on behalf of the MAGIC collaboration

Technical Solutions for the MAGIC Telescope

M. DORO

and E. LORENZ

on behalf of the MAGIC collaboration

a

University of Padova & INFN, Padova, Italy

b

Max Planck Institute for Physics, Munich, Germany

a

michele.doro@pd.infn.it,

e.lorenz@mac.com

∗

http://wwwmagic.mppmu.mpg.de

The atmospheric Cherenkov telescope MAGIC for ground-based gamma-ray astronomy is operating since late 2003 on the Canarian island La Palma. The telescope’s 17 m diameter mirror is composed of 964 light weight, square all- aluminum mirrors of 234 m

total area, which allows to lower the energy thresh- old to a value far beyond that of past generation of telescopes. The trigger sys- tem, based on ultra-fast decision logics (topological, time-coincidences, etc.) is designed to cope with very fast PMT signals (2 ns). Details of the MAGIC telescope will be presented, technical problems and solutions will be discussed as well as a report on three years of operations will be given.

Keywords: Style file; L

TEX; Proceedings; World Scientific Publishing.

1. The MAGIC Telescope

The MAGIC telescope is designed and operated by a collaboration of 150 physicists from 22 institutes from 7 countries. The telescope is fully oper- ational since fall 2003 and is currently in its third observations cycle. The MAGIC telescope belongs to the Imaging Atmospheric Cherenkov Tele- scope (IACT) class of detectors and is currently the one with the world- largest mirror dish. IACTs allow to study from ground the Very High En- ergy (VHE) gamma–ray emission coming from cosmic sources of various classes, i.e. Supernova Remnants (SNR), pulsars, Active Galactic Nuclei (AGN), etc. Gamma-rays, as well as the many orders of magnitude more frequent charged cosmic rays, hitting the Earth atmospherte initiate at- mospheric particle showers with a typical shower maximum around 8-12 km asl. The charged shower particles, mostly electrons and positrons, emit Cherenkov light in case their speed exceeds that of light in the atmosphere.

The Cherenkov light illuminates nearly uniformly a dish of ∼ 120 m radius.

If a telescope is located inside this Cherenkov-light pool, the light hitting

the mirror can then be reflected and focused onto a multi-pixel camera composed of a large number of photomultipliers. An image reconstruction algorithm allows the determination of major parameters of the primary par- ticles, such as energy and direction, as well as its likeliness to be a hadron or a gamma-ray. As lower energy gamma-rays produce a lower density of photons at ground, a larger telescope mirror area is required to decrease the energy threshold of the telescope. In case of MAGIC, the 17 m reflector allows the reconstruction of primary gamma-rays above ∼ 70 GeV (zenith angle dependent) with a sensitivity of 10

ph cm

s

in 50 hours of ob- servation and with an effective area, which is of the order of 10

m

(slightly depending on the energy).

Fig. 1. The MAGIC telescope. La Palma (Spain) (28.75 N, 17.89 W)

2. Technical Solutions

MAGIC is located on the Canary island of La Palma (28.75 N, 17.89 W) at 2225 m asl. The observation conditions on site are amongst the best in the world, even if occasional strong winds, winter snowfalls and high humidity demand anyhow a strong technical effort to prevent damaging and ageing as the MAGIC telescope is too large to be protected by a dome.

The telescope construction incorporates a number of “firsts” used in IACTS:

a) It has the worldwide largest mirror dish. b) It is the first time that a

lightweight mirror dish has been constructed from carbon fiber reinforced

plastics (CFRP). c) First use of diamond turned light weight all-aluminum

sandwich mirrors. d) First use of an active mirror control to counteract

small deformations of the 17 m φ mirror during positioning and tracking,

e) First use of low gain hemispherical PMTs with diffuse lacquer coating

and special light catchers resulting in increased quantum efficiency and allowing prolonged operation during modest moon shine. f ) First use of the transmission of the fast analog PMT signals over 160 m by means of optical fibers to a distant counting house. g) First use of a 2 GHz F-ADC digitization of the fast PMT pulses.

In the following, an overview on the technical solutions adopted for each subsystem is reported and discussed. A detailed design report of the initial design ideas can be found in Barrio et al.

2.1. The Mounting and Drive System

The reflector frame satisifies at the same time three main demands: it is very large, it is light-weight and it allows for fast repositioning. The space frame is made from carbon fiber reinforced plastic (CFRP) tubes and has a weight of only 5.5 tons. The CFRP construction is about three times stiffer and has a less than a third of weight of an equivalent steel construction. The structure of an alt-azimuth design is mounted on a circular rail of 19 m φ.

The telescope can be moved from − 80

to 105

in declination and 450

in azimuth. The camera at around 17 m distance from the reflector is carried by a single aluminum tubular arc. The weight of the camera is around half a ton, and the small bending, unavoidable during the telescope tracking, is corrected via a re-orientation of the mirror. Two motors control the motion in azimuth and one motor the zenithal motion, with a maximum power consumption of ∼ 7 kW per motor. The angular positions are controlled by absolute shaft-encoders of 14-bit precision/360

. In addition, a starguider camera, mounted at the centre (offset by 1 m) of the reflector, monitors the positioning of the telescope by viewing both the camera of the telescope and the corresponding section of the sky star-field.

The lightweight structure allows for very fast repositioning of the telescope to any position on the sky within ∼ 30 s. This challenging feature was designed to instantly react to Gamma-Ray Burst (GRB) alerts from dedicated satellites detecting GRBs in the KeV/MeV domain.

2.2. The Reflector and Details on its Construction

The 17 m φ reflector (17 m focal distance) follows a parabolic profile which was chosen to maintain the temporal structure of the shower light flashes.

The reflector of MAGIC I is tessellated and comprises 956 mirrors of a

total area of 234 m

. Each mirror is a square of 0.495 m side length and has

a spherical profile whose radius of curvature is optimized for the position

in the telescope to best approximate the paraboloid. MAGIC I mirrors are grouped onto panels of 4 or 3 elements and each panel can be moved by the Active Mirror Control system (AMC).

The AMC was designed to correct small deformations of the mirror support dish during telescope positioning and tracking. The mirrors are an all-aluminum, light weight sandwich construction composed of an Al-skin and an Al-box and filled with a Hexcell honeycomb structure.

A heating wire mesh, embedded in the sandwich, can be switched on in case of dew or ice deposits on the mirrors.

The total power consumption for heating the entire reflector is 40 KW. The reflecting aluminum surface of the mirror elements is diamond turned using the so-called fly-cutter technique, which provides an average roughness of 4 nm and a mean reflectivity of 85%. The surface of the mirrors was coated by a thin layer of quartz (with some admixture of carbon) for protection against corrosion and acid rain. Very little degradation (< 3%/year) of the reflectivity was observed after 4 years exposure the atmosphere at La Palma. The well-adjusted mirror has, for an infinite point-like source, a point spread function (PSF) of the reflected spot of ∼ 10 mm φ at the camera of the telescope. New solutions will be adopted for MAGIC II, whose outermost rings of mirrors will be made of recently designed glass mirrors, produced by a cold-slumping technique. The central part of the reflector will be equipped by a modified version of the MAGIC I diamond-turned mirrors but of increased dimension (1m

area), improved water-tightness and lower weight. Also the surface quality of these new Al-mirrors is enhanced and should result in a PSF of 5 mm.

2.3. Camera and read-out

The camera of the MAGIC telescope of a 3.6

field of view (FOV) comprises

576 photomultipliers with 396 1

PMTs from Electron Tubes Ltd of type

9116 covering the inner section (up to 1.2

radius) and 180 PMTs of type

9117 and 1 1/2

for the outer part of the camera. The PMTs of a hemi-

spherical cathode were enhanced in quantum efficiency (QE) by a diffuse

lacquer doped with P-Terphenyl shifting the short wave UV component of

the Cherenkov light into the spectral range of maximum sensitivity. In or-

der to minimize losses due to dead space between the densely packed PMTs

we used hex-to-round light concentrators, which added a further increase

of the QE by deflecting many photons such that their trajectory passed

the semitransparent photocathode twice. The mean QE of the entire sys-

tem (mirror, camera window, cone, effective PMT QE) between 300 and

600 nm was 15%. The 6 dynode PMTs were operated at a gain of ∼ 3 · 10

in order to allow also observations at modest moon shine.

Finally, the fast PMT signals are converted back into optical signals by means of large dynamic Vcsels operating in the analog mode and routed via 160 m long optical fibers to the 80 m away counting house. At the counting house the optical signals are converted back into electronic signals, split and routed to the trigger branch and to the fast digitization and recording system.

The trigger is two-fold: in level–0 the PMT analog signals are converted into logical signals once the signals exceed a 5-8 overlapping photoelectrons threshold. Level–1 is dedicated to the time-coincidence topological recon- struction of the signal in the camera: when a number of pixels (adjustable by software) are activated at the same time according to the next-neighbor logic (2NN–6NN) a level-1 trigger signal is generated. The typical trigger rate is about 150-250 Hz interleaved by frequent calibration events. Upon a trigger the data received by the telescope camera are digitized by ultra-fast Flash-ADCs of 2 GHz sampling rate. This system has replaced since a year the original digitizing system of only 300 MHz sampling rate and allows a much better timing measurement and a considerable reduction of the night sky light background.

3. Future Prospects

The IACT technique is already well-established. The so-called III gener- ations of IACTs will likely start observation around 2012-2015 (CTA in Europe and AGIS in USA). These new instruments will have a factor 10 increase in sensitivity by using a large array of IACTs of both large and small mirrors to allow observations on both the high energy and the low en- ergy domain of VHE (hundred of TeV and few GeV respectively). Chances for additional improvements are promizing as new technologies are emerg- ing such as new high quantum efficiency photon detectors, low power GHz digitizers for each pixel, higher level triggers and better mirror optics.

References

1. J. Albert et al., astro-ph/0705.3244 2. Barrio et al., MAGIC Design report, 1998 3. R. Mirzoyan et al., ApJ. 27:509-511,2007.

4. D. Bastieri et al., astro-ph/0709.1380

5. A. Biland et al., Procs. ICRC 2007, astro-ph/0709.1574.

6. M. Doro, Procs. 6th RICH, Trieste, 2007 7. D. Paneque, NIM A518:619-621, 2004.

8. J. Albert et al., arXiv:astro-ph/0702475.