  The VERITAS gamma-ray observatory, situated in southern Arizona, is an array
of four 12m diameter imaging Cherenkov telescopes, each with a 499-pixel
photomultiplier-tube camera. The instrument is designed to detect astrophysical
gamma rays at energies above 100 GeV. At the low end of the VERITAS energy
range, fluctuations in the night sky background light and single muons from
cosmic-ray showers constitute significant backgrounds. VERITAS employs a
three-tier trigger system to reduce the rate of these background events: an
initial trigger which acts at the single pixel level, a pattern trigger which
acts on the relative timing and pixel level, a pattern trigger which acts on
the relative timing and distribution of pixel-level triggers within a single
telescope camera, and an array-level trigger which requires simultaneous
observation of an air-shower event in multiple telescopes. This final
coincidence requirement significantly reduces the rate of background events,
particularly those due to single muons. In this paper, the implementation of
all levels of the VERITAS trigger system is discussed and their joint
performance is characterized.
