Performance of the prototype of an improved sum-trigger system for the MAGIC telescopes

The two MAGIC telescopes located on the Canary Island La Palma detect Cherenkov light from air showers induced by high energy cosmic rays impinging on the atmosphere. Among all air showers Gamma-ray induced showers are rather rare and strongly masked by night sky background light and hence require special efforts concerning the trigger system. In 2007 a prototype of a novel analog Sum-trigger was installed in MAGIC I which enabled to lower the trigger threshold from 55 GeV down to 25 GeV and achieved excellent results in the detection of the pulsed Gamma radiation from the Crab pulsar. To eliminate the need for manual tuning and maintenance demanded by that first prototype, a new setup with fully software-controlled, automatic calibration was designed recently. The key element of the new circuit is a novel, electronically controlled continuously variable analog delay line that enables the temporal equalization of the signals from the camera photo sensors, which is crucial for the reduction of false triggers from background signals. In this talk the basic working principle of the improved Sum-trigger concept will be presented as well as main results of the new prototype circuit tested on the MAGIC I telescope.