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Sorry, my bad.
So yeah, compare a 102mm f/5 achro to a 200mm f/5 Newt and you see the vast difference between them. One fits in a backpack, the other is a meter long and 11kg. Same focal ratio but very very different telescopes,one costing about triple the other. To keep a fast focal ratio as you increase the focal length gets very expensive very quickly. Same for camera lenses - compare prices for f/2.8 zooms to f/4 zooms.
Point remains. The faster your ratio, the shorter exposures you can take, and that's a good thing - but with long focal lengths the fast tubes cost a lot more.
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