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For DSO 1-2 arc second per pixel is the usual range. 1 arc sec s considered ideal by Roland Christen of AP fame based on average seeing of around 3 arc secs.
At my dark site I find on nights of good seeing .42 arc secs gives sharp images but on weaker seeing its better to go to .89 arc secs.
But this is for DSO not planetary where long focal length is used.
Greg.
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