The History of Ground-Based Very High Energy Gamma-Ray Astrophysics with the Atmospheric Air Cherenkov Telescope Technique

The History of Ground-Based Very High Energy Gamma-Ray Astrophysics with the Atmospheric Air Cherenkov Telescope Technique

Razmik Mirzoyan

Max-Planck-Institute for Physics, Munich, Germany

Abstract

In the recent two decades the ground-based technique of imaging atmospheric Cherenkov telescopes has established itself as a powerful new discipline in science. As of today some ∼ 150 sources of gamma rays of very di ff erent types, of both galactic and extragalactic origin, have been discovered due to this technique. The study of these sources is providing clues to many basic questions in astrophysics, astro-particle physics, physics of cosmic rays and cosmology.

The current generation of telescopes, despite the young age of the technique, o ff ers a solid performance. The technique is still maturing, leading to the next generation large instrument known under the name Cherenkov Telescope Array.

The latter’s sensitivity will be an order of magnitude higher than that of the currently best instruments VERITAS, H.E.S.S. and MAGIC. This article is devoted to outlining the milestones in a long history that step-by-step have given shape to this technique and have brought about today’s successful source marathon.

1. Introduction

Several very interesting papers, along with the clas- sical book of Jelley [1], have been devoted to the his- tory of Cherenkov emission ([2], [3], [4], [5], [6], [7]) and its use for ground-based very high energy (VHE) gamma astrophysics. It is not the intention of this pa- per to repeat those comprehensive articles, but rather to give a personal impression about the main developments that have played a key role in evolution of the ground- based VHE gamma astrophysics. The author is involved for three decades in the VHE gamma astrophysics and could in person observe some of the relatively recent important developments.

In recent years the Imaging Atmospheric Cherenkov Technique (IACT) has made giant steps in establishing itself as a very successful new branch of astrophysics, see, for example, [8] for a recent review. With the dis- covery of very high energy (VHE) gamma rays from the Crab nebula with 9 σ significance in 1989, the Whip- ple team, operating the 10m diameter IACT in Arizona,

laid the foundation for the new science [9]. The dis- covery of the second source of extragalactic nature, this time Mkn-421, followed in 1992, again by the Whipple team, led by Trevor Weekes. This important discovery stopped the speculations that it was a science of a single source only. In the meantime other telescope installa- tions were built, like HEGRA and CANGAROO, who very soon independently confirmed the Crab nebula as a source. It took another few years until MKN-501 was firmly discovered as a third source, in 1996. The designs and parameters of the telescopes were improving. For example, an optimal pixel size of ∼ 0.25

, based on bet- ter and faster PMTs, started to be used in the imaging camera, at first by the Whipple team, but a few years later also by HEGRA [47]. CANGAROO used PMTs of angular aperture of 0.11

[11]. The CAT telescope, put into operation in late autumn 1996 [12], on the same site as the previous ASGAT [13] and THEMISTOCLE [14] instruments, started operation using a pixel size of 0.12

.

But before going into the details of relatively recent

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important developments that lead us to today’s success, I want to go back in time to the last century for showing how the things have began.

2. Very early days

Oliver Heaviside calculated the movement of an elec- tron in a transparent medium with a speed higher than that of light (please note that until the beginning of the 20th century scientists believed that the space was filled by “ether”, transporting the electromagnetic waves). He showed that such movement would be accompanied by a specific conical emission see, for example, [15].

Though he published a series of papers on this issue, these stayed unnoticed; his genius has advanced under- standing of the problem by half a century. Also Arnold Sommerfeld calculated the task of a charge moving in vacuum with a speed v ≥ c. The relativistic principles prohibit such a motion in vacuum but in a medium with a given refraction index n his equations gave a valid so- lution. In his paper in 1904 he calculated this hypo- thetical task and came to very similar conclusions about the necessity to emit special emission [16]. The first experimental notices about the bluish glow from bot- tles with liquids containing radioactive radium salts in her dark cellar, is attributed to Madame Curie in 1910, who thought of it as some kind of luminescence [17].

The first systematic study of the e ff ect was performed by the French scientist Mallet, who published three pa- pers in the years 1926-1929. Mallet described the phe- nomenon and even could measure its continous emis- sion spectrum that, because of the absense of emission lines and bands, contradicted the previously assumed fluorescence explanation. But he failed to reveal the most significant polarisation and anisotropy effects of the unknown emission [18].

In 1932, young Pavel Cherenkov became a PhD stu- dent of Sergey Vavilov, who in 1934 became the direc- tor of the newly created Lebedev Institute for Physics of the Academy of Sciences of the former Soviet Union. S.

Vavilov assigned the task of studying the bluish “lumi- nescent” emission to Cherenkov as a topic for his PhD thesis. Soon afterwards Cherenkov found out that he would spend many hours in the dark, cold cellar. That was necessary for accommodating his eyes, the mea- suring instrument, to the darkness, for seeing the ex- tremely faint emission. Soon after starting Cherenkov formally complained about the unusual working condi- tions and the unusual task, but after detailed explana- tory work of Valilov he agreed to continue the stud- ies. He started varying the temperature and the pres- sure of liquids in large margins, but still the emission

was there. He added special additives to the liquids that should have quenched the luminescence, but that did not happen. Importantly, he failed to find the characteris- tics of luminescence spectral lines, and instead detected light emission with a continuous spectrum. Cherenkov could measure light emission even from solvent liquids.

Based on these results, in 1934 he wrote an article about his studies that S. Vavilov declined to coauthor [19].

Instead Vavilov wrote his own explanatory paper that appeared next to Cherenkov’s paper in the same issue of the journal [20]. Vavilov interpreted that continuous spectrum as bremsstrahlung emission of electrons. It took another three years until Cherenkov could show in a simple, elegant experiment the anisotropic character of the emission; light was emitted only within a certain angular range in the forward direction. He submitted his discovery paper to the journal Nature. For an un- known reason the journal declined publishing it. After that he submitted it to the Physical Review, where fi- nally it was published in 1937 [21]. In the meantime the theoreticians Igor Tamm and Ilya Frank put forward a theoretical explanation of that phenomenon [22].

Let n be the refraction index of a transparent, dielec- tric medium, and c / n the speed of electromagnetic inter- actions in it (c is the speed of light in vacuum). When a charged particle moves with a speed higher than c / n, along its path it asymmetrically polarizes the medium, that is a somewhat too slow for following the fast es- caping particle. Short afterwards the medium relaxes by emitting anisotropic radiation in the forward direction.

Sergei Vavilov became the president of the Academy

of Sciences of the Soviet Union in 1946. The work of

Cherenkov, Tamm, Frank and Vavilov got the highest

recognition in the Soviet Union and they were awarded

the then most prestigious Stalin prize. Vavilov was liv-

ing in permanent fear about his very famous botanist

and geneticist brother Nikolai, who was first arrested in

1940, then sentenced to death and subsequently com-

muted to imprisonment of 20 years; Nikolai died in

prison in 1943, presumably from hunger. As a result

of multiple heart attacks Sergei Vavilov died in 1951. In

1956, the works of Cherenkov, Tamm and Frank were

awarded the Nobel prize. Vavilov was not nominated,

only alive people can be awarded the Nobel prize. It is

interesting to note that in Russian literature some pref-

erence in this discovery is given to Vavilov, in fact the

effect is known under the name of Vavilov-Cherenkov

emission. Some reflection of this could be the fact that

while being one of the very few Nobel prize winners

in the former Soviet Union, only very late, in 1964,

Cherenkov was accepted as a corresponding member in

the Soviet Academy of Sciences.

Let us now have a look into the next events that have played a key role in the evolution of the IACT tech- nique.

2.0.1. Discovery of Cherenkov emission in atmosphere In 1948, Patrick Blackett, while studying the emis- sion of light from the night sky and aurorae, estimated that there should be a 10

part coming from air shower elementary particles producing Cherenkov emission in the atmosphere [24]. During his visit of Harwell in 1952 he met with J.V. Jelley and W. Galbraith and learnt that they were also experimenting with Cherenkov light emission, but in water. Blackett mentioned to them about his estimation of the Cherenkov light contribu- tion in the atmosphere. Very shortly after that Jelley and Porter built a simple setup consisting of a 25cm diameter parabolic signaling mirror of a short focal length, fixed it into a dustbin and put a single 5cm PMT in its focus. They did not had to wait for long un- til, the telescope was counting pulses, one every two minutes. They had to prove that these were in fact Cherenkov pulses coming from air showers. For this purpose they put the small telescope into coincidence with their large area Geiger counters for measuring air showers and observed coincident events. Within one year they improved the performance of their telescope and succeeded to measure the polarization of Cherenkov light as well found out that, compared to the green part of the spectrum, there was more light in the blue. Gal- braith and Jelley published a discovery paper in Nature in 1953 which has marked the beginning of atmospheric Cherenkov technique and measurements. Except for relatively small-scale test measurements not very much happened in the following few years.

2.1. First ideas on astronomy by means of gamma-rays In a seminal paper Philip Morrison suggested to mea- sure gamma rays from possible cosmic sources in 1958 [25]. He considered that measurements in the energy range (0.2 − 400)MeV using detectors on high flying aircrafts or balloon flights below ∼ 25g / cm

could pro- vide a real clue to cosmic rays and sources. Di ff erent type detectors could be e ffi cient for measuring gamma rays in that energy range. He mentioned the Crab Neb- ula as a source of gamma rays but, interestingly from today’s point of view, at lower energies; the expanding gas shell in the nebula contains radioactive debris from the original explosion. He predicted that a flux of spe- cific nuclear gamma rays, mainly of

Ra, should be detectable, with an incoming flux of 10

gamma / cm

· s.

Guiseppe Cocconi suggested to measure gamma ray sources at TeV energies at the International Cosmic Ray

Conference (ICRC) in Moscow in 1959 [26]. He sug- gested constructing an air shower array at a high moun- tain altitude with an angular resolution of ∼ 1

for mea- suring ∼ 10

eV gammas. He made some very op- timistic prediction on the possible flux of gamma rays from the Crab Nebula, which should have been about 1000 times higher than the background. He acknowl- edged the over-optimistic nature of his estimate, men- tioning that even if the efficiency in the production of gamma rays were substantially lower, still the Crab Nebula could be detected.

Very soon afterwards, Georgy Zatsepin, who was considering Alexander Chudakov as an expert for atmo- spheric Cherenkov emission, approached him and dis- cussed the possibility of detecting gamma sources.

For understanding the development of air showers in the atmosphere Alexander Chudakov and Natalia Nesterova were measuring the lateral distribution of Cherenkov light on Pamir mountain at a height of 3800ma . s . l . in 1953-1955. They were using large size Geiger counters and eight Cherenkov light receivers as detectors[27]. Chudakov became excited by the prospects of the new possible science and started plan- ning systematic study of cosmic gamma ray source can- didates in the sky.

3. First generation atmospheric Cherenkov tele- scopes

3.1. Chudakov’s telescopes in Crimea

Chudakov and his colleagues built a system of 4 telescopes in Katsiveli, Crimea, near the shore of the Black Sea in 1960 [23]. One year later, they increased the number of telescopes to 12. These were based on parabolic mirrors of 1.55m diameter and focus of 60cm, previously used by militaries for securing the Black see border. 4.5cm diameter PMTs were set in their foci, with a diaphragm limiting the field of view to 1.75 FWHM. Chudakov calculated a lens with a special form for reducing aberrations and improving timing and placed it in front of the PMTs. Every 3 telescopes were rigidly connected and adjusted to observe the same di- rection. The 4 independent mounts, with 3 mirrors each, were able to independently rotate and observe sources with a pointing precision of 0.2

in elevation and 0.4

in azimuth. Four-fold coincidences between these tele- scopes were required. A rate-stabilizing circuitry was developed for getting rid of count-rate instabilities due to variation of the background light emission.

The experiment was continued for ∼ 4 years. By us-

ing flashes of Cherenkov light initiated by muons in a

piece of Plexiglas of a given thickness, the researchers estimated a photon threshold of 280ph / m

(for the wavelength range 300 − 600nm). The shower count- ing rate was slightly more than 3Hz and the estimated energy threshold was ∼ 3.4T eV for gammas.

The observations were performed in the drift-scan mode: the telescopes were pointed to a direction through which, after a short while, the targeted source would pass. Then such scans, each of ∼ 7 minutes in duration, were repeated many times over the nights.

It is impressive to see the list of sources observed by Chudakov’s crew. One should mention that back in that time nothing was known about the pulsars and their periods, about X-ray sources. The common be- lieve was that radio sources should be good targets as gamma emitters.

Together with the Crab Nebula (47 scans), Cygnus A (191 scans), Cassiopea A (20 scans), Virgo A (20 scans) also Perseus A (7 scans) and Saggitarius A (7 scans) were observed. The long observations of Cygnus A were due to initially small indications of a possible signal that the researchers were trying to confirm. Some not very systematic observations were performed for clusters of galaxies like Ursa Majoris II, Corona Bore- alis, Bootes, Coma Berenices. In the end, they could not observe signal from any of the sources. The signal upper limit from the direction of the Crab nebula for a total of

∼ 5 . 5 hours of observations was 5 × 10

ph / cm

· s for the threshold of ∼ 4 − 5T eV. Today we know that the integral flux of the Crab nebula above 4TeV is

∼ 2.5 × 10

ph / cm

· s. This means that the upper flux limit set by Chudakov and his colleague was ∼ 20 times higher than the current flux from the source. So, for measuring a significant signal from the Crab nebula they had to measure ∼ 400 times longer and that was of course very improbable.

An important consequence of this non-detection of the Crab Nebula was that it turned down the ∼ 1000 times higher flux originally estimated by Cocconi. Also, non-detection of a signal cast strong doubts on the ori- gin of electrons in the Crab nebula as secondaries pro- duced in cosmic proton interactions with the nebula;

pp → π → μ → e (the common believe was that gam- mas were produced due to pp → π

→ 2 γ ). That meant that in fact the electrons could have been accelerated in the nebula.

3.2. Photographing showers with Image Intensifiers An important measurement was realized by Hill and Porter in 1960 [28]. They coupled an image inten- sifier to a 25cm diameter Schmidt telescope of a 24

field of view and shot the first images of air showers in

the sky. Because of the small size of the mirror, only images of showers with energies ≥ 0.5PeV could be recorded. The trigger came from a 5 inch PMT in the focus a conventional mirror that was set in close prox- imity. Bright background stars could be identified on images. The event rate was ∼ 7/ hour. Researchers re- alized that measuring the shape of an air shower could give clues to the true direction of the shower as well as to its impact point on the ground. The estimated high angular resolution of ∼ 0.2

was considered as a very important feature that could allow one to largely (100 times!) suppress the background [29]; moreover, one can read there that “the stereoscopic technique with two separate telescopes would greatly enhance the po- tentialities”. This technique convincingly demonstrated the power of imaging but because of the small mirror size and the related high energies, because of the bulky image intensifier and the relatively long integration time of the phosphor screen (∼ 1μ s), and some other disad- vantages it turned out to be impractical.

3.3. Monte Carlo simulations and ”stereo” observa- tions

Member of the Chudakov’s crew Victor Zatsepin published a remarkable Monte Carlo study paper in 1964 [30]. By simulating on the Soviet “Ural”-type computer (operated by a specially trained staff) he ob- tained the first equal photon density contours of air shower images produced by gamma rays as well as their angular distributions and radial photon densities. In par- ticular, he mentioned in his paper that ‘”since the maxi- mum intensity of the light from a shower does not coin- cide with the direction of arrival of the primary particle, in researches in which the determination of the angu- lar coordinates of the primary particle is made by pho- tographing the light flash from the shower one should seek improved accuracy in this determination by pho- tographing the shower simultaneously from several po- sitions”. In a recent discussion he mentioned that almost 50 years ago he was intensively considering if there was any practical means of photographing showers from many directions, or more explicitly, which type of cam- era could do that [31].

As one can see from the above, already ∼ 50 years ago some researchers clearly understood the potential of the coincidence measurements, better known today as stereoscopy.

The experiments with image intensifiers continued

for some more years, but no further breakthrough hap-

pened. In the following years many experiments as

a rule, of smaller scale than the one from Chudakov,

though, were built and operated. Except for the 10m di- ameter Whipple telescope, which played an absolutely central role in giving birth to gamma astronomy (this will be discussed in some detail below), no major tech- nical improvements were achieved until 80’s. As a rule, researchers used (0.6 − 1.5)m diameter military search- light mirrors of parabolic shape of F /0.5 optics that were severely suffering from coma aberration. They used coincidence between a few of such mirror ele- ments, which allowed lowering the energy threshold of the instrument.

3.4. Harwell-Glencullen telescope

One should mention the compact telescope in Har- well, UK, which later on was moved to Glencullen val- ley near Dublin [29]. The telescope, set on a gun mount, consisted of two 92cm diameter back-silvered mirrors, each with a single PMT in its focus, viewing a ∼ 5

field of view. The coincident count rate was (0.7 − 1.7)Hz.

Observations of quasars, the Crab nebula, Cygnus A, M31, and some other sources, were performed in the winter seasons of 1963, 1964 and 1965. Data collected from 75 useful nights, showed no signal [32]. From today’s point of view it is interesting to note that the data taking was done visually, namely the operator was watching a pair of scalers and every minute writing down the numbers in a log book [5]. In 1967-1968, a new telescope of very fast electronic design was devel- oped and built in Glencullen that was shifted to Har- well for giving it a fast timing system for pulsar studies.

Later on it was moved to Malta, where in early 1969 it started observations. This telescope had a very fast performance, namely it was using four F /2 mirrors of 90cm diameter, fast PMTs, fast amplifiers and a 3.5ns coincidence resolving time. No positive detections were made, flux upper limits for certain pulsars in the range of (0.5 − 2.5) × 10

ph / cm

· s were set [33].

3.5. Double Beam Technique

Another interesting development in gamma astron- omy is related to two 6 . 5m diameter reflectors set at a large distance ( ∼ 120m) for stellar interferometry in Narrabri, Australia. In 1968, the researchers carried out observations of the Crab nebula and two pulsars [34].

Later on, Grindlay and colleagues made a step forward by using the above telescopes for the “double beam” ob- servation technique. Each telescope had 2 PMTs. While the main PMTs were inclined towards one another on

∼ 0.4

for observing the shower maximum region from a selected source direction, the other two PMTs were inclined to even lower angles of ∼ 1.3

towards each

other for measuring a signal from the “muon core” of the showers. The authors claimed that in this way they could reject ∼ 50% of the hadron background. That was not much but the principle was very interesting; modifi- cations of it will be widely used in future. For giving an impression about the imaginary spirit of some of the ob- servations about 40 years ago, it is interesting to shortly discuss one selected observation made with this instru- ment [35]. In that article a detection of a ∼ 5σ pulsed signal from the Crab pulsar is claimed on a flux level, for example, of 8 × 10

ph / cm

· s at 1T eV, and this is at the absence of any DC signal from the Crab nebula.

Today we know that the cited pulsed flux level makes

∼ 40% of the DC flux from the Crab nebula. Concern- ing the pulsed flux from the Crab pulsar at 1TeV we do not yet even know if there is flux at such a high energy.

But we know that the pulsar flux at ∼ 400GeV is about 100 times less than the DC flux from the nebula [36].

Because of the space limitation of this article the au- thor cannot give a fair description of all the existing tele- scope installations and their details like, for example, for the Haleakala telescope in Hawaii, the Potchefstrum telescope in South African Republic, the telescopes of the Durham group, the telescope installations in India.

These are reflected in the proceedings of the workshop series of “Towards a Major Atmospheric Cherenkov De- tector” as well as in proceedings of the international cosmic ray conferences.

4. The second generation telescopes

4.1. The 10m Whipple telescope

In 1967, Giovanni Fazio and colleagues began con-

structing a 10m diameter, F /0.7 telescope on mount

Hopkins, at the Whipple observatory at a height of

2300m a.s.l. [37]. The large diameter of the telescope,

together with fast PMTs in the focal plane provided

a low threshold not achievable earlier. The telescope

started operating in 1968, initially with a single 5 inch

PMT in the focus that somewhat later was increased

to two and afterwards to ten PMTs for simultaneous

ON and OFF source observations. Trevor Weekes has

joined the Smithsonian project in 1966. In 1968 he co-

authored G. Fasio and two other colleagues in an in-

teresting observational paper about13 sources, among

which were, for example, Crab nebula (of course!),

M87, M82, IC443 [38]. The set flux upper limits above

the threshold of 2T eV were modest, but today we know

that the above listed sources in fact are gamma-ray emit-

ters. In 1977 Weekes and Turver suggested to use two

telescopes, each equipped with a 37-pixel imaging cam-

era, at a separation of 100m. They expected that such a

stereoscopic imaging system shall strongly suppress the background [39]. A 37-pixel imaging camera, covering a field of view of 3.5

in the sky, was built and installed on the 10m telescope in ∼ 1983.

The next real milestone was the image parameteri- sation suggested by Michael Hillas in 1985 at the La Jolla conference [40]. A few years later that has helped the Whipple team to measure the famous signal from the Crab nebula (see below). Since then this set of pa- rameters are known under the classical name “Hillas”

parameters and are widely used by all IACTs.

Though in the following years the Whipple team could measure marginal signals from Crab on a signif- icance level of ∼ (3 − 4)σ, the real breakthrough hap- pened in 1988 with the observation of the 9σ signal!

This measurement [41] has marked the beginning of a new era and is considered as the birthday of the ground- based gamma astronomy. The persistent work of Trevor Weekes for over 20 years finally paid o ff , a new branch of science started its marathon!

In the meantime other telescope installations were built, like HEGRA and CANGAROO, which very soon independently confirmed the Crab nebula as a source.

4.2. GT-48 in Crimea

Since the late 1960’s the group in Crimean Astro- physical Laboratory (CrAO) led by Arnold Stepanian, used two parabolic searchlight mirrors of 1 . 5m diameter in coincidence for studying gamma sources (this must not be mixed up with the group of Chudakov from Lebe- dev’s institute in Moscow, who for several years was experimenting in Crimea). They reported detections of Cassiopea and Cyg X3 in the beginning of the 1970’s.

Especially the latter has made a big resonance. In the 1980’s, the group started constructing a set of two large telescopes, separated by some distance, named GT-48.

Large gun mounts from a Canadian ship were used for these telescopes. On each mount they have built six telescopes, three of the imaging type with 37 pixels and another three operating a single UV-sensitive, so- lar blind PMT. Every telescope had 4 mirrors of 1 . 2m diameter and 5m focus. The goal of the Crimean group was to profit from the stereo observations see, for exam- ple, [42]. Because they did not want to sacrifice neither the threshold nor the coincidence rate, they put the tele- scopes at 20m distance from each other. Their relatively small mirrors and low altitude of the location provided a threshold of ≥ 900GeV. The proximity of the telescopes did not allow them to fully exploit the differences in im- age parameters otherwise seen from largely separated detectors. In 1989, this installation was put into opera-

tion and in subsequent years it measured quite a number of sources.

4.3. HEGRA

The first telescope of HEGRA [43] was designed in 1990, as a somewhat modified version of the Yerevan Physics Institute (YerPhI) first Cherenkov telescope.

The latter was reported at the first dedicated interna- tional workshop on VHE gamma astronomy organized by Arnold Stepanian in Crimea in 1989 [44]. It was the prototype of the planned five telescope “stereo”array (the experimental proposal was submitted in February 1985), installed at 100m distance from each other [45].

Each telescope was planned to have a 3m diameter tes- sellated mirror of 5m

area and to be equipped with a 37-pixel imaging camera in a 5m focal plane. The first telescope was built at Nor Amberd cosmic ray sta- tion (2000m a.s.l.) on mount Aragats in 1989. The commissioning of the telescope showed a shower count rate of ∼ (0.3 − 0.5)Hz near zenith. The pixels used UV-transparent light guides of a conical form (focons), made of Plexiglas and subtending an angular aperture of 0.41

in the sky. The imaging camera was based on the most advanced Soviet FEU-130 type PMTs with a GaP first dynode of very high amplitude resolution. The equatorial mechanical mount of the first telescope was re-designed with the help of the YerPhI group engineer and built in the workshop of the Max-Planck-Institute for Physics in Munich. A 37-pixel imaging camera, al- most a clone of what was built in Nor Amberd, was re- built in the collaborating institution in Kiel, Germany.

The very high quality glass mirrors were produced in

YerPhI. In mid 1991 the main parts for the planned five

telescopes, including the mirrors, PMTs, shaft encoders,

etc., were sent from Armenia via Munich to Canary is-

land La Palma. The first telescope was installed on the

Roque de los Muchachos observatory in La Palma in

late fall 1991. A ∼ 5σ hint of the first signal from Crab

appeared after two months of data taking, in late fall

1992. In the following year the second telescope with

the same pixel size but with one more ring in the cam-

era (61 pixels) and a larger reflector of 4.2m was built

and put into operation at 100m distance from the first

one. The stereo observations, the power of which has

been predicted in a dedicated Monte-Carlo study paper

in 1993 [46], could start. In the following years four

more telescope of the same size as the second telescope

but with 271 pixels of size of 0.25

were added. In the

end, the second telescope, too, was given a 271-pixel

camera and the array was completed in 1997. The last

upgrade in the same year was the increase of the mirror

area of the first telescope to 10.3m

. HEGRA, operated

till 2002. It has convincingly demonstrated the long- awaited power of stereo observations [47].

These second generation imaging telescopes pro- vided only a handful of sources, but it became clear that still there was a big potential in this technique that was just waiting to be explored.

4.4. 7-Telescope Array

The Japanese 7-Telescope Array was originally planned as a detector including two arrays, each of 127 imaging telescopes, operating in coincidence [48]. Each telescope had 3m diameter mirror and a 256 pixel cam- era. In 1996-1997 three out of seven such telescopes were built and installed in Dugway, Utah, USA, the rest four were planned to be installed within one year. The telescopes started taking data on several interesting ob- jects as, for example, measuring the flaring MKN-501.

Unfortunately, by a wrong turn of a 20 foot long un- armed missile that hit the two data taking containers, the operation of this array was discontinued short before the end of 1997.

5. Solar power plants as gamma-ray telescopes

It was recognized rather early that mirror-based solar power plants can o ff er large mirror area of several thou- sand m

that could be used for collecting scarce pho- tons from sub-100 GeV gamma showers. In the begin- ning of 90’s the threshold energy of the 10m Whipple telescope of ∼ 75m

reflecting area was estimated to be (300 − 400)GeV. The common believe was that for low- ering the threshold energy of a telescope by a factor of n one needs to increase its mirror area by n

times. So, for example, for lowering the threshold energy of ∼ 1T eV of the ∼ 10m

HEGRA telescope by a factor of 20 down to ∼ 50GeV one needed to increase its mirror area by 400 times! i.e. one needs a mirror area of 4000m

! Not a single telescope can offer such a huge mirror area. But the solar power plants with distributed mirror area can do that. Several solar power plants were rendered into gamma-ray detectors. The technique of doing that was quite di ff erent for di ff erent research teams. For exam- ple, while the GRAAL instrument [49] was attempting to collect Cherenkov photons from heliostats in the field into a ∼ 1m-size Winston cone, the other detectors were trying to organize a kind-of imaging in the central light collection tower, directing light from given heliostats to their own PMT channels. For a review of converted so- lar power plants please see [50]. Some interesting mea- surements were performed by using these arrays. The French CELESTE instrument tried to measure flux from

the Crab nebula down to ∼ 60GeV [51]. The compari- son with today’s more precise measurements show that their reported flux was ∼ 2.5 times too low. With the operation of the MAGIC telescope it became obvious that the “classical” imaging method can provide much higher efficiency than the solar power plant detectors, so several years ago they ceased their operation.

It is interesting to note that the MAGIC-I telescope, that had only 236m

mirror area, could perform suc- cessful measurements also in the sub-100 GeV energy range. This is in striking contrast with the above as- sumption about the threshold dependency on the mirror area. The author believes that the above mentioned de- pendence of the threshold on the mirror area, that even now is circulating in publications, is not correct. The lower threshold of a telescope is simply proportional to the used mirror area. This can be explained by the fact that for an imaging telescope it is not the light of the night sky that sets the lower threshold, but the higher- level requirement that for analyzing an image one needs some minimum amount of charge, something of the or- der of ∼ 100 photo electrons. To the knowledge of the author in the first time this was recognized in [52].

6. The 3rd generation telescopes

The third generation telescopes were designed well

before the potential of the second generation telescopes

was fully exhausted. Already in 1995 the first pre-

sentations on the concrete concept of 17m diameter

MAGIC were made [53]. These were followed by the

VERITAS proposal in fall 1996 and one year later by

H.E.S.S.. While both of these arrays were following

the goal of doing astrophysics with a stereo system

of 10m diameter telescopes, which were well-known

thanks to the Whipple telescope and the experience of

HEGRA, the MAGIC design instead was aiming to go

towards the sub-100GeV energy range, down to 30-

40GeV, into “terra incognita”. Obviously this task was

more demanding and challenging, several novel tech-

niques and technologies were necessary for making it

possible. When HEGRA stopped operating in 2002, the

collaboration split into two parts. One part together with

the scientists from France, largely people from the CAT

experiment, created the core of the H.E.S.S. collabora-

tion and built their instrument in Namibia. The other

part stayed in La Palma, in the original site of HEGRA,

and together with scientists from Spain and Italy created

the core of the MAGIC collaboration.

6.1. H.E.S.S.

The H.E.S.S. collaboration could get supported by the German and French financial agencies, while the VERITAS team had to wait for financial support for sev- eral more years. The H.E.S.S. telescopes were built and operated in Namibia in 2002-2004. Already in the be- ginning the H.E.S.S. team has performed a scan along the galactic plane and made a very reach harvest of galactic sources. This array has turned out to be a very e ffi cient instrument, making a very high number of im- portant discoveries and measurements above the ener- gies (160-200)GeV. Recently a very large telescope of 28m diameter has been set in the center of H.E.S.S. that shall allow them to perform observations also in the very low energy range of few tens of GeV.

6.2. VERITAS

The VERITAS telescopes, unlike H.E.S.S., which are operating imaging cameras of ∼ 5

aperture, are us- ing cameras of 3 . 5

field of view. Otherwise both in- struments are similar and both have increased the orig- inally planned 10m diameter of their telescopes up to 12m. VERITAS was built in Arizona next to the ad- ministrative building of the Harvard-Smithsonian center and inaugurated in 2007. Not surprisingly, also VERI- TAS turned out to be a very successful instrument that in the recent several years has made a high number of important discoveries and measurements.

6.2.1. CANGAROO

CANGAROO was a collaboration between several universities from Japan and the university of Adelaide.

The collaboration started operating a 3.8m size single piece telescope of parabolic shape that was used earlier for lunar ranging.It started operating in 1992 at a thresh- old of a few TeV and in the following years has dis- covered several new sources of gamma rays. Ten years later four telescopes of 10m size were built. These had some differences in the design. Along with technical problems, mostly related to the chosen type of mirrors, there were also technical and organizational problems related to the data analysis. In few occasions detections of new sources could not be confirmed by the H.E.S.S.

telescope [54]. A couple of years ago this array has ter- minated its operation.

6.3. MAGIC

Initially, mostly because of financial reasons, MAGIC was planned as a single telescope. Several in- novations were necessary for operating a single tele- scope also in the very low energy range ≤ 100GeV.

It was necessary to provide a very fast response time for the telescope, so a reflector of parabolic design was chosen. Along with this, very fast hemispheri- cal PMTs were developed for the needs of MAGIC by ElectronT ubes from England. In combination with the light guides and a mat lacquer coating, these provided an enhanced quantum efficiency. The PMT analog sig- nals were converted into light and by using optical fibers transported to the electronic room where they were con- verted back into electrical pulses practically in the ab- sence of any degradation of time features. The MAGIC- I telescope was built and put into operation in 2003- 2004. The fast signals were initially read out by us- ing 300MS ample / s custom-built FADCs that starting 2007 were exchanged against 2GS ample / s fast mul- tiplexed FACDs [55]. The latter allowed MAGIC to suppress further down the hadron-induced background by a factor of two for energies ≥ 100GeV and by a factor of three for energies ≤ 100GeV [56]. In fact, MAGIC could do observations of some selected sources as, for example, the Crab Nebula, at energies as low as

∼ (50 − 60)GeV. By developing a special trigger con- figuration, the so-called SUM-trigger, the researchers could operate the telescope even at a very low threshold of ≥ 25GeV. This allowed them to discover a pulsed signal component from the Crab pulsar. This has made a strong impact on the pulsar theory models. The next serious improvement of MAGIC’s sensitivity was due to the construction and operation of the second telescope at 85m distance from the first one. This has essentially doubled the sensitivity of the first telescope.

7. CTA: the 4th generation major instrument

7.1. CTA

A few years ago the researchers understood that one

needed to unify the efforts of different collaborations

and move towards one major instrument. Finally the

series of “Towards a Major Atmospheric Cherenkov

Detector” workshops, taking place between 1989 and

2005 (the last one), served its purpose. A few years

ago, a new collaboration was formed for building the

Cherenkov Telescope Array [57]. This collaboration in-

cludes practically all the researchers worldwide work-

ing with the atmospheric Cherenkov technique and

many newer groups who are interested in exploring the

sky in gamma rays with unprecedented sensitivity. Now

the CTA collaboration is moving from the prototyping

into the construction phase. Large number of differ-

ent size telescopes are planned to be built in southern

and northern observatories. This is going to be the ma- jor ground-based instrument for doing astrophysics by means of gamma rays for the next few decades.

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