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It's all about thermal management.
Refractors will consistently work straight out of the box..
A large Newtonian requires one to take care of convective plumes in the light path, both from the thermal mass of the optic and the ground upon which it sits. Once you get that right, there is only one thing that will beat a large Newtonian for planetary and that is a large binocular Newtonian.
And yes, I used to own a 7" AP so I have that as a reference point.
~c
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