Development of a new computer controlled analog sum trigger for MAGIC with a large trigger area

The MAGIC telescopes located on the Canary island of La Palma detect Cherenkov light pulses from air showers produced by high energy particles entering the atmosphere. To distinguish between those faint events and noise triggers from night sky background special efforts are necessary. Besides the standard trigger in MAGIC I with a threshold of 55 GeV a prototype analog sum trigger has been installed in October 2007 that enabled to lower the triggering energy threshold significantly down to 25 GeV. This new trigger system amplifies and sums up signals of a patch of pixels using analog electronics. The discriminator is applied to this sum of pixels rather than each individual pixel. Having achieved excellent results in the detection of the Crab pulsar the sum trigger now is being further optimized. The prototype setup has a ring shape trigger topology used for pulsar searches whereas regular observations are performed in wobble mode requiring a large homogeneous trigger area which will be one main alteration. Another improvement is an adjustable signal delay line that compensates the different signal transition times of the PMTs and optical fibers. Moreover the amplifying and summing stages are being redesigned to allow computer controlled adjustment of delay and gain per pixel. The new sum trigger will further decrease the triggering threshold and thus permit to detect even fainter events.