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Air Cherenkov telescopes are used to conduct gamma ray astronomy from the Earth's surface, using the Cherenkov radiation generated when high-energy gamma rays strike the upper atmosphere.

Earth's atmosphere is opaque to many wavelengths of electromagnetic radiation, including all photons more energetic than ultraviolet light. This is because high-energy photons are absorbed by atoms and molecules, which are in turn ionized. However, at extremely high energies, gamma rays will strike the upper atmosphere, and undergo pair production -- the creation of an electron and a positron by a photon more energetic than about 1 MeV (one million electron volts). You then have a free electron and a free positron moving along nearly the same path as the gamma ray. The positron will then interact with another electron, and generate a gamma ray via pair annihilation. The process continues until the last gamma ray generated has an energy below the threshold for pair production. The higher the original gamma ray's energy, the greater the number of particles produced.

This process can be observed; because the particles involved are travelling faster than the speed of light in the atmosphere, they generate Cherenkov radiation directed along the path of the moving particle. The particles produced in the air shower spread out within a cone with its apex at the top of the atmosphere, and the base on the ground. The Cherenkov photons produced lie within that cone -- they follow the path of the generating particle to within 1°. Here on the ground, what we would see is a blue circle of light centered on the source of the gamma rays, lasting for less than a nanosecond. And, as with the number of particles generated, the brightness of the flash is proportional to the energy of the incident gamma ray.

Air Cherenkov telescopes are mainly used to study extremely high energy gamma rays, those with energies greater than 1014 electron volts. For comparison, a photon of visible light has an energy of 1 eV, an X-ray in a dentist's office is around 1,000 eV, and gamma rays emitted by radioactive material are usually below 107 eV. These are incredibly high-energy particles, and are primarily generated in high-energy phenomena like supernovae and their remnants, and in the centers of active galaxies and quasars. They also strike the Earth very infrequently -- an air Cherenkov telescope may stare at a source for weeks and only observe a few events.

Air Cherenkov telescopes are simply large arrays of mirrors, each focused upon a detector, usually a photomultiplier tube. The telescope is pointed at an object believed to emit high-energy gamma rays, and the observers wait for a characteristic pulse of light characteristic of gamma-ray induced air showers. The operation in principle is very simple, but there are many things which complicate the observations. First, the detectors are sensitive to stray light flashes, such as those by meteors, airplane lights, and over-the-horizon lightning. Second, the observations must be done from very dark skies, because the pulse of light is usually very faint, and will be overwhelmed by city lights. Third, and most tricky, is that gamma rays are not the only source of air showers.

Hadronic air showers are generated by very high energy cosmic rays, usually by protons moving at very close to the speed of light. The difference is that such showers usually generate many different subatomic particles, like muons and pions. While interesting, they are less useful for studying individual sources because their electric charge makes them susceptible to magnetic fields which change their direction. Thus, a high energy proton will appear to come from a different direction from which it originated. Air Cherenkov detectors can distinguish between gamma ray and hadronic events either by using additional detectors to see if the Cherenkov radiation is accompanied by other particles (usually muons), and by the shape of the particle cascade; gamma ray-induced showers are usually tightly constrained to a narrow cone, but hadronic showers are more spread out.

The oldest operating air Cherenkov detector in the world is the Whipple Observatory, in southern Arizona, built in 1989. It consists of a single, 10-meter, parabolic dish composed of several dozen individual mirrors, each of which is focused on an array of photomultipliers in the focal plane. Several other observatories have been built using the same principles as Whipple. A few projects have also used the mirror arrays of solar energy generating stations to make observations. In daylight, these mirror arrays focus sunlight onto a tower, which makes steam. At night, a photomultiplier array is placed in the focal plane, and the mirrors pointed at sources in the night sky.

Unfortunately, very few sources of high-energy gamma rays have been observed, mainly because it takes incredible amounts of energy to make them. Such powerful sources are few and far between in the universe. The brightest constant source of high-energy gamma rays is the Crab Nebula, a supernova remnant. This source is so bright, it is used to calibrate the detector, and statistically significant detections can be made in less than an hour of observations. Other sources are primarily quasars and active galactic nuclei, including the Markarian galaxies Mrk 421 and Mrk 501, both BL Lacertae objects.

Sources:
http://egret.sao.arizona.edu/
"Gamma Ray Telescopes" and "Cosmic Rays: Extensive Air Showers", Encyclopedia of Astronomy and Astrophysics, Institude of Physics Publishing, 2001 (v. 1)

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