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 100 m (330 ft) 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. Each of the individual telescopes has a 12 m diameter aperture and a 3.5 degree field of view. The telescopes are built on a Davies-Cotton optical design, which uses a spherical reflector and is straight forward to construct and align. This design does cause some time spread in the arrival of Cherenkov photons at the camera, but this spread is small (~ 4 nanoseconds). The reflector consists of 350 individual mirror facets, hexagonal in shape, mounted on a rigid optical support structure. The camera on each telescope has 499 individual pixels (high-speed 26 mm-diameter photomultiplier tubes). VERITAS, like other IACTs, is sensitive to primary particles that produce sufficient atmospheric Cherenkov light to be detectable at the ground. Its full range of sensitivity is from 50 GeV to 50 TeV (although the spectral reconstruction does not start until at least 100 GeV, depending on source strength). 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 (65% containment radius). The entire array has a peak effective area of 100,000 square meters above 1 TeV. A very weak astrophysical source with a gamma-ray flux only 1% of the Crab Nebula can be detected by VERITAS in under 25 hours of observation. Stronger sources can be detected in significantly less time.
In order to distinguish between the background events (i.e. hadronic showers and muons) or noise (i.e. starlight and moonlight) and the targeted data (i.e. electromagnetic showers produced by gamma rays), VERITAS uses a three-level trigger system. Level one corresponds to a level crossing on each pixel using constant fraction discriminators. Level two is a pattern selection trigger, which selects photon-like showers, which have compact shapes, and eliminates most of the background showers, which produce more random shapes in each camera. Level three is the array trigger which looks for a coincidence in the arrival time of the shower at multiple telescopes. The Cherenkov light that is produced by gamma rays in the upper atmosphere is very dim, so VERITAS observes best under clear, dark skies. Observations are not possible under cloudy or rainy skies, or when the Moon is very bright. However, observations are regularly made when the Moon is dim or moderate in brightness (typically less than 60% illumination). The total yearly observation time is typically around 1,200 hours (of which around 200–250 hours is during brighter moonlight with illumination between 20 and 60%). The observatory does not generally collect data in July or August due to local monsoon conditions.
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