It depends on the target. With planetary video, I use a few seconds of video, and the stacking program I use splits the video into individual frames. Deep space objects I might take 50-100 1 minute exposures for good detail.
If the objects change over time how do you know that doing this isnt giving a false impression of the object? Taking a video of a spinning wheel, for instance, and then mashing a bunch of frozen frames together, is going to give an innacurate representation of the object
The only time that would really be a concern is with small and close objects like planets, but even then they are so bright you only need a few seconds to a few minutes of video for a sufficient amount of light frames. Sure, Jupiter's moons might be slightly different from night to night, but there's no way you can see detailed change in its clouds in a few minutes time. As for DSOs, I'm pretty sure some of the nebulae we see today look almost exactly the same as they did a century ago (for amateur astronomers at least) because they are light years wide and many light years away. There's no way I would see a change even with several weeks worth of imaging sessions on some objects.
Stars twinkle.
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