The effect that Rob decribed is not confined to narrow band filters and can occur with RGB filters or OSC cameras as well. Did you check the calibrated frames as I suggested? If clipping is the cause, it will be obvious in the histograms (of the green channels).
Bernd
yeah - i was actually commenting on jkmorse's reply, not necessarily trying to explain this situation. but i suppose whenever the exposure is short enough this problem can happen regardless of filter.
rob
