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Subs can be substandard for various reasons. FWHM is only one which would indicate the focus went off, the guiding got worse or was wind affected, clouds came over, dew formed, you bumped the mount in the middle of an image being taken etc etc.
Subs affected by cloud will generally degrade the stack noticeably and give odd background colours that are hard to remove.
Slightly worsening focus if bad can damage a stack, If slight it will a bit. You can always compensate with some deconvolution. Out of focus in one colour can cause coloured rings around stars in that colour.
But for best possible resolution yeah be somewhat strict on the standard.
It comes down to how hard you want to work to get the best possible image under the circumstances of clear skies or clouds, available time, position of the object etc.
Best subs will generally be when the scope is near the zenith.
Greg.
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