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A Newtonian telescope uses a large mirror, this is a weighty item and holds a lot of heat. A scope like this works better if the mirror is the same temperature as the surrounding air.
So to aid cooling the mirror down you can force air onto the mirror with a fan.
In reality even with a fan the mirror will almost always be a few degrees warmer than the surrounding air.
A warm mirror produces convection air currents in the tube and disrupts the path of light. Another thing is a "boundary" layer of warm air on the surface of the mirror that produces a mirage effect on it's surface.
I have found blowing air across the mirror surface and cooling it with a rear fan really sharpens up the image detail especially when viewing planets.
Hope this helps
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