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What does the image look like before you apply the flats etc?
I can get odd results with my large telescope with flats. It can be quite fussy.
Here's what I do for what its worth:
1. Darks. I use a temperature regulated CCD but still applies to DSLR. I take about 6-10 darks (my camera is very clean anyway but still is needed). I combine these in CCDStack using clip max/min combine.
2. Bias. I take only a few. Perhaps there is a gain to taking many but perhaps that applies more to DSLRs - I am not sure. Bias though are good because you want to use adaptive darks with DSLRs as the noise level will vary quite a bit as the temp you took the shot at will vary in different time periods. The bias is used to scale the darks to match your shot and it works quite well. I even use this occassionally with my astro CCD and I get good results. Best practice is darks matching lights for temperature. So for a DSLR try to take your darks under similar temperatures as your lights to minimise the difference. To get this in perspective if you look at the specs of many astro CCDs they double in noise with every 6C increase in temp. A DSLR would be similar.
3. Flats.
I have different approaches to taking flats. Both ways work. I take flats with a cotton sheet cover over the end of the scope. The scope is in focus and the camera is cooled and stable. I keep everything exactly the same as when taking the lights (you can't remove the camera, rotate it, change filters -anything).
I take 3 flats per filter. In your case there are no filters unless you are using a light pollution filter so take several. I am not sure what the gain is from taking 60 flats but perhaps for DSLRs they need more.
I do not dark subtract from my flats now. I used to take a dark for the flat duration and subtract that from the flat. But I am getting better results by only doing max/min clipping combine in CCDstack no subtractions. Then when I apply the flat I use the bias to subtract from the flat. Not sure if this is best practice but this is simply what works best on my setup.
I go for 20,000 to 40,000 ADU brightness in the flats. On a DSLR I think that equates to 1/3rd of a histogram exposure. For me I think I will implement Richard Crisps advice on flats (he has written a paper on it) where he recommends 40,000 ADU. Sometimes I find dust donuts are stubborn to disappear. I either take flats at dusk (you have to be fast) or in my observatory the next day as there is enough random light that filters in that flats work quite well.
Its best to keep everything spotless than rely on flats only as a result.
Dust donuts are more of an issue with long focal length scopes.
I find 2 mirror plus a corrector/reducer the toughest format for flats. Refractors are not fussy and often hardly vignette much at all.The CDK17 is tough work and the most difficult part of image processing with it. I see some weird effects like that if the darks do not match well or there is no bias for the flats or some combination is slighly mismatched.
So my advice is fresh darks, fresh bias and flats all at the closest you can get to the temperature when you imaged, don't change anything when you do flats.
Cameras do change slowly over time and a DSLR will be quite different between seasons. In Sydney we have high temps at night already now so that would throw things off if your darks were taken in winter.
Greg.
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