A friend and I were talking about this last night, do flats have to be in focus, if so, why? The results of our thought experiment are: flats are recording variations in illuminance caused by a few things:
I would think “yes” as focus setting may affect the vignetting profile. As an extreme example, if you were so out of focus the secondary became visible then that would impact the quality of your flats.
Even a refractor has an image circle profile that is a function of distance from the lens where some portion of the distant sky is concentrated and the rest never makes it down the tube. You want that profile to match your imaging.
The foregoing is only a product of my musings but I think it makes sense.
Roy
I have not found any difference taking flats at varying focus positions. Actually most systems focus drifts so each frame will like be slightly different. In practice, this is not a problem. That said, I do return the average focus position ie: 4150 when I take flats. That should stay the same for a given setup. Out of your list, vignetting is the only variable that theoretically changes in different positions, but I would imagine that it would take a pretty big difference in focus position to see it. (like inches from focus).
Thank you, Mystic_Hill and pmumbower. For the record, I haven’t tried to take flats when I was inches from proper focus. I tried taking sky flats at dusk prior to refocusing after my last trip a couple of weeks before. From week to week, my focus might shift a millimeter or two as the temperature changes. Not inches.
I think I see your point about vignetting, but that said, my thought was the shape and edges of dust-mote-shadows might change, introducing artifacts in pre-processed images. I may get curious and add a few out-of-focus flats to my next session so I can see what being several millimeters away from proper focus looks like.