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The very highest energy photons, gamma-rays, are too energetic to be detected by standard optical methods. In fact they rarely actually make it to the surface of the Earth at all but interact with molecules in the Earth's atmosphere. The high energy gamma-ray telescopes, such as Veritas and Hess use air Cherenkov telescopes to observer these photons. How does an air Cherenkov telescope actually work to measure the incoming gamma ray?

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A very high energy gamma ray spontaneously pair-produces a particle and anti-particle, the idea being that the gamma ray has enough energy that a decay into matter is feasible.

The particle and anti-particle which are created are still very high energy - they have velocities near the speed of light in a vacuum. Whenever a particle flies through a substance at a velocity higher than the speed of light in that substance, it emits Cherenkov radiation. The typical analogy given is that this is like a sonic boom: in a sonic boom, distinctive waves are produced when something flies through a substance at higher than the speed of sound in that substance; with Cherenkov radiation, the waves are produced by flying through at more than the speed of light.

The particle and antiparticle might collide with stuff in the atmosphere, producing high-energy photons; these high-energy photons can pair-produce again. This way, the Cherenkov radiation amplifies; a small burst of Cherenkov light is produced whenever a gamma ray enters the atmosphere.

The Cherenkov radiation produced by gamma rays has a distinctive pattern that can be detected using a photomultiplier tube. With an array of these detectors, you can observe the shower from several points. Then, you work backwards to figure out where the original gamma ray came from and what energy it had (much easier said than done!).

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Definitely much easier said than done. I built an array of six of these telescopes as part of my Ph.D. thesis. –  dagorym Jun 8 '11 at 2:41

Cherenkov telescopes do not detect gamma rays directly. A gamma ray has a lot of energy compared to a visible light wave. Collision of a high energy gamma ray with an atom in the atmosphere results in emission of electrons, positrons, and/or photons - each of which may have enough energy to cause a similar emission upon subsequent collisions. This results in a cascade effect, which may be larger or smaller depending on the magnitude of energy in the original gamma ray. The cascade of light is collected in a parabolic mirror, sent through photo-multiplier tubes, and measured. See the link for more details.

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