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RGB filters have a bandwidth of around 100nm so you could expect a 7nm filter to receive about 1/14th the number of photons in a given time from a broad band target or sky glow.
That would suggest exposures 14 time as long are needed.
My RGB subs are usually 30s long and my NB 7nm subs 300s. But for the NB I increase the gain to reduce read noise and therefore the required exposure but its still 10 times. Thats with an ASI1600 but the same principles apply.
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