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In my observatory during the day there is a small amount of light that filters in through the gap between the roof and the walls. This is handy as I use it to take flats quite successfully. I put the cotton cover my wife made over the end of the scope which is pointed to a neutral lighted area and then take flats. They seem to work well. Perhaps you can do that in your observatory.
I take just 3 of each filter and no dark subtract. I subtract a bias when I callibrate the image. I don't know about the CDK12 but the 17 vignettes a fair bit and tends to have a hot spot in the centre of the image so I have experimented with flats and this method was advised to me by Don Goldman of Astrodon and works best I am finding.
Greg.
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