Yes the blue circle/donut will be redrawn on each frame so you can adjust it and take a single frame and see in on screen.

I think it will be fine for targets in the centre where the only detail out to the edges will be stars as these can be dealt with in post processing. For bigger targets (like nebulae) that extend to the edges then you will loose detail at the edges of the frame (as they won't be in as sharp focus as the centre) but then if its looks OK no problem, or you can crop.

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