Not sure if this is the best subforum, but here goes: I tried to photograph YU55 when it flew by, and have been combing through a pair of photos, looking for a dot in one and a blank spot in the other. So far there have been at least 5 instances where something appeared in one photo only, but checking the locations with google sky and SIMBAD, each location indeed has a faint (magnitude 10+) star present. Each photo was taken with a 1.3 second exposure time, which I had thought was sufficiently long to average out any atmospheric turbulence (the twinkle). My reference text, Roggemann and Welsh "Imaging through turbulence", has a long chapter devoted to atmospheric turbulence in this specific context, and while they have a detailed discussion about the spatial extent- the correlation length as compared to the entrance pupil- there does not appear to be a similar discussion about timescales: short-time imaging gives speckle, long-time imaging gives uniform blur, but I can't find a derivation about the correlation time (which divides the two regimes). I'd appreciate any pointers, references, etc. on this, thanks.