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As Laurie said, the heat build up inside an unfiltered scope is immense, a filter placed near the focus point is subjected to temps as high as 600 Celcius in a matter of seconds (tested in a 6" F10 scope) , a glass filter will crack, a plastic one will melt and then your eye will suffer Photochemical retinopathy in an instant.
For refractors there is the option of a Herschel Wedge, for all others an objective filter of optical density 5.0 is required.
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