Array performance

Extensive simulation studies have been used to determine the optimum configuration for VERITAS and to characterize its performance. The VERITAS design is optimized for maximum sensitivity to point sources in the energy range 100GeV - 10TeV, but with significant sensitivity in the range 50GeV - 100GeV and from 10TeV - 50TeV. Also, the chosen field of view (3.5 $^\circ$ ) is large enough to conduct an efficient sky survey and study extended sources while maintaining a fine pixellation of the camera for a low energy threshold. Here we give a brief justification for the chosen configuration and concentrate on describing its capabilities.

**Table 1:** Performance of VERITAS
Characteristic	E	Value
Energy threshold^a		75 GeV
Flux sensitivity^b	>100 GeV	9.1 $\times$ 10^-12cm^-2s^-1/15mCrab
	>300GeV	8.0 $\times$ 10^-13cm^-2s^-1/5mCrab
	>1TeV	1.3 $\times$ 10^-13cm^-2s^-1/7mCrab
Angular resolution	50GeV	0.14 $^\circ$
	100GeV	0.09 $^\circ$
	300GeV	0.05 $^\circ$
	1TeV	0.03 $^\circ$
Effective area	50 GeV	1.0 $\times$ 10³m²
	100GeV	1.0 $\times$ 10⁴m²
	300GeV	4.0 $\times$ 10⁴m²
	1TeV	1.0 $\times$ 10⁵m²
Crab Nebula	>100GeV	50/minute
gamma-ray rates	>300GeV	7/minute
	>1TeV	1/minute
Energy resolution^c		<15%
^aEnergy at which the rate of photons per unit energy interval from the Crab Nebula is highest for a 4.2 photoelectron trigger threshold.
^bMinimum integral flux for a 5 $\sigma$ excess (or $\ge$ 10 events) in 50 hours of observations of a source with a Crab-like spectrum.
^cRMS $\Delta E/E$

The baseline configuration for VERITAS is described in §3. The performance characteristics of this configuration are outlined in Table 1. The performance characteristics listed are for a telescope trigger requiring 3 adjacent PMTs to record more than 4.2 photoelectrons (p.e.) within 14ns (see §3 for details on telescope triggering). The 4.2p.e. trigger threshold level is set by the sustainable single telescope trigger rate at which VERITAS can operate (1MHz) and by the requirement that the trigger rate from fluctuations of the night sky background (NSB) not exceed 10% of the total cosmic-ray rate of 1kHz. We emphasize that our definition of the energy threshold is the point where the rate of gamma-rays per unit energy interval from the Crab Nebula is highest. This definition is quite conservative because VERITAS has good sensitivity to gamma-rays at energies well below this (see Figure 17).