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Yep in general dew isnt a problem for a dob style scope, although some large truss dobs users have a heater on their secondary mirror.
A tube extension however is a good idea for blocking stray light.
Stray light can be seen from the focuser from out the end of the tube in many dobs (including GSO dobs) and it can also illuminate the spider vanes which can also be seen from the focuser.
A nicely blackened tube extension will improve contrast.
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