section_path: By electromagnetic spectrum/Gamma ray
section_name: Gamma ray
paragraph_text: Gamma ray
  
  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 installed on high-flying balloons or Earth-orbiting satellites since the Earth's atmosphere is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is the Fermi Gamma-ray Space Telescope which was launched in June 2008.
  
  The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. Such detections can be made either with the Imaging Atmospheric Cherenkov Telescopes (IACTs) or with Water Cherenkov Detectors (WCDs). Examples of IACTs are H.E.S.S. and VERITAS with the next-generation gamma-ray telescope- CTA, currently under construction. HAWC and LHAASO are examples of gamma-ray detectors based on the Water Cherenkov Detectors.
  
  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.