Trigger Levels:

For practical operation, VERITAS must trigger at a rate $\le$1kHz. A higher rate produces unacceptable dead-time in the acquisition systems. To facilitate this rate, while keeping the energy threshold of the detector at a minimum, a trigger scheme of four levels has been developed. The levels are defined as:

Level 1

Each PMT is connected to a discriminator which has a programmable threshold. The night-sky background light produces rates (singles) given by the dotted curve in Figure 14. At low thresholds the dominant contribution to the trigger rate comes from fluctuations in the number of single photoelectrons (p.e.) produced by night-sky light. At higher thresholds the background rate is dominated by PMT afterpulsing. At a threshold of 4.2p.e., the Level 1 trigger rate is $\sim$1MHz for each channel.

Level 2

A hardware pattern trigger at each telescope based on Level 1 triggers can reduce the background by discriminating between photon-initiated showers images, which are compact, and background triggers caused by sky noise or afterpulsing, which have random locations in the camera plane. The dashed curve in Figure 14 shows the expected rate of these background events when the pattern requirement is that the 3 nearest neighbors in the 499 pixel camera are hit. The Pattern Trigger will likely follow the scheme presently being used at the Whipple Telescope.

Level 3

The central station receives the Level 2 triggers from each telescope. The Level 3 trigger system selectively delays each Level 2 signal to account for the wavefront orientation and determines if the overall array trigger condition is satisfied. The trigger condition will depend on the array configuration and the observing strategy, but will typically require the time coincidence of several telescopes. The solid curve in Figure 14 shows the expected background rates for this trigger if 3 of 7 telescopes fire within 40ns coincidence time. If the Level 3 trigger condition is met, readout of the telescope event information will be initiated.

Level 4

The background trigger rate can be reduced by demanding that the individual telescope trigger clusters conform to the predicted parallactic displacement of the gamma-ray images. Thus, a high level trigger, implemented either in hardware or in software, can be used to significantly suppress the hadronic background. This system is currently in the development stage.

  

Figure 14: Expected rates from VERITAS. The various curves are discussed in the text.