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I just checked a few subs I have. With my Microline 8300 the background seemed to be around 3300 (I simply moved the cursor around the image in CCDstack and the ADU is displayed in the bottom right corner).
For my Proline 16803 it was more like 2100.
So if I took 20 luminance subs like that with the Microline that is 20 x 3300=66,000. 10X rule gives me 10 x 66,000 = 660,000. If my flat subs are around 30,000 ADU then I need 660,000 divided by 30,000 =22 flats.
So an approximate rule there would be 1 flat for every light. This is all 1x1 binning.
For the Proline though it would need 1/3rd as many so say 15 flats for every 20 lights.
I did approximately 12 flats in my last imaging run.
But if I increase my subexposure length to 30 minutes I'll have to measure the background ADU (not sure if it increases with exposure length). It may mean longer exposures require less number of flats?
Another argument for longer exposures so long as bright areas don't blow out?
Greg.
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