We define the energy threshold of VERITAS as the energy at which the differential rate of reconstructed gamma-rays from the Crab Nebula per unit interval of energy reaches its maximum. Reconstructed gamma-rays are those whose images pass an image cleaning procedure (to eliminate pixels whose light is dominated by fluctuations of the NSB light), a reconstructed direction cut, and an image shape cut. This rate curve is shown in Figure 17 for a 4.2p.e. pixel trigger threshold and indicates an energy threshold of approximately 75GeV. This threshold is somewhat higher than the trigger energy threshold, which is the peak in the gamma-ray rate for events triggering the telescope, because low energy Cherenkov events which trigger the telescope often do not produce enough light to be used in the analysis. However, events at energies below the array threshold which trigger the array can still be successfully reconstructed, as shown by the extent of the rate curves below the peak. Thus, objects with spectral cut-offs below the array energy threshold may still be detectable.
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An energy threshold of 75GeV is a factor of 4 lower than that reached by existing imaging Cherenkov telescopes. This improvement comes from the combination of stereoscopic reconstruction of events, which permits effective background rejection at lower light levels than a single telescope, and the multi-telescope trigger, which permits the operation of the individual telescopes much further into the region where fluctuations in the NSB dominates the telescope trigger rate.