Shooting Flats

let me add a caveat,  I only get error messages or failure when I am trying to create a BPM, otherwise the flats seem to work fine.

I shoot my flats using a flat-field generator. These aren't necessarily super expensive, the only real requirement is that the panel is able to produce a very even illuminated surface. I personally just lock my focuser and everything when the session is done in my garden, take it back inside and when I feel like it, I put on the flat-field generator, hook it up to Sequence Generator Pro and I start with using the flat-field wizard. If you don't have SGP, try to see if you can get the ADU count on the flat taken with something else, when working in 16-bit (which SGP always does) you target that ADU to be around 30.000. Which is the same as half-way the maximum spectrum your camera can take. Never do this by looking at the spectrum on the camera itself, those spectra are usually not lineair. If you reaaaaally have to look at that, aim for a peak at about 75% of the max you can see on there.

If you can only take sky-flats, things get a bit more complicated (I think). You cover the scope with some very even layers of white fabric (pulled tight) and use it under the early morning sky, preferably either clear or with an even layer of clouds. You want to aim for the same illuminations as described above. It's best to do this rather quick, the sun is rising and changing the light coming in fast. Personally I made this to work pretty nice, but I was getting a bit tired of having to get up super early just for that. 😉 And I didn't really like the nature of it, being less concise, which is why I went for a generator. Loving that ever since.

For a very good BPM, use very long darks (like 10 min subs), lots of good bias frames and indeed flats. What error do you get when you want to use them?