The goal of triggering on 3-pixel tracks may be ambitious because of phenomena that cause coherent pulses in adjacent pixels. Alternatively, a smart event trigger might be able to recognize those same phenomena by virtue of adjacent pixels triggering simultaneously. Making every pixel smart would seem to be more expensive since you only need one smart event trigger system for a large cluster of pixels. I am concerned, however, that a smart enough event trigger might not be feasible if the pixel trigger is simple. Noisy signals and time slewing cause unpredictable variation in pixel trigger times, and timing information seems to be the only additional data that could be used to discard the frequent 3-pixel spatial tracks that arise from coherent background phenomena.
More work needs to be done in establishing the rates of the coherent phenomena.
Perhaps their combined rate is more tolerable than I fear, in which case we could get by with simple pixel triggers and simple event triggers. Expected rates due to Poisson fluctuations of the night sky background are certainly manageable. 3-pixel events may increase dramatically, however, if pixel thresholds do not accurately track changes in the light background due, for example, to stars entering the pixels' fields of view. We should explore ways to have the threshold respond directly to the background light level rather than the pixel's trigger rate. My guess is that an analog pixel trigger could be made cheaper than a digital equivalent.