49.5cm or 99x99cm; f/D = 1.03;
  • Support frame of carbon fibre made for minimum weight and maximum stiffness;
  • Approximately hexagonal camera of 1.05 m diameter, with 1039 PMTs of 1" (or 0.1 degree) diameter each; all PMTs have an effective quantum efficiency of 25 to 35%, depending on wavelength;The camera is kept as light as possible, held by an aluminium support arc, stiffened by a web of thin steel cables;
  • The maximum repositioning speed is more than 7 degrees per second, meaning the telescopes can be pointed to any point on the observable sky in less than 25 seconds (due to a weight of only around 60 tons);
  • Analog signals are transmitted from the camera to the counting house via optical fibres; only the amplifiers and laser diode modulators for transmission are inside the camera housing;
  • Digitization is achieved by the Domino Ring Sampler (DRS4) chip with a sampling frequency of 1.64 GHz, to make use of the timing information in the pulse;
  • The lower threshold for gamma-ray detection is around 50 GeV.
 

Technological innovation in MAGIC

MAGIC-I was innovative in several key technological aspects, preparing the ground for future experiments. The following is a list of features of the first MAGIC telescope. Some were further improved upon by first adding MAGIC-II in 2009, and with a major hardware upgrade in 2012.

  • At the time of construction, MAGIC had the largest collection surface of any existing or projected gamma-ray telescope world wide. To this date MAGIC is the the largest pair of Cherenkov telescopes;
  • An assembly of 974 individual mirror elements for MAGIC-I, and 246 panels for MAGIC-II, resulting in two parabolic dishes with 17 meter diameter. The diamond-grinding and polishing of the individual aluminium mirrors and their mounting on a light-weight carbon fiber structure are technological challenges not solved at this level before MAGIC;
  • Elaborate computer controlled mechanisms to maintain the individual mirror elements in their optimal place to collect the maximum possible amount of Cherenkov light, counteracting effects of mechanical distortion due to gravity and weather;
  • A very fast (7 degrees per second) repositioning of the telescope is an important design parameter achieved by minimizing the device weight;
  • A high resolution camera, initially 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. A major upgrade of the camera using next generation photo detectors further improved the performance;
  • The detailed time analysis of the camera output is another key element. This is achieved by permanent digital sampling of the photomultiplier signal. Initially the sampling speed was 300 MHz using FADCs. After the upgrade the speed was increased, currently the signals are sampled at 1.64 GHz;
  • MAGIC was innovative 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 for applications in medical imaging and high-energy physics.
 

Why MAGIC has an edge over other Cherenkov telescopes

In this section some key aspects in designing the MAGIC telescopes which make it a superior instrument for VHE gamma ray physics, particularly at lower energies, are listed:

  • MAGIC was, at the time of its construction, the telescope with the best light collection, it had the largest mirror with an active surface of 236 square meters, combined with the best available photomultiplier tubes with a quantum efficiency around 30%. As a result, MAGIC is more sensitive to electromagnetic showers of lower energy. MAGIC is able to nearly close the gap towards satellite-borne detectors that can measure gamma-rays up to some 10 GeV energy. MAGIC has a threshold trigger energy of around 50 GeV, and an analysis threshold of around 70 GeV at small zenith angles. This also permitted MAGIC to observe sources with higher red shift than ever before.
  • MAGIC is constructed to maximize the repositioning speed in order to quickly react to alerts for transient events like Gamma Ray Bursts. Such alerts are broadcast by satellite experiments seconds after observing a signal, and MAGIC is able to react to them within a very short delay of around 30 seconds. This includes redirecting the telescope axis and reloading software and trigger tables.
  • MAGIC initially was a single telescope, but was later extended by a second, making it the world's largest pair of Cherenkov telescopes. This stereo observation of multiple telescopes taking data synchronously on the same source increases the sensitivity due to the added surface. Additionally, the determination of the shower impact point is improved;
  • With the highest priority given to light collection and therefore lowering the energy threshold, MAGIC was able to probe earlier parts of the universe than other experiments.