You are beginning to understand how difficult it is to take a good flat, but you can get one. The twilight flats taken by experienced shooters offer the best potential, but are the most difficult to successfully get. I'd suggest that you consider other types of flats initially.

I use a "light box" when using my 10" newt or cassegrain. The box is about 14"X14"X9", with a circular hole that fits over the business end of the scope. The box uses six 12" long 12V light tubes (bought at Home Depot), separated from the scope by 4 layers of 1/8" thick light diffusers, each separated by 1/2" of air to create a relatively flat (uniformly illuminated) field at the scope aperture. I use a 6"X8" GEPE slide film viewing "table" (a 1/2" thick uniformly illuminated "white light" slide viewer) for the "flat" source with my smaller refractor. GEPE makes these in sizes up to about 12"X15", but they get expensive as the size goes up.

I use 20 flat exposures each @ about 3/4 full well of the camera, 20 bias exposures each @ minimum exposure (~50ms). Subtract the median combined (master) bias from both the flat exposures (each one) and the light exposures (each one). Then form a median combined (master) flat, and divide each bias corrected light by the master flat, and renormalize before combining your lights. A good imaging program like MaximDL will do all of this for you.

Robert

I use a monochrome cam + filter wheel, flats should be taken for every filter used for lights, especially if you don't keep things VERY clean (gets rid of dust doughnuts).

I suppose if you are using a cooled one shot color, or a DSLR, you'd just shoot to the desired exposure, then convert the flat exposures to greyscale. You should convert the RGB light exposures into and be working with individual (greyscale) RGB layers, so use the greyscale flats and biases on the individual (greyscale) RGB lights

Maybe someone who does proper flats/biases/darks witha DSLR will chime in.

Robert