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Originally Posted by pk3lee
Is Dob the only type need to cool down? I am very confuse about this point.
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let us forget about the mount ie the dob bit for a second.
the tube that the mirror is a cylindar that has the mirror at one end. the other end is open. The system is open to the elements, so that the air that is in the cylindar can be colder than the mirror as the temperature at night cools down.
this leads to what i can best describe as a small heat haze that distorts the view. not a lot when you are a beginner, but as you get more experienced, the more finicky you get about it.
so to get around this you let your scope cool for say one hour so that hopefully your mirror and the surrounding air are the same temperature