Found out my telescope mirror was 20% smaller than I thought

Got my used 8 inch dob last month and was measuring stuff just for fun. Checked the primary mirror with a ruler, turns out it's actually only 6.4 inches across. The tube says 8 inch but the optical diameter is way less. Looked it up on a cloudy nights forum thread, apparently some budget makers give you the tube size not the mirror size. Has anyone else measured their scope and found a big difference like this?

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veramiller1mo ago

The tube says 8 inch but, funny thing, that mirror size probably affects your focal ratio more than you think.

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