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Glen, while they might be closed tube scopes they are usually not airtight - I only speak with experience with my SCTs of course.
Despite their reputation, I've never had a case of internal fogging - with one exception...a particularly humid Sydney evening when I left my Tempest fans on when using my C11 HD, the fans drew the moist air into the tube and my dew heater was insufficient.
It'd be an interesting experiment of course, but I'd suspect the external surface of the corrector plate is the one you need to worry about most, and an oversized dew shield (by length) can go a long way...
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