The Level 2 Trigger of the H.E.S.S. 28 Meter Cherenkov Telescope

Abstract

The High-energy Stereoscopic System (H.E.S.S.) is an array of atmospheric Cherenkov telescopes dedicated to GeV-TeV γ ray astronomy. Four 12 meter diameter telescopes (CT) are currently in operation and a fifth 28 meter diameter telescope (LCT) should be completed by 2011. This will lower the system's energy threshold in stereoscopy down to ≈ 50 GeV. Concurrently, the LCT will also be used to detect monoscopic lower energy showers without a coincident detection on the smaller telescopes. This is made possible thanks to the higher sensitivity and the faster electronics of the LCT. But lowering the L1 trigger conditions on the LCT results in a larger number of spurious background detections the reduction of which motivates the second level trigger system of the LCT. An efficient L2 selection algorithm was designed based on intensive off line simulations of air shower images from different background events and their morphological differences as seen by the LCT. The present contribution reports on the flexible and reconfigurable hardware solution designed for the L2 trigger, and on the real-time implementation of the proposed L2 algorithm.