THE MAGIC TELESCOPE, STATUS AND FUTURE PLANS

THE MAGIC TELESCOPE, STATUS AND FUTURE PLANS

Eckart Lorenz (for the MAGIC Collaboration)

Max Planck Institute for Physics, Foehringer Ring 6, D80805 Munich, Germany and the ETH Zuerich, Hoenggerberg CH8093 Zuerich, Switzerland

Abstract. The status of the 17 m diameter imaging air Cherenkov telescope MAGIC will be presented. The telescope is in the final phase of commissioning and first (test-) data have been recorded. Also, the next steps for improving the telescope and its integration into a future multi- telescope Cherenkov observatory will be discussed.

INTRODUCTION

Currently, a number of high performance, imaging air Cherenkov telescopes (IACT) for ground-based gamma (γ) astronomy are put into operation, amongst them the 17 m diameter MAGIC telescope, figure 1. This telescope has been built by an international collaboration formed by 14 institutions from 7 countries.

SOME GLOBAL DESIGN CONSIDERATIONS

The basic design considerations for the new telescope were:

- To build a telescope that has a threshold as close as possible to the upper energy limit of EGRET in order to close nearly completely the unexplored energy gap between 10 and 300 GeV

- To develop new technical solutions to reach the goal of a very low threshold and to further increase the sensitivity beyond that of contemporary IACTs but with the priority for a lower threshold.

- To build a telescope that can be very rapidly repositioned in case of a GRB alert.

Also, the telescope should allow one to track sources up to the horizon,

respectively look down to the sea (a special condition for the La Palma site)

It was decided to bundle all the resources and build at first only a single telescope

and not follow the common concept of a multi-telescope system, which could not be

justified when using many promising but unproven technologies. Only after the

successful demonstration of the novel ideas the next steps towards a multi-telescope

observatory will be taken.

FIGURE 1. Photograph of the MAGIC telescope with the completed mirror, August 2004.

KEY PARAMETERS AND SOME TECHNICAL PERFORMANCE DATA OF MAGIC

The MAGIC design follows very closely the concept outlined in the design report [1]. The main design parameters of the 17 m diameter telescope are:

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A mirror of 241 m

surface with a parabolic (isochronous) profile and an f/D of 1. The main mirror is composed of 964 individual elements (light weight all-aluminium elements with diamond turned reflective surface and quartz coated protection layer).

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A carbon fiber-Epoxy tubular space frame of low thermal expansion and low weight. The telescope has an alt.-azimuth mounting.

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An active mirror control to counteract the residual deformation of the space frame and to adapt the mirror to different focusing requirements.

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A fine pixelised camera of 3.6° FOV (inner section with 396 pixels of 0.1°

diameter and an outer ring of 180 pixels of 0.2° diameter). Photosensors are 6 dynode hemispherical PMTs followed by a fast transimpedance preamplifier. Specially shaped light collectors result in close to 100% light collection and improved QE [2].

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Fast analog signal transmission by optical cables to the central counting house and signal digitization by 300 MHz FADCs.

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Use of a multilevel trigger and a maximal event readout rate of 1 kHz.

The telescope is installed on the Island La Palma (28.8°N, 17.8° W, 2225 m asl).

The price of the telescope including the counting house and some infrastructure is ≈ 4.5 M €. Construction started in 2001. Apart from some staging (15 % of the main mirror) the telescope was completed late 2003.

All the novel concepts and components such as the carbon fiber space frame, the

new mirror construction, the active mirror control (AMC), the improved

photondetectors (PMT), the optical analog signal transmission, the trigger concept and

the use of FADCs for the signal digitization, contributed to the improvement of the

telescope although some parts of the technical realization were not without problems.

It has to be mentioned that in some areas, namely the DAQ, the originally bold proposals have already been outdated by better concepts (see below).

Because of limited space I can only mention a few of the technical achievements : - One of the physics goals of the project is the possible observation of γ-

emission from GRBs, which requires rapid positioning of the telescope. With non-optimal drive conditions it was possible to turn the telescope within 25 sec by 180° in both axis, and the design goal of 20 sec is within reach.

- The tracking precision is currently around 0.02° without a starguider and should eventually reach a value of ≤0.05°(with a starguider). The tracking algorithm requires including a series of small corrections taking into account camera mast bending and telescope deformations.

- The point spread function of the mirror when focusing on a point source is 0.025° and we hope to improve the PSF to < 0.02°.

- For the camera support a single curved arc of a 20 cm Ø Aluminum tube with prestressed transverse steel cables is used. Oscillation amplitudes of ≥3 mm deflections are highly damped within a few sec (transverse oscillations τ ≈ 2.2 Hz, oscillations along the camera mast τ ≈ 1 Hz). The reason for fast damping is the use of the CF tubes, which have an inherent higher damping than steel tubes. Even during gusty winds the maximal amplitudes rarely exceed 5 mm.

To highlight some technical achievements of the novel components I show some figures about the point spread function when observing a point source at 970 m distance, the active mirror control function, the increase of the PMT QE by overcoating with a diffuse, wavelength shifter loaded lacquer and a comparison of the PMT signal attenuation in an optical fiber and a coax cable of around 160 m length.

The figures 2, 3, 4 and 5 are self-explanatory.

A major concern was the response of a single telescope to muons (the so-called muon wall preventing by excessive trigger rate and poor γ/muon discrimination a low telescope threshold for γs). First measurements showed that the muon rate was not a problem for the current trigger setting and that the background in the ALPHA plot for the

all cuts; it also showed that the muon images did not peak around a possible γ contribution. A trigger rate of typically 200-300 Hz was obtained. It was found that the focusing of the telescope with 0.1° pixels is quite critical for triggering on small size events, thus a low threshold requires a very efficient AMC and good optical parameters.

The threshold for the new telescope is rather difficult to determine. It will take us at least another year to determine the value with, say, 10 % precision. With the current conservative settings for the trigger we estimate that the present threshold is ≈ 50 GeV.

For the first observation results of MAGIC during the commissioning phase I refer

the reader to the contribution of R. Boeck in these proceedings [3].

NEAR-TERM PROJECTS AND PLANS Adding A Second Telescope

Following the general design philosophy, as outlined in chapter 2, the MAGIC collaboration has started the construction of a second telescope, MAGIC II. It will basically be a copy of MAGIC I. While the overall design parameters are conserved we plan to improve some components to make full use of the experience gained with MAGIC I and also by profiting from the progress of technology in the last eight years:

- Use of larger all-aluminum mirrors of 1 m

size. This is possible due to progress achieved in the diamond turning technique.

- Use of an improved AMC working permanently in the far IR not disturbing the camera operation.

- A camera with high QE photosensors, which allow lowering the threshold by

≥2 (see below). A copy of the MAGIC I camera will be used as a backup.

- A signal digitization with a ≥ 2 GHz sampling rate and higher dynamic range (see below).

FIGURE 2. Point spread function from observing a point source at 970 m distance.

FIGURE 3. Demonstration of the Active Mirror Control. FIGURE 4. Comparison of a PMT Left: defocused mirror when observing a point signal after ≈ 160 m cable, Narrow source; right: focused mirror signal for optical cable, Wide signal

for RG 58C cable

The construction of the mechanics of the telescope has already been started and the

site use permission has been granted. We expect that MAGIC II will be operational in

2006, i.e. be ready at the start of GLAST. It is planned to operate the second telescope

either in a stand-alone mode for observations of other sources, or in combination with

MAGIC 1 forming a stereo system for dedicated studies requiring better energy and

angular resolution and/or higher γ/h separation. MC simulations predict an increase in sensitivity by a factor 1.4-1.8. The distance between MAGIC II and I will be around 80 m. A first discussion of the impact of the earth magnetic field on low energy showers can be found in the contribution of R. Boeck [3] in these proceedings.

FIGURE 5. Improvement of the QE by overcoating the PMTs with a diffuse lacquer. Left: photograph of a naked and overcoated PMT, Right: gain in QE, also for photon trajectories passing cathode twice.

Developments Of High Quantum Efficiency Photosensors And Improvements Of The Readout And DAQ

The current IACTs have a rather low conversion of Cherenkov photons to photo- electrons, with a typical mean effective Quantum Efficiency (QE) ranging between 10 and 18%. The main limiting elements are the currently used photomultipliers. Light collecting elements (mirrors, light funnels) cannot be improved much more and optical imaging limits the mirror diameter to a diameter of 25-35 m. New photosensors with higher QE and very fast photon detection response have the largest improvement potential. The MAGIC collaboration pursues two directions:

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The use of hybrid PMTs with a GaAsP photocathode with a 40-50% peak QE, a wide spectral sensitivity range, high photoelectron collection efficiency (≈100%), very fast single photoelectron response (pulses with <1 nsec risetime and 1.5 nsec FWHM) by means of using electron bombarded avalanche diodes as amplifying anodes [4].

•

The possible use of so-called Geigermode Avalanche Photodiodes (G- APD), also sometimes called Silicon PMTs (SiPMT) [5].

The development of the hybrid PMTs is already fairly advanced. It is not unlikely

that around 2006-2007 a camera for MAGIC II can be realized. Still some

manufacturing problems have to be solved, amongst them an extension of the limited

lifetime. The alternative, the G-APDs, still need a considerable development time. The

expected QE is currently somewhat below that of the hybrid PMTs; also the ambient

temperature noise rate is unacceptably high, thus requiring cooling. On the other hand,

some operation parameters are very promising, such as a low operation voltage, no

need of high quality, ultrafast preamplifiers and extreme robustness (the G-APDS can be exposed permanently at full bias voltage to daylight without any damage).

Nevertheless, it is expected that the operation of either of the new photosensors will be much more complex compared to classical PMTs. A reduction in threshold by a factor

≥ 2 is expected.

Another development concentrates on the improvement of the pulse digitization with a ≥ 2Ghz digitization rate of the very fast signals, either by using a multiplexer based on an optical fiber system or a 2 GHz switched capacitor array followed by a slower multiplexed FADC. The aims are: a) minimizing the impact of the night sky light and b) improving the γ/hadron separation as well as the γ/muon separation on the base of the expected differences in the photon arrival time distribution.

THE LONG-TERM PLAN: THE EUROPEAN CHERENKOV OBSERVATORY ECO

The long-term plans proceed along two different concepts: a) adding an ultralarge IACT with 1000 m

mirror area and a high QE camera for a threshold below 10 GeV and b) by adding a wide angle telescope with at least 10-15° FOV. While the design for the 1000 m

telescope, dubbed ECO, is relatively straight forward (basically using the concept of MAGIC), the construction of a wide angle IACT poses major technical challenges. A solution based on a single mirror is not possible because of optical imaging problems. Also the costs for a wideangle camera are well beyond that of ECO 1000. Various alternatives are under discussion. Nevertheless, the construction of an ultra large telescopes is only justified after the outcome of the new telescopes has justified their investments.

ACKNOWLEDGMENTS

I would like to thank my colleagues from the MAGIC collaboration for the provision of many detailed information and many useful discussions and comments.

Also I thank Sybille Rodriguez and Ina Wacker for assistance in preparing this report.

REFERENCES

1. Barrio, J. A. et al.: The MAGIC design report. Max Planck Institute report MPI-PhE/98-5, 1998 2. Paneque D. et al., Nuc. Inst. Meth., A 504, 2003, pp.109-115

3. Boeck, R., Recent results from the MAGIC telescope, contribution to this conference 4. Lorenz, E. et al., Nuc. Inst. Meth., A 504, 2003, pp 280-285

5. Buzhan, P. et al., Nuc. Inst. Meth., A504, 2003, pp 48-52