33ND INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE The VERITAS Upgraded Telescope-Level Trigger Systems: Technical Details and Performance Characterization

Abstract: VERITAS is an array of imaging atmospheric Cherenkov telescopes sensitive to gamma rays in the energy range between ∼100 GeV and ∼50 TeV. The instrument underwent an upgrade of the camera triggers in November 2011. The new systems use 400 MHz Xilinix Virtex-5 FPGAs for the pixel neighbor coincidence logic necessary to produce a camera-level trigger. The upgraded systems are capable of time-aligning individual triggering pixels to within ∼0.2 nanoseconds, allowing for an operational pixel-to-pixel coincidence window of ∼5 nanoseconds. This reduced coincidence window provides improved rejection of night-sky background (NSB) which permits a reduction of the energy threshold at the trigger level. The use of FPGAs allows for the future implementation of a topological trigger capable of discriminating events based on an image moment analysis of a bit-wise hit pattern. As part of the commissioning phase for the trigger upgrade, the hardware was initially installed in a single telescope in ”parallel ” to the (then) current system. This allowed for the detailed performance characterization of the new system relative to the pre-existing trigger. Here we present technical details of the upgraded VERITAS camera trigger system and outline the details of these performance studies