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