VERITAS

VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a major ground-based gamma-ray observatory with an array of four 12 meter optical reflectors for gamma-ray astronomy in the GeV – TeV photon energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma rays that cause particle showers in Earth's atmosphere that are known as extensive air showers. The VERITAS array is located at the Fred Lawrence Whipple Observatory, in southern Arizona, United States. The VERITAS reflector design is similar to the earlier Whipple 10-meter gamma-ray telescope, located at the same site, but is larger in size and has a longer focal length for better control of optical aberrations. VERITAS consists of an array of imaging telescopes deployed to view atmospheric Cherenkov showers from multiple locations to give the highest sensitivity in the 100 GeV – 10 TeV band (with sensitivity from 50 GeV to up to 50 TeV). This very high energy observatory, completed in 2007, effectively complements the Large Area Telescope (LAT) of the Fermi Gamma-ray Space Telescope due to its larger collection area as well as coverage in a higher energy band.

== Specifications and design == VERITAS is constructed of four 12 m diameter Imaging Atmospheric Cherenkov Telescopes with an approximate separation of between each adjacent telescope. Each telescope comprises a large, steerable optical reflector and a high-speed photomultiplier tube camera. Multiple telescopes in an array are needed for stereoscopic observations of the Cherenkov light produced in extensive air showers. These stereoscopic observations allow precise reconstruction of the particle shower geometry, thus giving greatly improved angular and energy resolution compared to a single telescope. The angular direction of the incoming shower is determined by finding the central axis of the spread of the shower on each telescope and tracing those axes until they cross. The intersection of these axes determines the incoming direction of the primary particle (cosmic ray or gamma ray) that initiated the air shower in the upper atmosphere. It also determines the shower core position, i.e. the extrapolated position of the primary particle on the ground had it not interacted. The energy of the primary particle is determined from the total amount of Cherenkov light measured in each telescope, along with the distance of that telescope from the shower core.