Thanks for all the quick answers!
I looked at some older data on the Helix Nebula. A 4 min lum image through the LPF gave me an average backgroud of 1339 and the nebula average of 1545. (measured using CCDStack). I guess my initial question of this thread is, restated now differently, by looking at these two numbers for a 4 min exposure, can I make a guess as to how much longer I could expose before the background would become too bright, or when the background would begin to interfer with the nebula?
Peter
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