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VERITAS is an array of imaging atmospheric Cherenkov telescopes sensitive togamma rays in the energy range between 85 GeV and 30 TeV. The instrumentunderwent an upgrade of the camera triggers in November 2011. The new systemsuse 400 MHz Xilinix Virtex-5 FPGAs for the pixel neighbor coincidence logicnecessary to produce a camera-level trigger. The upgraded systems are capableof time-aligning individual triggering pixels to within ~0.2 nanoseconds,allowing for an operational pixel-to-pixel coincidence window of ~5nanoseconds. This reduced coincidence window provides improved rejection ofnight-sky background (NSB) which permits a reduction of the energy threshold atthe trigger level. The use of FPGAs allows for the future implementation of atopological trigger capable of discriminating events based on an image momentanalysis of a bit-wise hit pattern. As part of the commissioning phase for thetrigger upgrade, the hardware was initially installed in a single telescope in"parallel" to the (then) current system. This allowed for the detailedperformance characterization of the new system relative to the pre-existingtrigger. Here we present technical details of the upgraded VERITAS cameratrigger system and outline the details of these performance studies.

The VERITAS Upgraded Telescope-Level Trigger Systems: Technical Details and Performance Characterization