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Chrys
Had a similar issue ( not quite the same ) 2 weeks ago with the same target NGC 5128 ( read my post in Beginners Astrophotography ) Had to refocus just over halfway through my subs ( only because I noticed it , if I hadn’t checked all my subs would have been slightly out of focus )
I put my problem down to the focus drifting caused by atmospheric diffraction / dispersion and Jetstream as the object ascends from lower altitudes ( 30 degrees ) up to higher altitudes ( 70 degrees ) ?
Doesn’t happen that often
Martin
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