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The focal length of the ED100 and ED120 are the same - 900mm. At prime focus the ED100 is F9, the ED120 F7.5. I use Bintel's Orion 0.8x reducer to get down to F7.2. Of course the focal length then drops to 720mm for the ED100. The ED120 with the same reducer becomes F6 at 720mm.
The ED100 at F9 doesn't need a field flattener. The stars are sharp to the edge, although a flattener can help smooth the center hot spot.
But it's not all about focal ratio and focal length. That extra aperture of the 120mm gets you fainter stars than the ED100. Not a massive difference, about 40% more I think it is.
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