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You want to see something like a parabola with decently distinguishable minimum when plotting fwhm against focus, as with focnext ( see http://sabzi.lowell.edu:8080/confluence/display/LIGS/Focus+Log+Plotting
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In principle, a focus setting once made will be maintained through the night by the AOS automatic update, irrespective of elevation angle. If this were strictly true, only one focus run would be needed per night, and the best place to do that would be at an elevation of 60-70 degrees from the horizon, where the seeing is better. This is probably not true as yet, and the jury is out as to how often focus runs need to be done. This question interacts with the similar question of how often the AOS needs a WFS correction update.
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