such telescopes, which can be typically located 70 to 120 meters apart. The primary advantage of this mode of operation is that the energy threshold (the peak sensitivity) of the telescope can be lowered as local muons produced by cosmic ray induced showers can be eliminated. This is because the narrow Cherenkov light cone produced by local muons will only be recorded by a single telescope. The shower reconstruction and background rejection offered by an array of telescopes provides an order of magnitude increase in sensitivity and improved angular and energy resolution as compared to a single telescope. This advantage has been used to great effect by the H.E.S.S. telescope array which has detected several new sources of very high energy gamma-ray photons in recent years. History The IACT was pioneered by the Whipple collaboration and led to the discovery of TeV emission from the Crab Nebula in 1989. The Whipple 10m telescope also discovered the first extra-galactic source of TeV emission with the detection of very-high-energy gamma-ray emission from the active galaxy Markarian 421. The HEGRA telescope array was the first system to use multiple telescopes, a technique known as stereoscopy. The MAGIC system at the Roque de los Muchachos Observatory in La Palma began with one 17 m telescope in 2004, to which a second 17 m telescope was added in 2009. The First G-APD Cherenkov Telescope (FACT) at the Roque de los Muchachos Observatory became operational in 2011. The H.E.S.S. collaboration has added to their system in Namibia a new 28 m IACT, currently the largest IACT telescope, which has been operational since July 2012. See also List of telescope types Pavel Cherenkov References External links Aspera European network portal Astroparticle physics news on Twitter Official web page of CTA NASA Goddard Space Flight Center - Air Cerenkov Detectors Observational astronomy Gamma-ray astronomy