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VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a major ground-based gamma-ray observatory with an array of four 12m optical reflectors for gamma-ray astronomy in the GeV - TeV energy range. VERITAS uses the Imaging Atmospheric Cherenkov Telescope technique to observe gamma-rays that cause particle showers in Earth's upper atmosphere. The telescope design is based on the design of the existing 10m gamma-ray telescope at the Fred Lawrence Whipple Observatory. It consists of an array of imaging telescopes deployed such that they permit the maximum versatility and give the highest sensitivity in the 50 GeV - 50 TeV band (with sensitivity from 100 GeV to >10 TeV). This very high energy observatory, completed in 2007, effectively complements Fermi due to its large collection area as well as its higher energy bound. ==Specifications & Design== VERITAS is constructed of four 12m Imaging Atmospheric Cherenkov Telescopes with an approximate separation of 100 meters (330 feet) from each adjacent telescope. The four telescope array is needed for stereoscopic observations. These stereoscopic observations allow the reconstruction of the particle shower geometry, thus giving precise angular and energy resolution. The angular resolution is acquired by finding the center axis of the spread of the shower on each telescope and tracing those lines until they cross. The intersection of these lines is the direction of the source of the gamma ray. This is depicted on the figure to the right. Each telescope sees the particle shower at a specific region in its camera (shown as grey ellipses), and thus the source of the shower can be found. Each of the individual telescopes have a 39 feet aperture, 350 mirrors on each dish, and a 3.5 degree filed of view. The telescopes are built on a Davies-Cotton optical design, which uses a spherical reflector and is easy to construct and align. This design does cause a small time spread in the signal, but this spread is very small (~ 4 nanoseconds).〔 The camera on each telescope has 499 individual PMTs. VERITAS, like other IACTs, are most sensitive to the highest energy cosmic rays. Its full range of sensitivity is from 85 GeV to >30 TeV (although the spectral reconstruction does not start until at least 100 GeV). The energy and angular resolution depend on the Energy of the incident gamma ray but at 1 TeV the energy resolution is ~17%, and the angular resolution is 0.08 degrees. The entire array has an effective area of 100,000 square meters. In order to distinguish between the background noise (e.g. hadronic showers, starlight, moonlight, and muons)) and the targeted data (i.e. electromagnetic showers produced from gamma rays), VERITAS uses a three-level trigger system. Level one corresponds to the discriminators on each pixel using constant fraction discriminators. Level two is a pattern selection trigger, which selects only photon-initiated showers, which have compact shapes, and background pulses, which have more random distributions. Level three is the array trigger which looks for coincidence between multiple telescopes. The Cherenkov light that is produced by cosmic rays in the upper atmosphere is incredibly dim, so VERITAS can only observe under clear, dark skies. Observations are not possible under cloudy or rainy skies, or when the moon is bright. This shrinks the observation time to approximately 70–100 hours each month from September until June. The observatory does not collect data in July or August due to local monsoon conditions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「VERITAS」の詳細全文を読む スポンサード リンク
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