Development of the new analog sum trigger for the MAGIC Telescope: Concept and realization

A new analog trigger concept which allows to reduce significantly the energy threshold of Cherenkov telescopes has been designed and implemented for the MAGIC telescope. The analog signals of a patch in the camera consisting of 18 pixels are summed up in an analog way. In order to prevent accidental triggers from PMT-afterpulses signals are clipped at a certain amplitude. Short pulses, induced by cosmic showers, allows us to keep the coincidence time window very short and strongly suppress accidental triggers from night sky background (NSB). Also, unlike the usual digital trigger designs, all low amplitude signals in the given patch in the camera are summed up and fully contribute to the trigger decision. This improves signal to noise ratio. This trigger concept provides a low cost trigger installation and very effective and stable trigger for Cherenkov telescopes. The new trigger system was installed and commissioned in October 2007. The Crab pulsar was observed from October 2007 until February 2008 with a threshold of 25 GeV - the lowest trigger threshold ever achieved for Cherenkov telescopes. The detailed description of the design of the novel trigger system and the analysis results are presented here