Trigger conditions and effective areas of imaging air Cherenkov telescopes

The contributions to the trigger rate of an imaging atmospheric Cherenkov telescope and the optimization of the trigger conditions are discussed. Among the trigger options studied are cameras with trigger regions of different sizes, triggers requiring a coincidence of 2, 3 and 4 pixels as well as triggers imposing additional conditions on topology of the trigger pixels. A two-pixel trigger based on neighboring trigger pixels is shown to provide full efficiency, at considerably reduced random trigger rates. Numerical examples of effective areas and rates are given based on the parameters of the new HEGRA Cherenkov telescopes. Especially addressed are trigger conditions for the observation of extended sources with angular size approximate to 1 degrees.