  A fast trigger system is being designed as a potential upgrade to VERITAS, or
as the basis for a future array of imaging atmospheric-Cherenkov telescopes
such as AGIS. The scientific goal is a reduction of the energy threshold by a
factor of 2 over the current threshold of VERITAS of around 130 GeV. The
trigger is being designed to suppress both accidentals from the night-sky
background and cosmic rays. The trigger uses field-programmable gate arrays
(FPGAs) so that it is adaptable to different observing modes and special
physics triggers, e.g. pulsars. The trigger consists of three levels: The level
1 (L1.5) trigger operating on each telescope camera samples the discriminated
pixels at a rate of 400 MHz and searches for nearest-neighbor coincidences. In
L1.5, the received discriminated signals are delay-compensated with an accuracy
of 0.078 ns, facilitating a short coincidence time-window between any nearest
neighbor of 5 ns. The hit pixels are then sent to a second trigger level (L2)
that parameterizes the image shape and transmits this information along with a
GPS time stamp to the array-level trigger (L3) at a rate of 10 MHz via a fiber
optic link. The FPGA-based event analysis on L3 searches for coincident
time-stamps from multiple telescopes and carries out a comparison of the image
parameters against a look-up table at a rate of 10 kHz. A test of the
single-telescope trigger was carried out in spring 2009 on one VERITAS
telescope.
