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Hello Stephen,
I would be interested in understanding why you feel you need a primary mirror heater.
What type of scope do you have in mind?
On group observing nights in heavy dew conditions I have regularly seen steel tube newts with closed bottom ends dewing up their primaries. My experience with wooden/sonotube scopes is the same and as John B's, if your primary is dewing up, then the conditions are probably so bad you might as well give up anyway. I regularly do (or at least try to) all nighter sessions in conditions of heavy dew and/or frost with wooden truss tube dobs of various sizes and I haven't had a dewed up primary for many years.
Regards,
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