reduce a heuristic threshold for newer experiments with shorter CS period
reduce a heuristic threshold for newer experiments with shorter CS period

We assumed the interval from CS-on to US-on would be approx 5 seconds
as a sanity check and guard against glitchy light-level detection
where we might see global illumination step up over multiple frames,
rather than jump all at once.  Changes in a shorter interval triggered
the script into fallback modes for older, broken data.

To accomodate newer experiments, reduce the threshold from 5s to 1.5s.

Note, this is also an approximation as we guess a framerate (20fps)
due to a legacy of movies w/o proper framerate metadata. Some movies
may be higher fps depending on the instrument...

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