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Always had to do longer flats with longer FL and a lot more of them or they're grainy. Apart from that longer FL fov is almost always better illuminated than shorter FL. The exceptions to that would be if you have a large imaging circle (like an FSQ) or a small one (like a C11). I also found that flats shot with an OSC might not correct every color the same way. It might work for some channels but over/under correct for others. So in practice I'd shoot flats for each channel with different durations then debayer to channels respectively. But it's a lot of work.
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