Having proper light is essential to chronograph function. The challenge is to design the skyscreens to use available light to your advantage.
We won't get into the design and operation of skyscreens. We are talking about trying to measure the effectiveness of the skyscreen/chronograph system. To measure the effectiveness of the skyscreens, we consider that the second velocity reading on the same shot is essential. It is just about in the same league as trying to measure rifle accuracy by firing one-shot groups on various targets at various ranges. When you fire a two-shot group on a single target you at least get an indication of where the third shot will fall. If the first two shots are close together, you then expect the third shot to hit nearby. If the first two shots are widely separated, you have no idea where the third shot will hit. If you can make two chronographs read very similar velocities on a single shot, you can reasonably assume that both chronographs are accurate. (We'll assume that you've eliminated all the gross error sources such as screen spacing, alignment, oscillator frequency, timer resolution, totally inappropriate light, et cetera.)
KenO
