Question: Which telescopes are gamma ray telescopes?
About the article "Telescope", section "Gamma-ray telescopes".

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Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum.
Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.
The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.
Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away.
Only 10% of gamma-ray sources are non-transient sources.
These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei. 
Higher energy X-ray and Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.X-ray and Gamma-ray telescopes are usually on Earth-orbiting satellites or high-flying balloons since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum.
However, high energy X-rays and gamma-rays do not form an image in the same way as telescopes at visible wavelengths.
An example of this type of telescope is the Fermi Gamma-ray Space Telescope.
The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization.
An example of this type of observatory is VERITAS.
Very high energy gamma-rays are still photons, like visible light, whereas cosmic rays includes particles like electrons, protons, and heavier nuclei.
A discovery in 2012 may allow focusing gamma-ray telescopes.
At photon energies greater than 700 keV, the index of refraction starts to increase again.
Gamma rays are absorbed high in the Earth's atmosphere so most gamma-ray astronomy is conducted with satellites.
Gamma-ray telescopes use scintillation counters, spark chambers and more recently, solid-state detectors.
The angular resolution of these devices is typically very poor.
There were balloon-borne experiments in the early 1960s, but gamma-ray astronomy really began with the launch of the OSO 3 satellite in 1967; the first dedicated gamma-ray satellites were SAS B (1972) and Cos B (1975).
The Compton Gamma Ray Observatory (1991) was a big improvement on previous surveys.
Very high-energy gamma-rays (above 200 GeV) can be detected from the ground via the Cerenkov radiation produced by the passage of the gamma-rays in the Earth's atmosphere.
Several Cerenkov imaging telescopes have been built around the world including: the HEGRA (1987), STACEE (2001), HESS (2003), and MAGIC (2004). 
Gamma ray telescopes collect and measure individual, high energy gamma rays from astrophysical sources.
These are absorbed by the atmosphere, requiring that observations are done by high-altitude balloons or space missions.
Gamma rays can be generated by supernovae, neutron stars, pulsars and black holes.
Gamma ray bursts, with extremely high energies, have also been detected but have yet to be identified. 
The High Altitude Gamma Ray Telescope (HAGAR) is an atmospheric Cerenkov experiment with 7 telescopes setup in 2008.
Each telescope has 7 mirrors with a total area of 4.4 square meters.
The telescopes are deployed on the periphery of a circle of radius 50 meters with one telescope at the center.
Each telescope has alt-azimuth mounting.