||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
- 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
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
technology side, MAGIC innovates in several key aspects, most likely
preparing the ground for future experiments. The following is a list of
- 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.
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
background, are foreseen, e.g. by faster sampling and intelligent
- 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
||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
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.