Commissioning and performance of a fast level-2 trigger system at VERITAS

Abstract

We have built a new three-stage FPGA-based high-speed trigger for VERITAS, an array of ground-based imaging atmospheric Cherenkov telescopes (IACTs) located in Arizona. This trigger has the ability to recognize patterns of Cherenkov light generated by atmospheric air showers initiated by incident extra-terrestrial gamma rays. The new trigger has programmable coincidence recognition timing and programmable delay compensation over 499 pixel channels in an IACT camera. Measurement of and compensation for systemic variations is achieved through the use of an FPGA-based time-to-digital converter (TDC) and FPGA-based programmable delay elements. The trigger pattern is the coincidence of any three adjacent pixels within the camera. The minimum coincidence window of 3ns with low dead time provides sensitivity at lower energies than were achieved previously. The new trigger has now been successfully installed on all four of the IACTs of VERITAS, replacing the previous system. We present measurements of the performance of this new trigger in comparison with that of the previous system and of the effect of the new trigger upon overall array performance.