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Shouldn't need to refocus that often in a session - but best to check the subs every so often if there's a significant drop in temperature over the imaging run.
Consensus is that hairdryers are not the best option as you might be blowing dust onto the lens surface at speed and heated!!!
That said, I've never done any damage to my lenses with the hairdryer method - but got dew straps now just in case.
Do you have a link to the Barnard's Loop pic you mentioned?
Doug
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