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I do between 3 and 6 and they are dusk flats with a white cloth over the end of the scope. I have to move fast as the light fades and you don't have that much time to capture flats for 7 filters some of which have both 1x1 binning and 2x2 binning.
I tend to only do one set of 3 to 6 for RGB usually the red filter.
I don't see any downside.
Total exposure time is more important than a little bit of noise coming from your flats.
I also tend not to use bias for flats or if I do its a separate bias not a flat with a bias subtracted so I can not use it if I am not getting a good flat correction.
On my CCDs I go for around 20,000 ADU. That seems to work well.
I tried using 30,000 but they usually overcorrect. 17-20,000 ADU seems to match the lights nicely and correct the vignetting well and any dust bunnies.
I also prefer to clean my filters and CCD glass rather than rely on flats to remove dust bunnies because they don't always without fail do a good job of that.
Cleanliness is next to perfect imagingness!
Greg.
Greg.
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