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Eric, I have observed with 8 and 12 inch tube dobs in Winter here in Melbourne over the last few years and with a cooling fan at the rear, I can only recall 2 or 3 times which dew only started to form. I think the tube design did contribute to protecting the mirror solmewhat. I had more problems with dew on the eyepieces. I had more issues (naturally) with SCT's on the corrector plate (but that's to be expected).
Other may provide better evidence, but I think it has a lot to do with the environment around you. i.e. out in an open paddock in the country, or a suburban block.
regards,
Stephen
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