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I was pondering the other day whether it was worth using some data I grabbed when the moon was up to add to data I had from a moonless sky, and was just doing a quick back of the envelope calculation when I saw this thread.
You can use essentially the same calculation to decide which of your subs to throw away Rodney, ie how bright does the background have to be in an image before using that image in your stack would make the stack’s final SNR worse?
With some (perhaps oversimplifying!) assumptions, it seems that the answer is that if the background brightness of the new image is less than twice the background brightness of the majority of the images, then it’s beneficial to use it. (This is if you have a lot of images and are only looking at using a few extra bright ones – the more general case is the one I was originally looking at, where you can be adding quite a few images, and there you can get away with using even brighter images).
Note: by brightness I mean average ADU above the dark frame –not sure what that would be in your case Rodney.
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