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Richard, a cooling fan is a must for Newtonian scopes. The mass of the mirror is quite large and the sooner it is cooled toward ambient temp the better. A mirror who's temp is close to the air temp will show better images as it is not radiating warm currents of air (like what a mirage over a hot desert or road.)
I would always use a fan on a Dob or any scope with mirrors with any mass.
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